Download power board and battery selection guide

POWER BOARD AND BATTERY
SELECTION GUIDE
Made for students,
by students
Table of Contents
1
Power Distribution and Safety Board ................................................................................................... 4
1.1
Overview ....................................................................................................................................... 4
1.1.1
Power System Requirements ................................................................................................ 4
1.1.2
ModBot Example Power Requirements................................................................................ 5
1.1.3
ModBot Example High Level Requirements for Safety, Runtime, and Modularity .............. 8
1.2
Motor Safety Circuitry................................................................................................................... 8
1.2.1
Motor Protection Overview .................................................................................................. 9
1.2.2
Noise Protection ................................................................................................................. 10
1.2.3
Over-Current Protection ..................................................................................................... 12
1.2.4
Motor Circuit Over-Voltage Protection............................................................................... 14
1.2.5
Motor Circuit Reverse Voltage and Current Protection...................................................... 15
1.2.6
Motor Circuit Reverse Polarity Protection .......................................................................... 16
1.2.7
Motor Circuit Under-Voltage Protection ............................................................................ 17
1.2.8
Battery Stress Mitigation through Local Power Sources .................................................... 18
1.3
Electronics Safety Circuitry ......................................................................................................... 19
1.3.1
Electronics Protection Overview ......................................................................................... 19
1.3.2
Electronics Circuit Over-Voltage, Under-Voltage, and Reverse Polarity Protection........... 20
1.3.3
Electronics Circuit Regulation ............................................................................................. 20
1.3.4
Switching and Linear Regulators ......................................................................................... 20
1.3.5
Electronics Circuit Over-Current Protection ....................................................................... 22
1.4
Sensor Circuit .............................................................................................................................. 24
1.4.1
Designing Sensor Outputs to Meet the Input Needs of Your Microcontroller: Electronics
Voltage and Current Sensing............................................................................................................... 24
2
1.4.2
Battery Voltage Sensor ....................................................................................................... 27
1.4.3
Battery Current Sensor ....................................................................................................... 27
1.4.4
Electronics Current Sensor .................................................................................................. 27
1.5
Power System Interface .............................................................................................................. 28
1.6
Battery, Power Board and Battery Charger Interface ................................................................. 30
Battery Selection ................................................................................................................................. 33
2.1
Battery Chemistry ....................................................................................................................... 34
Page 2 of 45
2.2
Protection Circuit Module........................................................................................................... 35
2.3
Cell Balancing .............................................................................................................................. 36
2.4
Battery Charging ......................................................................................................................... 36
2.5
Separation of Power Circuits ...................................................................................................... 37
2.6
ModBot Battery Selection Process ............................................................................................. 38
2.7
Battery Pack Design .................................................................................................................... 40
2.8
Load Analysis ............................................................................................................................... 42
3
Things to discuss with teammates ...................................................................................................... 44
4
Board Components List ....................................................................................................................... 44
5
PCB Artist Power Board Layout ........................................................................................................... 45
Page 3 of 45
1 Power Distribution and Safety Board
1.1 Overview
One of the most critical but often overlooked sub-systems for any electronics device is the development
of a robust power distribution system. The power distribution system’s main responsibility is to provide
an interface between the power source and any electronic components. As various components may
have different power requirements, the power distribution system regulates the power source (which in
the case of the ModBot is a DC battery voltage) down to several different voltages required to power a
variety of components. The power distribution system must perform this function while simultaneously
protecting sensitive electronic components in the device from dangerous power conditions that could
occur, for example the counter-electromotive force (CEMF) from motors. For many first time designers
of power distribution systems, it is surprising that the development of robust safety circuitry often
requires the most continued effort.
In general, safety circuitry is defined as any measure used to protect sensitive components from
situations that could cause the components to operate outside of their safe operating ranges. Such
situations include:






Over-current
Over-voltage and under-voltage
Reverse polarity
Reverse voltage and current
Transient voltage and current spikes
Noise
The first step in designing the safety circuitry is to determine the power system requirements, which are
described in the next section. A well-designed power system will meet safety needs while taking into
consideration the overall system efficiency in terms of power dissipation and circuit footprint.
1.1.1
Power System Requirements
Designing a power system requires determining the power requirements of each component that will be
utilized in the overall system. At first, this can seem tricky because early on in the design process, not all
component choices will be finalized. The key to beginning your design is to identify the components that
will likely have the most constraining needs or be the largest sources of problems. Motors are some of
the most common components to define the safety circuitry requirements. Motors can require quick
changes in power and/or current and may also be the source of CEMF or current spikes that harm
sensitive components. Furthermore their effects may be additive; it is possible that all of your motors
may cause a current spike at their stall currents1 simultaneously. Common sensitive components are
your main processor board(s), such as an Intel Atom or Altera FPGA board.
1
For more on stall currents and motor specifications, please see Motor Selection Guide Section 1.9.
Page 4 of 45
In some cases, sensors also have very strict power requirements. Two examples of common sensing
circuits that could be at risk are voltage and current sensors. In the case of the ModBot, they were
sensitive due to the use of an Arduino, which operates between zero and five volts (see next section for
further explanation).
It may be impractical to develop a special safety circuit for every component, so it is often helpful to try
to group components where the requirements are set by that group’s most constraining members. Two
very common groupings are to have one safety circuit that handles the sources of CEMF but is more
tolerant to a variety of power inputs, for example your motors and their directly associated circuitry,
such as their H-bridges.2 This grouping is sometimes referred to as the unregulated motor power circuit.
The other common grouping is for those components that require a more specific and constant input
(i.e. regulated input) but are not a source of power fluctuations. This often includes items such as your
main processor board, any sensors, and some kinds of actuation, such as small servomotors that run at
low enough power and/or have their own internal power circuitry to prevent issues seen in normal DC
motors. This second group is commonly referred to as the regulated electronics power circuit. In many
cases, this group is split into more than one electronics power circuit because many common
components function at a specific voltage (12 V, 5 V or 3.3 V), so a separate, similar electronics circuit
may be developed for each one. The electronics portion may include special connections for sensors,
but it is also very common that these devices do not require these additional protection efforts.
Thanks to the electronic industry’s standardization of common inputs (like 12 V, 5 V and 3.3 V), the
process of developing the power regulation system and its safety circuitry is more manageable. The
regulated electronics circuit designs can often be very similar with only different value components
(resistors, capacitors, etc.). Similarly for the unregulated power circuitry, even if the exact motors aren’t
known, initial ranges can be established such as the maximum power the motors will require and the
maximum total stall current the motors will be allowed to operate at. Once these ranges are
established, then any motor—or set of motors—that are within that range will be acceptable to
interface with your motor power circuit.3
This establishment of interfaces is a key part of any subsystem design, as once they are established both
the people working on the power system and the people working on the exterior systems can work
more independently so long as they stay within the agreed interface requirements. The key to
establishing the interfaces is to examine what specific functions, information, and requirements each
subsystem needs to provide the others.4
1.1.2
ModBot Example Power Requirements
Following from the design process outlined above, the first step to developing the ModBot power
regulation and safety circuit system was to identify what were expected to be the most demanding and
constraining components. The major components that need to be powered are the four drive motors
2
For more on H-bridges and motor controllers please see the Motor Controller Guide.
More on establishing these ranges for motors can be found in the Motor Selection Guide Sections 1.3 and 1.5.
4
More on interfaces can be found in the Cornell Cup USA presented by Intel Interface Guide.
3
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and their motor controllers, the Intel Atom Board, as well as a myriad of common electronics such as the
common lower level I/O helper boards (like the Arduino, or the future Intel / Arduino Galileo) and their
accessories, common sensors like the motor’s encoders, and possibly smaller servo motors that can be
run in the 4-6 V range. Of these, the four drive motors are used to determine the unregulated motor
power circuit starting requirements. The Intel Atom board, helper boards, and the servo motors are
used for the regulated power circuit starting requirements. The sensors and the helper board
accessories’ needs were lumped into the helper board requirements since the helper boards were being
used to run these components; if the helper boards’ requirements could be met then so could these
component’s requirements. A similar argument could also be made for the small servo motors that were
to be driven by the helper boards. However, before these assumptions can be confidently made, testing
should occur to confirm it.
In order to begin the process of establishing the exact requirements, any designer should begin by
examining the components’ data sheets and then testing each of these components to determine its
characteristic performance, requirements, and power demands. The characteristic performance and
requirements of a component are different for every type of component. They are listed along with the
power requirements on the component’s data sheet, which can often be found online or in some cases
by contacting the manufacturer directly. However, it is not uncommon that some components do not
live up to the promises of their data sheets or the data sheet tolerance values are not very precise. Any
components considered critical to your system or considered to have wide tolerances5 should be tested
to verify that they comply with the specifications listed in the datasheet.
As mentioned above, for the ModBot system the most critical (and perhaps most constraining)
components were the four drive motors and the Intel Atom board, and hence they were selected for
testing. The motors were from a reliable vendor, Maxon, so it was likely that the data sheets could be
trusted—at least well enough for power system development to begin. However, it is still important to
test components in conditions as close as possible to their expected operation to see if there are any
additional characteristics you may have missed in the data sheet that are actually critical to your overall
system’s performance. This can be especially important if working with “cheaper” components.
The motors for the ModBot were tested under three different load scenarios by connecting a wattmeter between them and a battery that is used to monitor the motors’ current draws and voltages.
First, the motors were tested with no load by lifting the ModBot off the ground. Then they were tested
with the expected full weight6 of the ModBot to determine nominal load current. Finally the max stall
current was measured by holding the ModBot wheels stationary (using either a clamp or just the tester’s
hands) under load while running all the motors for a few seconds at the expected maximum input
voltage of the ModBot. Caution: performing this stall current test for too long can damage the motors
and may even cause them to burn out completely. The goal of this test is to observe the stall current
5
A general rule of thumb for a component’s characteristic tolerance to be considered “wide” is if the design or
operation of the overall device would have to be changed if the value of the characteristic falls on one end of the
range or the other; another rule of thumb is if the range extremes are greater than or equal to the median
characteristic value ± 20% (i.e. range = [. 8 ∗ 𝑚𝑒𝑑𝑖𝑎𝑛, 1.2 ∗ 𝑚𝑒𝑑𝑖𝑎𝑛]).
6
Please see the Chassis Design guide for weight estimate information.
Page 6 of 45
value. Once that spike is reach and the apex is clear, the test can end. The results of these three critical
tests are then listed in Table 1 below.
For the next set of tests, the Intel Atom Board was tested by first powering it with its provided DC power
supply and monitoring the current draw. The board was first tested with no additional components
connected to it to determine its normal current draw. Next the Arduino, as an example “helper board,”
was connected to the Atom board where the current was again measured. Finally the board was
connected with all other peripherals such as an external monitor, mouse, and keyboard. In all cases, the
current was monitored from boot up to shut down. The results of all these tests are summarized in
Table 1 below and they represent the initial power requirements of the ModBot. Please note that all
motor values are listed as combined needs for all four motors, therefore the current draw for a single
motor is 25% of the current draw listed for all four motors.
Table 1. Power requirements of components under various conditions
Module
Nominal
Voltage (V)
Current
Demand (A)
Nominal
Power (W)
Regulated
Intel Atom Board
12.0
1.0 – 1.2
12.0-14.4
Yes
Arduino
12.0
0.100
1.20
Yes
4 Steering Servos
5.0
1.0
5.0
Yes
4 Motors: Normal Load
24.0
6.0
144.0
No
4 Motors: Stalled
24.0
26.3
631.2
No
4 Motors: No Load
24.0
0.6
14.4
No
In addition to meeting these power requirements, the ModBot power system should be prepared to
accommodate multiple additional outputs, and provide 12 V and 5 V regulated voltage outputs to power
the Intel Atom board, and the Arduino/servos, respectively.7 The motors required a different voltage of
24 V, and hence an unregulated 24 V output was added to the board. Regardless of whether the motor
voltage was different from the electrical components, a separate output for the motors should be
included in the design for proper isolation needs. How to handle these isolation needs is discussed later
in Section 1.2 as part of the safety and circuit protection discussion.
7
Please also note that similar tests were performed for the Arduino itself and the servo motors as were done for
the other components but the results followed their data sheets well. Aside from requiring 5 V instead of 12 V,
their requirements were less significant than those of the drive motors and Intel Atom board.
Page 7 of 45
1.1.3
ModBot Example High Level Requirements for Safety, Runtime, and
Modularity
The other main sources of ModBot requirements came from the needs for safety, the expected runtime
for the entire system, and an overall goal for modularity. Of these, one of the most important
requirements of our entire system is safety, and with that can also come a need for redundancy. For
example, if wires are connected incorrectly or a short circuit is accidentally created, there needs to be
circuitry in place which will protect the electrical components of the entire robot. The goal to meet
these requirements is to design failsafe mechanisms to protect against all of the following potential
dangers in our circuit:







Overcurrent - Too much current can melt wires and components.
Overvoltage – High voltage can damage electronic components.
Under-voltage – Low voltage can result in poor performance.
Reverse polarity – Mixing up positive and ground can permanently damage batteries and
components.
Noise or power spikes – Power spikes can cause unpredictable or imprecise electronic control.
Battery mix ups – Switching batteries will result in incorrect voltages and poor performance.
Reverse current– Reverse current can destabilize the battery during operation.
These conditions could all damage the circuit, the battery, other components on the ModBot, or even
pose dangers to users in extreme cases. More on the safety and related redundancy solutions to meet
these requirements can be found in Sections 1.2 and 1.3. However, it is worthwhile to mention that
safety requirements like these are sometimes not given the attention they deserve, especially in
prototype designs, and hence they can potentially lead to frustrating (and expensive) consequences if
not dealt with properly.
Additionally, the ModBot is required to have a run time of at least 30 minutes on a single charge. This
will play a large role in the battery selection process described in Section 2.7, which in turn is related to
the power system. Furthermore, the entire ModBot had the design goal to be as modular as possible,
which for the power system meant that it should not only offer a variety of power outputs but also
make use of universal connectors. With the requirements for the power system distribution system now
defined, the actual design of the physical system could begin.
1.2 Motor Safety Circuitry
The following schematic shows the location of components and wiring for the power board. It was
generated using PCB Artist, a free software that can be found online. The purpose of the diagram is to
aid in visualizing the circuit. All of the elements in the diagram will be discussed in this guide, including
the rationale behind their use. The files used to create this figure can be found on the Cornell Cup USA
presented by Intel website, where this document is also posted.
Page 8 of 45
Figure 1. Power Distribution and Safety Board Circuit
1.2.1
Motor Protection Overview
As mentioned before, since it isn’t practical to design a safety circuit for every component, there will be
two groupings for the ModBot component power and safety circuitry: an unregulated motor circuit and
a regulated electronics circuit. This section describes the design of the first grouping, the unregulated
motor power circuit.
As described in the previous section, the motor power circuit has to safeguard against certain dangers
that could affect the overall circuitry like over-current, over-voltage, under-voltage, reverse polarity,
reverse voltage and current, transient voltage and current spikes, and noise. This section will also
explain the mechanisms that can be added to your circuitry to lessen those dangers.
To help understand the structure of the power system and safety circuitry, Figure 1 shows the
unregulated motor power circuit, surrounded by a bold red line to help the reader locate the circuit
components. 8 The main sets of components that make up the unregulated motor power circuit are:




8
a set of capacitors used for noise protection
a thermal circuit breaker for over-current protection (off board and hence not shown in Figure
1)
a TVS (transient voltage suppression) diode for over voltage protection
a positive channel metal oxide semiconductor field effect transistor (P-MOSFET) for reverse
polarity protection
See the Cornell Cup USA presented by Intel website for the source files used to create Figure 1.
Page 9 of 45


a battery protection PCB for battery under-voltage protection (not shown in Figure 1 but
included as part of the purchased battery)
a set of capacitors to reduce battery stress
Each of these aspects and its design selection process will be described in the sections below. Figure 1
also shows how the motor power circuit interfaces with the motor utilizing the Molex connector PL5 as
is described in Section 1.5.9
1.2.2
Noise Protection
One major drawback to working with DC motors is the large amount of electrical noise they produce. If
not protected against, this noise can interfere with your sensors and can even impair your
microcontrollers, causing voltage dips on your regulated power line. Large enough voltage dips can
corrupt the data in your controller’s registers or cause microcontrollers to reset. As a result, decoupling
capacitors will be used across power and ground to suppress the noise resulting from the motors. For
example, a bank of decoupling capacitors is connected between line and ground as shown in Figure 1 to
filter unwanted AC noise and pass DC voltage.
When selecting capacitors for this purpose, there are three key parameters that you need to consider:
capacitor voltage, capacitance, and material.
1.2.2.1 Voltage
As a rule of thumb, choose a capacitor that is rated at least twice your input voltage to tolerate noise
spikes. In the case of the ModBot, the maximum input voltage was 24 V, so a minimum voltage rating of
48 V would be recommended.
1.2.2.2 Capacitance
The transient current spikes produced by the motors can be used to approximate the correct
capacitance. Using the equations of operation for a motor below, one can approximate an upper bound
on the current spike that will be produced when the motors enter stall or when the motors change
direction.
𝑑𝐼
𝑉 = 𝐿 𝑑𝑡 + 𝑅𝐼 + 𝑘𝑏 𝜔
(1)
𝑀𝜔 = 𝑘𝑇 𝐼 − 𝑣𝜔 − 𝜏
(2)
In these equations,



9
𝐿 is the motor’s inductance
𝐼 is the current through the motor’s windings
𝑅 is the motor’s winding resistance
For more information on power system interfaces see Section 1.5.
Page 10 of 45




𝑘𝑏 is the motor’s back electromagnetic force (emf) constant
𝑀 is the rotor’s moment of inertia
𝑘 𝑇 is the motor’s torque constant
𝑣 is the motor’s viscous friction
Typically, all of these above constants can be found on the motor’s data sheet. The rest of the terms are
variables,



𝑉 is the voltage applied to the motor
𝜔 is the rotor’s angular velocity
𝜏 is the torque of the rotor due to its applied load
All of these variables are dependent on the motor’s operation point.10 When the motors change
direction from no load speed, the current will approximately spike according to the equation below:
𝑉
𝑉𝑘𝑇
𝐼𝑟𝑒𝑣𝑒𝑟𝑠𝑎𝑙 ≈ − 𝑅 − 𝑘𝑏 (𝑅𝑘
𝑏 𝑘𝑇
)=−
2𝑉
(3)
𝑅
A good approximation for the upper limit of the current spikes is ±
2𝑉
.
𝑅
As an example, the required
capacitance will be calculated for four motors that have approximately 3 A current spike each (for a
combined 12 A) and a transient spike time of around 10 ms. The transient spike time was estimated
using an oscilloscope with a data recording feature to monitor the motor circuit. Using the capacitance
equation below, the needed capacitance will be around 2500 μF to sufficiently help mitigate the stress
caused by current spikes.
𝑖𝑐 = 𝐶
𝑑𝑣𝑐 (𝑡)
(4)
𝑑𝑡
𝑖𝑐 = 𝐶 (
2∗𝑉
𝑡
12 A = 𝐶 ∗ (
)
(where V is the voltage across the motor)
2 ∗ 24 V
)
10 ms
𝐶 = 2500 μF
1.2.2.3 Material
Based on the calculated capacitance, it is recommended to use an electrolytic capacitor, since
electrolytic capacitors are known for their large capacitance. A capacitor bank of three capacitors was
used in order to achieve the desired total capacitance needed to eliminate any voltage noise from the
motors. The capacitors were connected in parallel as shown in Figure 1 to maximize the capacitance of
the capacitors bank, and hence filtering as much noise as possible (recall 𝐶𝑠𝑢𝑚 = ∑ 𝐶𝑖 for capacitors in
parallel).
10
For more on determining these last three variables please see Motor Selection Guide Section 1.2 specifically, the
details on the motor’s operation point.
Page 11 of 45
1.2.3
Over-Current Protection
Over-current can occur when the motor enters its stall mode of operation, causing the motor to draw its
maximum current, i.e. the motor’s stall current. In DC motors, the current draw is proportional to the
torque thus when the torque is at a maximum, the current draw will be at a maximum. This commonly
occurs very briefly as the motor first starts turning from a stopped position, but can also occur over a
longer period of time if the motor is asked to lift something too heavy or if say, the ModBot were to
drive into a wall and continue to push up against the wall with full power. As described in the Motor
Selection Guide Section 1.9, this will put the motor under a large strain and can permanently damage
the motor. Therefore, it is desirable to limit the time spent in stall mode if the scenario occurs.
Another condition that can cause an over-current scenario is when the motor changes direction, which
could produce over-current even higher than over-current due to stalling. However, the change in
direction over-current will be transient, and in general, does not pose as much as a risk to the circuit
elements or the motors as the stalling condition. Together this means the system will have to both
protect against stall current issues while at the same time safely permitting transient motor directional
change current spikes. Protecting the circuitry for this is described in the Motor Circuit Breaker
Overview and the Motor Circuit Breaker Design sections below.
It should be noted that the motor battery can also be susceptible to over-current. However, this can
often be easily remedied by selecting a battery that has a built in PCB that protects the battery from
discharging too much current. For the ModBot’s LiFePO4 motor battery, the manufacturer data sheet
shows that the built in protection can handle a maximum discharge current of 16 A, which was also
confirmed by performing bench testing on the battery. The maximum of 16 A is well within the system’s
measured over-current of 10 A total for stall mode.
1.2.3.1 Motor Circuit Breaker Overview
Protecting the motor from an extended stall period, an off board thermal circuit breaker is utilized to cut
off the motor’s connection to the batteries, ending the stall condition and all power to the motor. A
thermal circuit breaker works so that when a certain dangerous current is reached, known as the trip
current, the thermal circuit breaker will begin to heat up rapidly. This rise in temperature is allowed for a
short period of time, which is referred to as the circuit breaker’s “time to trip” or “trip time.” Once this
time to trip is exceeded, the circuit breaker trips and shuts off the power.
Rather than allowing an instantaneous trip time, a finite trip time is used to accommodate the brief
transient over-current spikes that are present in many systems (such as when the motor changes
direction). This is very helpful for meeting the motor over-current requirements mentioned in the
section above.
Once the circuit breaker does trip, the breaker must be allowed to cool down briefly and then must be
reset manually by the user before operation can resume. In order for cooling to begin, the current must
drop below what is referred to as the hold current of the thermal breaker. The hold current is normally a
little less than the stall current in order to create a hysteresis effect. If the threshold was the same both
Page 12 of 45
for the hold current and the trip current, the breaker would rapidly flip back and forth between heating
up beyond the threshold and cooling down just below the threshold.
Figure 2 illustrates the behavior of thermal circuit breakers during over current situations. As shown, the
breaker starts to heat rapidly when it reaches 𝐼𝑡𝑟𝑖𝑝 . It heats up until it reaches 𝐼𝑆𝑇𝐴𝐿𝐿 , and when the
current reaches 𝐼𝑆𝑇𝐴𝐿𝐿 the breaker trips. Then breaker starts cooling until the current drops below
𝐼𝐻𝑂𝐿𝐷. After that, normal operation conditions resume.
Figure 2: Thermal Circuit Breaker Hysteresis Diagram
1.2.3.2 Motor Circuit Breaker Design
When choosing a thermal circuit breaker for the ModBot, the two major design parameters are the trip
current and time to trip. The trip current should be low enough to trigger the circuit breaker under stall
conditions with a time to trip short enough to prevent damage to the motors under a sustained stall
period. Yet, the time to trip should also be long enough to accommodate for the transient over-current
spikes so it doesn’t trip when the robot rapidly changes direction.
This careful tradeoff often makes choosing a thermal circuit breaker tricky and furthermore, once you
do find your ideal trip current and time to trip parameters, it is almost impossible to obtain a breaker
that is rated at your exact requirements. Part of the reason it can be difficult to find a breaker that
meets your requirements is that the time to trip is dependent on the current going through the breaker.
This means that if the input current is equal to a breaker’s rated trip current, the breaker will take longer
to trip than if the input current is much higher than the rated trip current.
As a good design rule, a thermal circuit breaker will trip in under a few seconds only if it encounters an
input current that is double its rated trip current value. Therefore, if you want a thermal breaker to trip
at 10 A in under a few seconds, you should choose a 5 A rated thermal breaker. In case of the ModBot,
Page 13 of 45
you want the breaker to trip at an 8 A stall current in under a few seconds, so a 4 A rated thermal
breaker should be used for the ModBot.
The largest problem with thermal breakers is if dangerous input currents are in the range between one
and two times the breaker’s rated trip current. In this case, the breaker will eventually trip, but most
likely not quickly enough to protect the components. Likewise, you should ensure that your circuit will
never operate in the one to two times the trip current range for any long period of time. If the operating
current was just over the breaker’s trip current, this can cause the unwanted and often unexpected
effect that the breaker trips after several minutes.
1.2.4
Motor Circuit Over-Voltage Protection
Transient over-voltages are also common, particularly when a motor switches direction. To protect
circuit elements from large transient over-voltage conditions, an appropriately named transient voltage
suppression (TVS) diode is utilized. When a transient over-voltage occurs in the circuit, the TVS diode
will enter breakdown. Breakdown in a TVS diode refers to the case when its voltage rating is exceeded
and causes the diode to act as a very weak resistor, or essentially as just a wire. Hence when a TVS diode
is connected in the circuit to ground as shown in Figure 1, the TVS diode becomes the lowest resistance
path to ground, effectively shorting out the rest of the circuit and preventing the voltage of the rest of
the circuit from exceeding the diode’s breakdown voltage. It is often said that the TVS diode used in this
manner “clamps” the transient over-voltage. TVS diodes are a useful way to handle voltage as they also
automatically “reset” once the over-voltage expires. Once normal operation resumes, the TVS diode no
longer prevents the current from escaping the circuit, and instead allows the rest of the circuit to be
powered as desired.
However, in some rare cases the over-voltage may not be transient; therefore, a thermal circuit breaker
is used in conjunction with the TVS diode. This topology is known as a crowbar configuration, where the
TVS diode is connected in parallel with the battery. This thermal circuit breaker is the same circuit
breaker used for over-current and hence acts as the main circuit breaker and power switch for the entire
ModBot. This design decision was made to ensure that in case of transient over-voltages, the TVS diode
would protect both the electronics and the motors. If the over-voltage condition continues for a longer
period of time, the circuit will depend on the thermal circuit breaker to trip, which would turn off the
entire ModBot, otherwise the TVS diode could be damaged. Thus, when choosing a TVS diode it is
important to consider the breakdown voltage and to recognize when it could be damaged.
TVS diodes may be described in terms of a number of different parameters on suppliers’ websites, and
may also be displayed as a graph. An example is shown in Figure 3, where the breakdown voltage is the
voltage at which the TVS diode begins to conduct current (𝐼𝑡 ) and becomes a low impedance path for
the transient overvoltage so that it can protect your circuit. 𝑉𝑛 is the maximum voltage at which the TVS
diode acts as an open circuit; any voltage over 𝑉𝑛 causes the diode to clamp and prevent the voltage
surge. 𝑉𝑐 is the maximum allowable voltage drop across the TVS diode; over voltage conditions that
exceed 𝑉𝑐 might damage the TVS diode.
Page 14 of 45
Figure 3: TVS Diode Characteristic Curve
In the case of the ModBot, the TVS diode breakdown voltage was chosen to be just above the battery
charging voltage (26.9 V). This voltage would be the absolute maximum voltage that the power circuit
would ever encounter under normal operating conditions since the motors are rated at 24 V ± 10%.
1.2.5
Motor Circuit Reverse Voltage and Current Protection
There are a few other scenarios that can cause reverse voltage and current in the power circuit that are
a result of a motor’s normal functional design that must also be considered. A motor can also function
as a generator because a counter electromotive force (CEMF) current can be created that opposes the
battery’s current direction when it is spun manually. This can occur if our ModBot was to roll down a hill,
or if it continued to coast after power was cut to the motors. In certain rare scenarios, the CEMF voltage
can become larger than the battery’s voltage, causing the current to change direction and recharge the
battery. In the case of the ModBot application (and many common applications), it is undesirable to
have a reverse current returning to the battery because the battery should only be charged through its
AC wall charger which allows the battery cells to balance properly.11 Given the ModBot’s 24 V battery, it
is very unlikely that under normal conditions the wheels will spin fast enough to generate above 24 V
and start charging the battery. When the motors are being driven by the battery, they could not build up
that CEMF voltage without first stalling, which would trip the ModBot’s circuit breaker. When they are
not being driven by the battery, that reverse voltage will disappear when the motors stop coasting and
the system will return to its rest state. Therefore, reverse voltage and current produced due to spinning
the motors manually does not pose risk of damaging components in the ModBot thanks in part to the
safety mechanisms implemented earlier.
Another source of reverse voltage and current that could possibly damage components in the ModBot is
the reverse voltage “kick” produced from the motor when it is disconnected from the battery suddenly
after being powered. Due to the inductive nature of an electric motor, when it is disconnected
suddenly, the motor creates a brief—but very large—reverse voltage, capable of arcing over a switch.
This can seriously damage the rest of the electronic components or the motor itself. The solution is a
simple and effective one: connect a “flyback” diode across the terminals of the motor so that any
11
Batteries that are improperly balanced can result in the batteries being severely damaged. For more on this,
please see Section 2.3.
Page 15 of 45
reverse voltage is dissipated across the internal resistance of the motor safely. The selected ModBot
motor controllers have this functionality built into them, to protect both the motors and the ModBot’s
circuitry, however it is not always true for all motor controllers. Hence it is important to check for when
selecting a motor controller and to use a flyback diode if necessary. 12
1.2.6
Motor Circuit Reverse Polarity Protection
If one were to accidentally reverse the connection of the battery, it would cause a reverse polarity
condition, which would seriously damage the circuit elements on the board. To prevent this condition, a
positive channel metal oxide semiconductor field effect transistor (P-MOSFET) is utilized. A P-MOSFET is
a three terminal (the terminals are known as the gate, source, and drain) device that can act as a switch.
Figure 4 shows what a P-MOSFET symbol looks like.
Figure 4: P-MOSFET Symbol
The P-MOSFET will conduct electrons from the source to the drain terminal by means of a conducting
channel when the gate-to-source voltage, VGS, is less than the P-MOSFET’s defined threshold voltage,
VTH, as shown in equation (5) below.
𝑉𝐺𝑆 < 𝑉𝑇𝐻
(5)
When VGS is less than the threshold voltage, VTH, the P-MOSFET is said to be “turned on” or “in the
saturation region of operation” (i.e. the voltage is at a level that will create a channel for current to flow
between drain and source). If VGS becomes too large, as in the case of overvoltage, the P-MOSFET will
“turn off” and protect the circuit. However, the P-MOSFET transistor alone is not sufficient to block any
reverse current because it allows current to conduct in both directions, although it is designed to carry
conventional positive current from the drain to the source.
In the ModBot design, the positive battery input is connected to the P-MOSFET drain, and the rest of the
output circuitry to the P-MOSFET source as shown in Figure 4. The gate is connected between a 10 V
Zener diode (which is connected to the positive battery side), and a resistor (connected to ground).
When the battery is connected properly, the gate restricts any current flow, and is pulled to 0 V at
ground, while the source remains at the battery voltage. This turns the MOSFET on. If the battery were
to be connected in reverse, the diode would turn on and maintain a +10 V 𝑉𝐺𝑆 , turning the MOSFET off
and effectively opening the circuit entirely.
12
For more information about motor controllers see Motor Controller Guide.
Page 16 of 45
When choosing a P-MOSFET for this application, there are a few parameters that need to be taken into
consideration, including:

𝑅𝐷𝑆(𝑂𝑁) , the resistance of the P-MOSFET when it is on



𝑉𝑇𝐻 , the threshold voltage at which the P-MOSFET turns on
𝑉𝐷𝑆 , the maximum voltage that is allowed across the P-MOSFET drain and source terminals
𝐼𝐷 , the maximum current that is allowed to pass through the P-MOSFET when it is on
So the P-MOSEFT that you choose would need to have a 𝑉𝐷𝑆 greater than or equal to the source voltage.
In case of the ModBot, the battery voltage is 24 VDC ± 10%, so a 30 V 𝑉𝐷𝑆 MOSFET would be suitable. It
is recommended to choose a P-MOSFET with 𝑅𝐷𝑆(𝑂𝑁) as small as possible to maximize the efficiency of
the circuit, roughly 1-10 mΩ. It is important when designing the circuit to know your circuit’s current
demand and make sure that 𝐼𝐷 of the P-MOSFET is greater than the maximum circuit current demand.
As mentioned before, a Zener diode will be used between the gate and source terminals to make sure
no reverse current is conducted through the P-MOSFET. When choosing a Zener diode, an important
parameter to take into consideration is the diode breakdown voltage, 𝑉𝑍𝑅 , which is the voltage when
the diode starts conducting in reverse mode. In case of the ModBot application, 𝑉𝑍𝑅 would need to be
bigger than 𝑉𝑇𝐻 to make sure that 𝑉𝐺𝑆 is always bigger than 𝑉𝑇𝐻 and therefore won’t conduct current
when the battery is connected in reverse mode. Also, 𝑉𝑍𝑅 has to be less than 𝑉𝐷𝑆 , which makes the
battery supply voltage sufficient to have the Zener diode conduct in reverse mode, when the battery is
connected improperly. Based on that, 𝑉𝑍𝑅 should be chosen to be within the limits shown in Equation 6:
𝑉𝑇𝐻 < 𝑉𝑍𝑅 < 𝑉𝐷𝑆
(6)
So in case of the ModBot, a typical P-MOSFET would have 𝑉𝑇𝐻 equal to 1-3 V, with 𝑉𝐷𝑆 around 24 V. A
𝑉𝑍𝑅 of 10 V should be sufficient to allow the Zener diode to prevent the P-MOSFET from conducting any
reverse current if the battery is connected improperly since 𝑉𝐺𝑆 will be held at 10 V, which is greater
than 𝑉𝑇𝐻 , and hence will keep the P-MOSFET in the off position.
The resistor that is connected to the gate terminal of the P-MOSFET is there to limit the current through
the diode, and can be any reasonably large value, for example in case of the ModBot a 18 kΩ resistor
was used.
1.2.7
Motor Circuit Under-Voltage Protection
An under-voltage scenario can occur when a battery has been in use for long periods of time or under
heavy stress. It will begin to lose its charge and fall below its rated supply voltage. Some of the
consequences of under voltage are premature shutdown of the ModBot circuit (which in turn can cause
issues like loss of data) and over draining of the battery which can permanently damage its cells.
Under-voltage protection on the ModBot is provided by the battery’s protection PCB. However, not all
batteries offer this, so establishing a battery cut-off voltage limit is another common method of
providing under-voltage protection. The battery cut-off voltage is the lower limit voltage at which
Page 17 of 45
battery discharge is considered complete. The cut-off voltage is usually chosen to be as small as possible
so that the maximum useful capacity of the battery is achieved; a high cut off voltage might cause the
battery to stop operating even with significant capacity remaining.
In case of the ModBot, the cut-off voltage was chosen to be when the battery voltage falls below 10% of
a motor’s rated voltage operating range, or below the electronic side voltages. This level does not have
to be established directly with hardware. Instead, a voltage monitoring system13 is also implemented on
the power PCB that allows software to control the cut-off voltage of the ModBot.
1.2.8
Battery Stress Mitigation through Local Power Sources
A battery's lifetime can be diminished if it is continually placed under high stress. High stress for a
battery refers to the rate at which it needs to provide large amounts of current, specifically current
levels that if sustained, will drain the battery in less than a certain threshold time; one hour is a common
threshold time. This can occur when a motor is switching direction or when a motor stalls for a short
period. One way to lessen battery stress is to utilize capacitors as local power sources. Luckily, the same
capacitors chosen to reduce noise earlier also serve this same purpose.14 Since the ModBot deals mostly
with low frequency transient spikes, very large electrolytic capacitors were used for noise reduction,
because they don’t need to have a fast response time. These same large capacitors are ideal for
buffering voltage spikes from the battery.
13
14
For more information about the voltage monitoring system see Section 1.4.
See Section 1.2.2 for more information on how appropriate type of capacitor was chosen for such application.
Page 18 of 45
1.3 Electronics Safety Circuitry
Figure 5: Regulated Electronic Circuit Components
1.3.1
Electronics Protection Overview
As discussed previously, there will be two groupings for components when designing the power and
safety circuitry: an unregulated motor circuitry and a regulated electronics circuit. In Section 1.2, the
unregulated motor circuitry was discussed. This section will discuss the regulated electronics circuit. The
electronics safety circuitry protects the following electronic components,



Intel Atom Processor Board
Arduino Microcontroller Board
Steering Servos
against over-current, over-voltage, under-voltage, reverse polarity, etc.15 The major difference between
the motor circuitry and the electronics circuitry is the maximum current drawn by the load. The
electronics side is designed for a total maximum continuous current draw of 6.3 A, divided into a 12 V
rail and a 5 V rail each limited to 3.15 A each, since the current draw for two rails is approximately the
same. The current drawn was calculated based on the 12 V and 5 V load components power
requirements in Table 1. Where the 12 V rail maximum load is 1.3 A (𝐼𝑖𝑛𝑡𝑒𝑙𝐵𝑜𝑎𝑟𝑑 + 𝐼𝐴𝑟𝑑𝑢𝑖𝑛𝑜 ) and the 5 V
rail maximum load is 1 A (𝐼4𝑠𝑒𝑟𝑣𝑜𝑠 ). So the actual total maximum continuous current draw is 2.3 A.
15
See the power requirements Section 1.1.3 for a complete list and definitions of these terms.
Page 19 of 45
Based on that, the circuit is designed for approximately three times the expected total current draw,
allowing for other electronics to be added later (such as from additional sensors or servos).
Figure 5 shows the regulated electronics circuit surrounded by a red line to help the reader allocate the
circuit components and understand the structure of the regulated electronics circuit. The main sets of
components that make up the regulated electronics circuit are:




a TVS diode for over voltage protection
a P-MOSFET for reverse voltage protection
a thermal circuit breaker for over current protection
5 V and 12 V switching regulators
Each of these aspects and their design selection process will be described in the sections below. In
Section 1.4, Figure 7 shows how the 5 V and 12 V rails interface with the electronic components utilizing
a separate Molex PL5 connector for both rails.16
1.3.2
Electronics Circuit Over-Voltage, Under-Voltage, and Reverse Polarity
Protection
Similar to the motor safety circuitry, the electronics safety circuitry needs to protect circuit elements
from over-voltage, under-voltage and reverse polarity. Since the ModBot is powered by a single 24 V
battery, the TVS diode used to protect the motor side also protects the electronics side from overvoltages. 17 The electronics and motor safety circuitries rely on the battery protection PCB for battery
under-voltage protection.18 For reverse polarity protection, both circuitries use the same P-MOSFET and
Zener diode circuits. 19 Figure 7 shows the location of each of these components.
1.3.3
Electronics Circuit Regulation
A major requirement that differentiates the electronics circuit from the motor circuit is the need to
regulate (or maintain) the voltage at a specific level. Mechanical elements such as the motors are robust
and do not require voltage regulation, thus regulators were not required in the motor safety design.
However, for the electronics 5 V and 12 V outputs, a LM2678 Texas Instruments switching regulator was
used. Switching regulators are known for their high efficiency, which in the case of the LM2678 can be
more than 90%, according to the manufacturer datasheet.
1.3.4
Switching and Linear Regulators
There are two common types of regulators, linear and switching regulators. A linear regulator is like a
passive circuit that can only step down (buck) input voltage and maintains a steady voltage output while
dissipating the difference between the input and output voltage as waste heat. The other type is the
16
For more information on power system interfaces see Section 1.5.
See Section 1.2.4 Motor Over-voltage protection for more details on the operation of the TVS diode.
18
See Section 1.2.7 for more information on battery under-voltage protection.
19
See Section 1.2.6 for more information on reverse polarity protection.
17
Page 20 of 45
switching regulator, which can be used to step down (buck) or step up (boost) the input voltage. The
switching regulator operates by controlling the switching duty cycle, which is the ratio of on to off time.
The chosen duty cycle determines the generated output voltage.20 The advantage of the switching
regulator is significantly less power dissipation resulting in higher efficiency. Linear regulators burn away
the difference between input and output voltages as waste heat to achieve their desired output, and the
amount of waste heat generated increases as the difference between the input and output voltages
increases. Switching regulators produce very little heat and often do not require any heat sink.
As a general rule of thumb, if your linear regulator is wasting more than 0.5 W of power, then a
switching regulator is the best option. Equation (7) can be used to calculate the amount of power that is
wasted in a linear regulator. Equation (8) can be used to calculate the amount of power that is wasted in
a switching regulator.
𝑃𝑜𝑤𝑒𝑟 𝑊𝑎𝑠𝑡𝑒𝑑𝐿𝑅 = (𝑉𝑖𝑛 − 𝑉𝑜𝑢𝑡 ) ∗ 𝐼𝐿𝑜𝑎𝑑
𝑃𝑜𝑤𝑒𝑟 𝑊𝑎𝑠𝑡𝑒𝑑𝑆𝑅 = 𝑃𝑖𝑛 − 𝑃𝑜𝑢𝑡
(7)
(8)
𝑃𝑜𝑢𝑡 = 𝑉𝑜𝑢𝑡 ∗ 𝐼𝐿𝑜𝑎𝑑
𝑃𝑖𝑛 =
𝑃𝑜𝑢𝑡
⁄𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦
In case of the ModBot, the expected power to be wasted in the 5 V and 12 V linear regulators can be
calculated as follows using Equation (7),
𝑃𝑜𝑤𝑒𝑟 𝑊𝑎𝑠𝑡𝑒𝑑5𝑉 = (24 − 5)𝑉 ∗ (1 𝐴) = 19 𝑊
𝑃𝑜𝑤𝑒𝑟 𝑊𝑎𝑠𝑡𝑒𝑑12𝑉 = (24 − 12)𝑉 ∗ (0.1 + 1.2)𝐴 = 15.6 𝑊
The power to be wasted in a 5 V switching regulator can be calculated as follows using Equation (8):
𝑃𝑜𝑢𝑡 = 5 𝑉 ∗ 1 𝐴 = 5 𝑊
𝑃𝑖𝑛 = 5 𝑊⁄0.9 = 5.5 𝑊
𝑃𝑜𝑤𝑒𝑟 𝑊𝑎𝑠𝑡𝑒𝑑𝑆𝑅 = 5.5 𝑊 − 5 𝑊 = 0.5 𝑊
The power to be wasted in a 12V switching regulator can be calculated as follows using Equation (8):
𝑃𝑜𝑢𝑡 = 12 𝑉 ∗ 1.3 𝐴 = 15.6 𝑊
𝑃𝑖𝑛 = 15.6 𝑊⁄0.9 = 17.3 𝑊
𝑃𝑜𝑤𝑒𝑟 𝑊𝑎𝑠𝑡𝑒𝑑𝑆𝑅 = 17.3 𝑊 − 15.6 𝑊 = 1.7 𝑊
Based on this calculation it is clear that the power waste produced by a linear regulator can be very high,
and the power waste increases as the difference between the input and output voltages increases.
20
See Motor Controller Guide for more on the duty cycle.
Page 21 of 45
Another parameter to consider is the regulator dropout voltage, which is the minimum difference
between input and output voltage for which the regulator can still supply the specified current. For
example, a low drop out regulator is designed to work well even with an input supply only a volt above
the output voltage. Switching regulators require a higher dropout voltage than linear regulators to
achieve the same stepped down voltage. In the case of the ModBot, the selected switching regulator
requires a 2 V maximum dropout voltage. However, this should not be an issue since the ModBot’s
battery voltage of 24 V is double or more than the required regulated voltages of 12 V and 5 V.
A disadvantage of utilizing switching regulators, however, is their large circuit footprint. They require
numerous filtering capacitors on the input and output of the chip along with a large inductor and
Schottky diode, as seen in Figure 5. The circuit footprint is manageable on a well designed PCB though.
For this example, the additional footprint size is well worth the gained efficiency and temperature
benefits as compared to using linear regulator. The manufacturer provides the normal operating
configuration in the datasheets. This is how you determine the proper size filtering capacitor, large
inductor, and Schottky diode.
1.3.5
Electronics Circuit Over-Current Protection
Over-current situations can result from even common mistakes or misuses, such as shorting the Arduino
microcontroller board I/O pins to each other, sustained over loading of the servo motors, and a short
circuit in the servo motors. High performance on-board thermal circuit breakers21 are used to
disconnect the electronics from the circuitry in case of an over-current situation. The thermal circuit
breakers on the electronics side are different from those used on the motor side.22 There are two
parameters that need to be considered when choosing a circuit breaker for this application, the trip time
and trip current. In case of the ModBot’s electronics side, its circuit powers sensitive electronic
components, which means that a breaker with fast trip time is necessary to reduce over loading of the
electronics as much as possible. For the trip current, choosing a current slightly over the rated trip
current will result in an extremely long trip time, possibly hours long. In order to obtain a trip time of a
few seconds, a good rule of thumb is to double the rated trip current. For example, a thermal circuit
breaker with a 3.15 A trip current will trip in less than a second if the current through it is roughly 6.3 A.
Manufacturer datasheets typically provide a logarithmic plot of the trip current versus trip time as
shown in Figure 6. A breaker with a trip current of 3.15 A will trip within 0.1-1 seconds based on the plot
in Figure 6
21
22
As compared to the motor’s circuit breaker which is larger and off board.
See the motor safety circuit (Section 1.2) for details.
Page 22 of 45
Figure 6: Thermal Circuit Breaker Trip Time vs. Trip Current
Regulators with included protection circuitry will clamp current if it exceeds a certain threshold value; in
the case of the ModBot regulators, a short-circuit current is clamped at 7 A. Because of this, a circuit
breaker with a trip current of 3.15 A was used on the output of the switching regulators to achieve a
near instantaneous trip time if an off board short occurs. Furthermore, the circuit breakers were placed
after the regulators, so that if a short circuit occurs across the electronics power outputs, the capacitors
adjacent to the output would discharge through the circuit breaker. This in turn would both buffer the
sharp spike on the regulator and help create a surge current though the circuit breaker that will cause it
to trip faster.
Page 23 of 45
1.4 Sensor Circuit
Figure 7: Sensor Circuit
Sensors are useful because it is important to know the real-time state of your battery. Two of the most
important (possibly most evident) things to keep track of with any battery are the current and voltage
outputs. The following sections show how to monitor power to your circuit, and can help you estimate
your battery life.
In the past, it was decided to order sensor devices from manufacturers, such as the Watts-Up sensor, to
avoid building sensor circuits from scratch. However, it was found that these sensor devices were rather
difficult to interface with—so the decision was made to design and build a sensor circuit, which turned
out to be relatively simple.
1.4.1
Designing Sensor Outputs to Meet the Input Needs of Your
Microcontroller: Electronics Voltage and Current Sensing
When designing a sensor circuit, it is important to consider the device that is interfacing with it. In the
case of the ModBot, the sensor circuit interfaces with the Arduino microcontroller which will perform
the actual “reading” of the sensor circuit output. The Arduino microcontroller, however, requires all
sensor input values fed to it to be within a 0-5 V voltage range. Therefore, in order to read something
like the true battery voltage of the 24 V battery, the battery voltage will need to be transformed into a
range that is acceptable to the Arduino microcontroller.
Page 24 of 45
Voltage dividing is a simple way of transforming the battery’s 0-24 V range into the Arduino's 0-5 V
voltage range. A typical voltage divider circuit can be shown in Figure 8, as shown it utilizes two
resistors, where the ratio between the two resistors controls the ratio between the input and output
voltage per the equation shown below.
𝑉𝑜𝑢𝑡 = 𝑅
𝑅2
1 +𝑅2
𝑉𝑖𝑛
(9)
Figure 8: Voltage Divider Circuit
For example, the voltage divider would need to maintain a ratio of,
5𝑉⁄
24𝑉 = 1/4.8
1
𝑅2
=
4.8 𝑅1 + 𝑅2
𝑅1
= 3.8
𝑅2
When designing the voltage divider circuit, selecting any two resistors that would maintain the above
ratio should be acceptable. The next stage after transforming the voltage to acceptable levels for your
microcontroller is to ensure that the voltage or current signal is measured in the most accurate and
noise free way as possible. A method of doing this is to utilize an operational amplifier (op amp) for both
amplifying the signal and reducing the noise. An op amp amplifies the difference between its two input
terminals (the 𝑉1 and 𝑉2 terminals), based on its closed loop gain (A); this is why op-amps are also called
differential amplifiers. Figure 9 shows how a differential amplifier is typically connected, where the
resistors are used to control the closed loop gain of the op amp. The equation below shows how the
output voltage of an op amp can be calculated.
Page 25 of 45
Figure 9: Differential Amplifier
(𝑅𝑓 + 𝑅1 )𝑅𝑔
𝑉𝑜𝑢𝑡 =
(𝑅𝑔 + 𝑅2 )𝑅1
𝑅1 +𝑅𝑓
𝑉𝑜𝑢𝑡 = (
𝑅1
) (𝑅
𝑉2 −
𝑅𝑔
𝑔 +𝑅2
𝑅𝑓
𝑉
𝑅1 1
𝑅
) 𝑉2 − 𝑅𝑓 𝑉1
1
(10)
This equation can be simplified by keeping in mind that the function of the op amp is to amplify the
signal without stepping up its input voltage. In other words, the op amp should be set up to have a
closed loop gain of 1. To achieve a closed loop gain of 1, Equation (10) can be simplified by first using the
following relationship of Equation (11) to create (12) below.
𝑅𝑓
𝑅1
=
𝑉𝑜𝑢𝑡 = 𝐴(𝑉2 − 𝑉1 )
𝑅𝑔
(11)
𝑅2
𝑅
𝑤ℎ𝑒𝑟𝑒 𝐴 ≜ 𝑅𝑓
1
(12)
From this simplified version, it is easy to see that for the gain A to equal 1, Rf must equal R1. This
simplifies Equation (12) further so that the equation for the output voltage is simply (13) below.
𝑉𝑜𝑢𝑡 = 𝑉2 − 𝑉1
(13)
You can choose any values for the resistors that meet the conditions above. One way of satisfying the
resistor requirement is to utilize a voltage follower orientation where 𝑅𝑓 = 𝑅1 = 0, or in other words,
don’t use a feedback resistor or a resistor at the op-amp input. The voltage follower is an extremely
simple circuit that outputs an identical voltage to the input voltage but the output signal has more
current. Put another way, the op-amp changes the high impedance input voltage to a low impedance
voltage output, and hence makes the output signal stronger. Figure 10, shows an op amp, which is
represented schematically as a triangle. The op amp is connected in a voltage follower orientation,
where the output feeds back into the input. Remembering that the physical op-amp chips are powered
components (i.e. they have a separate power input not typically shown on the schematic diagrams but
only on the wiring diagrams), it is the feedback setup of the voltage follower that is able to utilize this
power input to create the increase in output current that will help to create a stronger, cleaner, and
easier to read signal for the microcontroller.
Page 26 of 45
Figure 10: Voltage Follower
Once 𝑉𝑜𝑢𝑡 is calculated, the current flowing to the rest of the circuit can be determined. To do this, a
very small resistance is simply placed after the op-amp and the microcontroller is used again to measure
the voltage drop across it. Since the resistance is known and the voltage drop is now measured, Ohm’s
law says that the current can in turn be determined, or least in principle. In practice, measuring current
requires specialized components that are discussed in the following Battery Current Sensor section.
1.4.2
Battery Voltage Sensor
The battery voltage sensor circuit consists of a voltage divider, an op-amp, and resistors. The
unregulated voltage of the battery is sent through the voltage divider to decrease it to within the 0-5 V
range and can be determined by,
𝑅2
𝑉𝑖𝑛
1 +𝑅2
𝑉𝑜𝑢𝑡 = 𝑅
(9)
This output is then sent through the op-amp, where the voltage is amplified while staying within 5 V to
ensure that a safe signal is sent to the microcontroller. As a safety precaution, you can add an extra
small resistor (200-300 Ω) off board between the sensor and the microcontroller to protect against any
possible overcurrent.
1.4.3
Battery Current Sensor
Current sensing can be trickier than voltage sensing and requires special components specifically
designed for this purpose. The battery current sensor circuit consists of micro ohm resistors (“sense”
resistors) as well as other resistors, special current sensing op-amps, and a regular op amp. The micro
ohm resistors are needed so that the voltage across them can be sent into the current sensing op-amps,
which will in turn amplify this voltage. These special op-amps are needed because of their common
mode rejection voltage; in other words, they can accept the large voltage of the battery. The current
sensing op-amp output is then sent through a normal op amp to be further amplified by a gain of 1.3
using Rf and R1 resistors as shown in Figure 9 and bounded between zero and five volts to ensure a safe
signal is sent to the microcontroller.
1.4.4
Electronics Current Sensor
For the electronics current sensor circuit, the setup is the same as for the battery current sensor but it
instead senses the current across the regulated power outputs of 5 V and 12 V. The values for some of
Page 27 of 45
the components are slightly different but these can be looked up via the ModBot Power Board parts
list.23
1.5 Power System Interface
Figure 11: Power System Interface Connectors
The goal of the power system circuit as a whole is modularity: allowing many different uses and
powering a variety of devices. To achieve this, the power system would need to be equipped with the
proper hardware to allow it to interface with other devices. When designing the ModBot interface
hardware, two types of physical interfaces were taken into consideration: connectors and screw
terminals. When choosing an interface, it is important to take into consideration


the gauge of the wire connected to the connector or terminal
the number of positions available in terminal or connector
Choose a terminal or connector that can handle the wire gauge used to avoid having a wire that is too
thick to fit into the connector or terminal block, or a wire that is too thin and won’t be held properly in
the terminal. The wire gauge is the diameter of the wire, the bigger the diameter of the wire (i.e. the
lower the gauge), the bigger the current load that the wire can handle. In order to make the design
process easier for electrical designers, the American Wire Gauge table was developed (see Figure 16 the
table lists the nominal current that can be handled by every wire gauge. It is important to be aware of
the gauges of the wires that are used in the power system, which are highlighted in Figure 16, and
23
See the posted Power Board Parts List spreadsheet for more information.
Page 28 of 45
choose a connector or a terminal block that can handle these sizes of wires. The power wiring used in
the unregulated motor circuitry is 14-18 AWG. The power wiring in the regulated electronics circuitry is
16-21 AWG.
The other parameter you need to consider when choosing the interface hardware is the number of
positions that the screw terminals can have. It is advised to choose terminals with more positions than
you currently need to ensure that the power system is scalable and can interface with additional
components in the future if needed. For example, the regulated 12 V electronics bus interfaces with the
Intel Atom Board and the Arduino, so a two position terminal should be sufficient for the current use,
but a five position terminal was used instead to ensure scalability of the power system.
Based on the discussion above, Molex Saber Connectors were chosen (see Figure 12), for the high
current battery input and off board motor circuit breaker connection as shown in Figure 11. Molex Saber
Connectors are very robust, safe, and can handle high currents. The connector that was chosen for the
ModBot can handle 14-18 AWG wires with up to 18 A of current load; this is more than enough to
ensure reliable operation of the ModBot, since the nominal current load is 8.7 A based on Table 1.
Figure 12: Molex Saber Connector
One disadvantage of using Molex Saber Connectors is they are difficult to use as the universal output
interface due to the immense amount of soldering and crimping required to attach the connectors to
every device’s power wires. So instead, 3 position, 5 position, and 8 position simple screw terminals that
can handle 16-28 AWG wires with up to 12 A of current load were utilized for all of the power outputs,
making it very easy to connect any board or device to our system. See Figure 13 for an example of a
screw terminal.
Figure 13: Screw Terminal
For the interface between the battery terminals and the battery charger terminals, 30 A Anderson
Powerpole Connectors were used for their reliable wire-to-wire design. They are very easy to reconnect
Page 29 of 45
repeatedly, in the case that the batteries ever needed to be removed or replaced. See Figure 14 for an
example of an Anderson Power Pole connector.
Figure 14: Anderson Power Pole Connector
1.6 Battery, Power Board and Battery Charger Interface
As discussed previously in Section 1.2.4, an off board thermal circuit breaker will be used as a power
switch to turn power to the ModBot ON and OFF. In addition to the thermal circuit breaker, a master
power switch will be used to allow the ModBot battery to be charged while still connected to the
ModBot. In other words, the switch will provide the ModBot with battery power while in the ON
position, and allow the ModBot’s battery to charge while in the OFF position. To achieve that goal, you
would need to consider two parameters that characterize a switch,


the switch poles
the switch throws
The switch poles are the number of separate circuits that are controlled by the switch knob. For
example, a "2-pole" switch has two separate identical sets of contacts controlled by the same knob. The
switch throws are the number of separate positions that the switch can adopt. A single-throw switch has
one pair of contacts that can either be closed or open. A double-throw switch has a contact that can be
connected to either of two other contacts; a triple-throw has a contact which can be connected to one
of three other contacts, etc.
In case of the ModBot a double pole double throw (DPDT) switch is used since it will be controlling two
circuits, the power circuit and the battery charger circuit. The switch is wired as shown in Figure 15. The
main battery’s positive lead runs through one pole of a DPDT master power switch, while the other pole
of the DPDT switch is wired to the positive terminal of the power board. When the master switch is
turned ON, it will connect the positive battery lead to the power board (through the circuit breaker) and
directly power the electronics safety circuit and the motor safety circuit. When the master switch is
turned OFF, it will connect the positive battery lead to the charger port, disconnecting the power board
and allowing the battery to be charged while still inside the ModBot. The negative battery terminal and
the negative charging terminal should both be connected to the negative terminal of the power board,
to maintain a common ground at all times. When the ModBot is powered ON, the charging terminal is
disconnected from the batteries and the safety circuits are connected to the battery. Remember to wait
the battery manufacturer’s specified cell balancing time (30 minutes for the ModBot’s batteries) after
Page 30 of 45
the batteries are fully charged before powering on the system. This is explained in the Battery Selection
section below, and it is crucial to ensure your batteries will continue to operate as expected.
Figure 15: Battery, Power Board and Battery Charger Interface
Note that there is an additional port on the power board for a separate motor switch. This port can be
jumped if it is unwanted. In the case of the ModBot, the port is connected to a thermal circuit breaker.24
By using this port, it allows the user to keep the motor circuit off but have the electronics turned on and
therefore prevents the motors from turning on without the electronics controlling them. If the motor
controllers are active-low,25 then the wheels would immediately spin at full speed if the motor circuit
was to be turned on before the electronics circuit.
24
25
See Section 1.2.3.1 for more information about the thermal circuit breaker.
See Motor Controller Guide for more on motor controllers.
Page 31 of 45
Figure 16. American Wire Gauge (AWG) Current Load Limits
Page 32 of 45
2 Battery Selection
Electrical power is required by the ModBot to operate all of its components. This power can come from
batteries that are carried on the robot or from a power cord (also known as a tether) connected to an
external power supply. Either method could be used, but batteries are better suited for the ModBot
because tethers tangle easily, especially if multiple robots are on the field at the same time. Although
the batteries provide more flexibility, they are limited in the amount of power they can provide. For this
reason, the batteries must be selected carefully in order to meet the desired performance goals.
There are two main categories of batteries: single-use and rechargeable. One commonly thought of
advantage that rechargeable batteries have over single use batteries is the reduction of waste due to
disposal of single use batteries containing hazardous material. Rechargeable batteries are also ideal for
many embedded systems because once the batteries run out of charge, the system only needs to be
connected to a charger without having to remove the batteries from the system. This can be achieved
by disconnecting the battery’s wiring and then reconnecting it after charging, or by designing the
interface of the battery circuitry so that the battery can charge without disconnecting it.26 This section
will demonstrate the latter more complicated but more user friendly option, which is also featured in
the ModBot design. Regardless of whichever option you choose, there are many types of batteries to
choose from and therefore it is important to understand their various characteristics.
A battery pack generally contains several cells, each producing roughly 1.2 - 3.7 volts.27 The cells are
wired together in series to produce a higher desired voltage, and in parallel to produce a higher desired
capacity, or in other words be able to support higher loads. Figure 17 below depicts an example of a
series and parallel battery wiring diagrams.
Figure 17: Battery Series/Parallel Wiring Diagram
One of the most important characteristics that must be considered during the battery selection process
is battery chemistry, because it directly affects all of the battery's properties. Battery chemistry
describes the type of material used inside the battery to convert chemical energy to electrical energy,
and the material properties that affect the battery performance. In addition, battery chemistry affects
other variables that characterize a battery, including:
26
27
See Section 1.6 for more information on how the battery circuitry interfaces with the ModBot.
See Section 2.7 For more information on battery packs.
Page 33 of 45






Voltage: the characteristic electrical energy level of the battery. A battery will have a slightly
higher voltage when fully charged, and a slightly lower voltage when fully discharged.
Discharge Curve: the plotted change in voltage a battery experiences as it is charged and
discharged. The voltage drop will increase based on the amount of current drawn from the
battery.
Capacity: the total amount of electrical power a battery can hold
Energy Density: the battery’s energy capacity stored per unit of mass
Maximum Discharge Rate: the maximum amount of continuous current the battery can safely
produce at its defined voltage; it is often expressed as a C-rate, which is calculated as the
maximum discharge current divided by the battery capacity (mAh).
Cycle Life: the number of recharge cycles the battery can withstand before degrading to 80% of
its original capacity
Different types of battery chemistries also have their own safety and environmental considerations.
Some battery types are extremely dangerous and can catch fire or explode if overcharged or ruptured.
Others are made with very toxic compounds and must be disposed of properly. Be sure you know and
understand the limitations and safety procedures for your batteries.
2.1 Battery Chemistry
Chemical reactions inside batteries produce electrical energy; therefore, the specific battery chemistry
determines how the battery will perform. Different chemistries have different characteristics and
limitations, so it is important to select the correct chemistry for your power application. Several battery
cells can be connected together to change the voltage or capacity of a battery pack, but the
fundamental characteristics of the cell depend on the battery chemistry and cannot be changed.
Common rechargeable chemistries and their most notable characteristic are listed below. They can be
separated into nickel based, lithium based, and lead based chemistries.







NiCd (nickel cadmium) - older low-power consumer chemistry
NiMH (nickel metal hydride) - newer low-power consumer chemistry
LiCoO2 (lithium cobalt) - absolute highest density, but very explosive
LiMnNi (lithium magnesium) - safer alternative to lithium cobalt
LiNiMnCoO2 (NMC) - safer than lithium cobalt & better cycle life than lithium magnesium
LiFePO4 (lithium phosphate) - very stable discharge curve, very safe & reliable
Lead acid - cheapest chemistry, extremely low density
Battery chemistry is typically selected based on the application usage, rather than only on power
requirements. Lead acid batteries are often used in applications where durability is more important than
weight, such as car batteries. NiCd batteries are an older, low-cost technology found in consumer
rechargeable batteries. Currently, many systems which used NiCd batteries in the past are upgrading to
the more modern NiMH. LiCoO2 batteries have excellent energy density making them ideal for aerial
vehicles; however, these chemistries are much less safe compared to older technologies, especially if
Page 34 of 45
they are not protected by a protection circuit board. They are even deemed explosive, and they should
always be charged in a fire retardant safety bag. LiMnNi and LiNiMnCoO2, the newest cell chemistries
available, are both safer alternatives to the dangerous LiCoO2 chemistry. They both provide relatively
high energy densities. The LiFePO4 chemistry is one of the most promising modern chemistries, because
while it does not match the other lithium chemistries in density, it provides exceptional safety,
reliability, and it has the most stable discharge curve of all the chemistries the team tested. Table 2
briefly summarizes the characteristics of seven common cell chemistries.
Table 2. Characteristics of various battery chemistries
Chemistry
Voltage
per cell
Safety
Environmental
Cost based on
cycle life x Wh
NiCd
1.2V
Safe
Bad
0.7
NiMH
1.2V
>80
Wh/kg
-20 - 50 °C
Safe
Good
1.2-1.4
LiCoO2
3.7V
>200
Wh/kg
-20 - 60 °C 500-1000
Unsafe without
PCB or PCM
OK
1.5-2.0
3.7V
>160
Wh/kg
Unsafe without
-20 - 40 °C 500-1000 PCB or PCM,
better than LiCo
OK
1.5-2.0
LiNiMnCoO2
3.6V
>160
Wh/kg
Unsafe without
-20 - 40 °C 1000-2000 PCB or PCM,
better than LiCo
OK
1.5-2.0
LiFePO4
3.2V
>120
Wh/kg
Very Safe
Good
0.75-0.85
Lead acid
2.0V
Very Safe
Bad
1
LiMnNi
Energy Working
Cycle Life
Density
Temp.
>
-20 - 50 °C >1000
40Wh/kg
>500
-0-60 °C 1000-2000
>
-20 - 40°C
35Wh/kg
>200
2.2 Protection Circuit Module
By themselves, batteries are vulnerable to shorts, over charging, over discharging, and overdrawing
current; this led many manufacturers to offer battery protection circuit modules (PCM) built directly into
the pack. These PCMs are extremely important and it is highly recommended to only use batteries with
installed PCMs for safety. A PCM built into the battery pack adds protection that cannot be removed
from the battery pack and protects the battery from damage. This is especially important for the Li-Ion
chemistries, which can be explosive under the worst circumstances.28
28
See Section 1.2.3 for more information about PCMs.
Page 35 of 45
The PCMs within the battery pack should not be treated as the only source of protection for the power
system. The PCMs are meant to protect the battery from becoming damaged through improper use, but
additional safety circuits should be created to protect the other system components.29
2.3 Cell Balancing
The cells of a battery pack are not exactly identical and can charge and discharge to different voltages. If
the voltages of the cells differ too much, the battery performance can suffer. The pack will cut off when
the first cell reaches its critical drained voltage, so some batteries include a cell balancing feature which
trims the voltage in each cell in the battery pack so that they are equal.
Cell balancing greatly improves pack longevity, charge capacity, and performance of the battery pack.
External smart balancing chargers can be connected to a battery without a PCM to charge each cell
individually, but packs with a PCM and balancing function will do this internally without the need to
connect to each cell separately. The PCM balancing function of the ModBot system begins immediately
after the battery finishes charging and lasts approximately 30 minutes, during which time the battery
should not be used.
2.4 Battery Charging
Each battery chemistry has its own type of charger, and every charger is rated for a certain battery
voltage. When a battery is drained, its voltage is lower than normal, and when it is charged its voltage is
higher than normal. A battery charger outputs a voltage that is precisely a few volts higher than the
normal battery voltage, in order for energy to flow back into the battery cells. It is very important to use
a labeled smart charger when recharging a battery because it will limit the current flowing into the
battery pack. Without this limiting capability, the battery alone will draw too much current and could
overheat, damage itself, or even explode. Be sure to check the maximum charging current of your
batteries. To get the best longevity of your battery pack, it is best to recharge your battery after is has
been completely drained.
When a battery is ready to be recharged, first disconnect it from the circuit it is powering. Be sure to
plug the smart charger into the wall before connecting it to the battery. Finally, connect the like positive
(red) terminals together and the like negative (black) terminals together. More dangerous lithium
batteries should be charged inside special fire-retardant bags to reduce the risk of fires. The following
formula can be used to estimate the length of time required to charge any battery:
1.5∗(Capacity in Ah)
Time (min) = (Charging Current in A) + (Balancing Function Time)
(14)
Most importantly, be sure to read the instructions for your specific battery and battery charger and
ignore any of the recommendations given here if your battery/battery charger says otherwise.
29
See Sections 1.2 and 1.3 for more on safety circuits.
Page 36 of 45
2.5
Separation of Power Circuits
In some cases it makes sense to actually create two power circuits to simplify the power system and
make it more robust. The batteries supply the power system with energy for all the components of the
system, but these components can have different demands of the system. Embedded systems generally
contain two types of devices that need electrical power: sensitive electronics and robust mechanical
components. The CPU and microprocessors require a regulated power source that is free of electrical
noise. More robust components, such as the motors, do not require a regulated power source, and are
prone to introducing power spikes and noise in the system. Trying to create a single power system to
meet both system requirements can be difficult.
One solution is to create two circuits: an electrical power system and a mechanical power system. The
electrical power system powers the sensitive electronic components and the mechanical power system
supplies power for the robust components. This separation eliminates the conflicting demands of the
two systems, and isolating the more robust components also makes it easier to prevent electrical noise
and spikes from affecting the sensitive electronic components. Creating separate power circuits is a
common solution to make the system more robust.30
The batteries used by each power circuit can be different or the same, and both solutions provide their
own pros and cons. Electronic components generally require 5-12 V to operate, but electronic circuits
almost always require regulated power and voltage regulators. These regulator components usually
require a positive 1.5-3.0 V difference between their input voltage from the battery or other power
source, and the regulated output voltage that the regulators provide. Therefore, it is best to find a
battery with a voltage that is higher than the maximum required regulated voltage of the system. On the
mechanical power system side, motors often have a much higher voltage, such as 24 V, to reduce the
amount of current flowing through the power system. Since the two systems have very different power
input requirements, separate batteries for each power system may be a good solution.
However, two separate batteries can cause their own problems as well. While separating the two
circuits completely is a guaranteed solution to the problems of interference, it does pose other
problems to the overall system design. Some of the biggest issues that can result are regulation
inefficiency, wasted power storage, and battery charging risks. First, although having an electronics
battery with a nominal voltage only slightly higher than the 12 V required to power some electronics
(like the Intel Atom board) may allow for smaller and lighter batteries, batteries that are only slightly
higher than the required voltage restrict the design to using linear regulators on the 12 V electronics rail.
Linear regulators simply convert all the unwanted voltage directly to heat, and therefore are extremely
inefficient for high current systems such as many significant microprocessor boards. In some cases this
added heat can even be a larger problem that the inefficiencies.31
Second, using two batteries means that overall system is limited by whichever battery runs out of power
first. The weakest link will require the entire system to shut down, which means that the fuller battery
30
31
See Sections 1.2 and 1.3 for more detail about these separate circuits.
See Section 1.3.5 for more on linear regulators.
Page 37 of 45
will still be holding untapped power at that time. Finally, running two batteries causes charging to be a
more delicate procedure, where you must ensure the correct charger is plugged into the correct battery
and you must wait until both batteries are fully charged and balanced before powering the system on.
This may sound like a simple enough issue to avoid, especially if unique connectors are provided for
each battery, but operator errors of this type are unfortunately considered to be a common potential
problem.
The severity of these issues often depends on the exact application. For example, in a system where one
battery may be used to run “stand-by” electronics in between charging the second battery, the two
battery system may offer significant benefits that outweigh these issues. In other systems, like the
ModBot, the entire system often needs to be operational in order to gain any significant performance,
making two batteries more problematic than helpful.
Hence, in the ModBot a single 24 V battery was used in the design and additional steps were taken to
help ensure circuit isolation even though both the electronics and the mechanical system had the same
power source. To ensure circuit isolation, motor controllers were used as a first step to help isolate the
motors from the power board. Many motor controllers include circuitry to prevent reverse current and
inductive spikes when the motors stop or change direction. As an important safety tip, it is always wise
to test that the purchased motor controllers provide the protection their data sheet indicates, because
unfortunately, low cost motor controllers have a reputation for not performing as promised and have
high failure rates. The best low cost motor controllers found by the ModBot team and their operation
are discussed further in the Motor Controller Guide.
To provide further protection, there are also three large capacitors placed just before the high current
24 V output, to buffer any power spikes from the motors. Each electronics-side switching regulator also
has its own capacitor bank to help maintain a constant input voltage, along with a high 260 kHz
operating frequency to quickly adjust to fluctuations in both input voltage and load currents. Combined,
these design choices enable the system to isolate the motor and electronics circuits well enough to
utilize a single 24 V battery.
2.6 ModBot Battery Selection Process
Battery selection is one of the aspects of power system design that is often overlooked and yet can be
very involved and with a significant influence on your system performance. To begin with, there are a
number of battery vendors and even though they may promise the same battery, they are not all equal
(think of the reliability difference even between generic AA batteries and major name brands). As the
first step in the battery selection process, it is best to find a reliable manufacturer or distributer of
batteries. For the ModBot, the batteries had to be reasonably priced with very high performance and
high capacity. The results of the ModBot team’s search concluded with the company AA Portable Power
Corporation, which operates through their website www.batteryspace.com. They are a US company
based in California, who design and build custom battery packs and cells for a wide range of
applications.
Page 38 of 45
From the comparison of battery characteristics found in Table 3, several battery types were rejected due
to low performance for the given task. Weight is a significant attribute to consider for the ModBot,
therefore batteries with low energy densities would not be an efficient choice. Lead acid, NiCd, and
NiMH batteries were ruled out because of their very low energy density. Also, NiMH and NiCd have low
maximum discharge rates and therefore using either of these chemistries would be equivalent to
powering the ModBot with dozens of consumer AA rechargeable batteries. Since the overall ModBot
design is aiming for something with higher performance, those chemistries were also ruled out.
The four remaining potential candidates capable of high current throughputs and high energy densities
are LiFePO4, LiMnNi, LiNiMnCoO2, and LiCoO2. Lithium-cobalt batteries are some of the most widely
used batteries for consumer electronics because of their exceptional energy density. These batteries are
the most widely used batteries in laptops today, where they are housed in protective cases and are
charged and discharged behind highly regulated circuits inside the laptop. However, their explosive
chemistry poses serious safety risks and application limitations (they must be charged in a fire retardant
safety bag), and their poor cycle life does not make them a good long term investment. Given that it is
anticipated that students will be handling these batteries themselves and using them on a moving robot,
the safety concerns were too great to ignore. Their only sensible application is in small portable
electronics or aerial RC vehicles, where energy density and low weight are the absolute highest priority
and worth having to deal with the safety factors. However, the Modbot does not fit these cases, and for
that reason Lithium-cobalt was ruled out as a chemistry choice for our application.
Lithium-magnesium and the newer NMC batteries are both safer alternatives to the lithium-cobalt
chemistry, with slightly less energy density. Similar to the lithium-cobalt chemistry, they both have a
high cell voltage and they both require a PCM (protection circuit module) to ensure their safety. In fact,
the only major difference between these two chemistries is the higher cycle life and slightly increased
robustness of the NMC chemistry over the standard lithium-magnesium chemistry. Their discharge
curves were also found to be very similar as well, and it seems the newer NMC batteries are beginning
to replace the older lithium-magnesium chemistry in applications. Because of its higher cycle life,
impressive energy density, and increased safety, the NMC chemistry was found to be the best choice for
a regulated electronics power system. More information about discharge curves can be found in the
load analysis section below.32
Lithium-phosphate batteries are great for their reliability, cycle life, and safety, and they also have a
unique 3.2 V cell voltage. However, they have the lowest energy density of the four potential
chemistries. But this lack of energy density can be justified for one very important reason: lithiumphosphate batteries have an incredibly flat discharge curve. This means that they maintain almost the
exact same voltage from when they are fully recharged to when they are almost completely exhausted.
This extremely valuable property makes them a perfect choice for unregulated systems, such as the
mechanical power system, because the voltage output is close to a stable regulated voltage though the
entire discharge cycle. For mechanical systems, this means that the top speed of the motors at the
beginning of operation is very close to the top speed at the end of operation, which offers more
32
See Section 2.8 for load analysis details.
Page 39 of 45
consistent performance and allows better system controls to be obtained throughout the system’s
operation. In addition, lithium-phosphate is the most chemically stable and physically robust battery
chemistry available (i.e. it doesn’t explode if you smack it), making it ideal for applications with high load
currents. For these reasons, the lithium-phosphate battery was chosen as the best battery for the
ModBot’s mechanical motor circuit.
2.7 Battery Pack Design
Once the battery chemistry has been selected, the next task is to either pick out or custom design a
specific battery pack. The first step is to identify the target voltage needed from your battery pack.
Calculate the possible battery pack voltages by multiplying the characteristic cell voltage by any integer
n, where the characteristic cell is the basic physical unit available for that battery chemistry and n is the
number of cells in a pack. Characteristic cell voltages commonly range between 1.2 -3.7 V depending on
the chemistry type.33 By connecting those n cells in series you can achieve that higher voltage. When
connecting cells in series, the only variable that changes is the voltage, which adds up. All other
characteristics of the battery pack remain the same as for a single cell. The ModBot’s initial two battery
design (separate mechanical battery circuit and electronics battery circuit) had a motor battery of 25.6 V
(8 cells in series), and an electronics battery of 14.4 V (4 cells in series). The final one battery design only
used the 25.6 V battery.
Now that you have your target voltage specified, you can configure your pack to reach your desired
discharge rate and capacity. The runtime of a system can be calculated by using the following formula,
battery capacity (Ah)
Run Time (hours) = average current draw (Amps)
(15)
At this point it is important to consider different individual cell options for a given chemistry. Each cell
has different specifications; some cells are designed for higher power (discharge rate) and others are
designed for higher energy (capacity). Choose a cell that fits your needs based on your application
demands. Both the discharge rate and capacity of any pack can also be multiplied by an integer factor of
n, by duplicating the entire set of cells in series and connecting the multiple sets in parallel with each
other. Only discharge rate and capacity change when connecting the sets in parallel with each other; all
other characteristics of the single set of cells, such as the set’s voltage, remain the same. Table 3 details
several batteries that were chosen for the ModBot electronics system and motor system. All four of
these batteries were ordered and tested in the lab for load analysis to both identify the best choice and
have a backup alternative battery if needed.
33
The 1.2 -3.7 V range is taken from www.batteryspace.com.
Page 40 of 45
MOTORS
Table 3. Comparison of batteries for motor & electronics circuit
Batteryspace
Part#
6482
6539
Chemistry
LiFePO4
LiNiMnCoO2
Volts
25.6 V (working) 28.8 V (peak) 20.0
V ( cut-off)
25.2V (working) 29.4 V (peak) 19.25V (
cut-off)
Capacity (Ah)
2.5Ah
3.6Ah
Capacity (Wh)
64Wh
91Wh
Energy Density
62Wh/kg
120 Wh/kg
Max Discharge
Rate
40A
16A
Dimensions
(LxWxH)
168mm (6.6") x 108mm (4.3") x
55mm (2.2")
185mm(7.3") x 39mm (1.5")
x70mm(2.8")
Weight
1040g
760g
Safety
Very Safe
Better than Li-Co, Requires PCM
PCM Protection
Yes (with balancing function)
Yes (with balancing function)
Cycle Life
>1000 charge cycles
750 charge cycles
Cost
$165.00
$130.35
Page 41 of 45
ELECTRONICS
Batteryspace
Part#
6469
5184 (discontinued)
Chemistry
LiNiMnCoO2
LiMnNi
Volts
14.4V (working) 16.8 V (peak)
9.6 V ( cut-off)
14.8 (working) 16.8 (peak) 9.6 (cutoff)
Capacity (Ah)
3.6Ah
4Ah
Capacity (Wh)
51.84Wh
59.2Wh
Energy Density
121 Wh/kg
148 Wh/kg
Max Discharge
Rate
10A
10A
Dimensions
(LxWxH)
114mm(4.5") x 35mm (1.4") x 75
mm (3.0")
114mm(4.5") x 35mm (1.4") x 75 mm
(3.0")
Weight
430g
400g
Safety
Better than Li-Co, Requires PCM
Better than Li-Co, Requires PCM
PCB Protection
Yes (with balancing function)
Yes (with balancing function)
Cycle Life
750 charge cycles
1000 charge cycles
Cost
$59.95
$59.95
2.8 Load Analysis
After these batteries were designed and ordered, they were put through a load analysis test to plot their
discharge curves and measure their real world performance. This is perhaps one of the most important
steps in finalizing a battery choice because some battery packs, especially ones with smaller cells, can
perform very differently in real world tests compared to what is expected.
A load analysis is a measure of the battery pack’s voltage at continuous time intervals during a
controlled discharge. The easiest way to perform a controlled discharge of a battery pack is to use an
industrial chassis mount resistor capable of dissipating hundreds of watts of power. In lab tests, all
batteries were discharged at a 1 C rate for consistency,34 because the battery’s entire discharge curve
34
The C rate is defined as the current that would discharge the battery in one hour, so if the battery was a 3.3 A-hr
battery, the C-rate would be 3.3 A. Using a 1 C rate to discharge multiple different batteries places the same
physical stress on each battery, allowing you to more consistently compare different batteries.
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shifts down as the current draw increases. The resistor’s specifications for a 1 C discharge rate can be
calculated with the following formulas,
Resistance (Ω) =
battery pack voltage (V)
battery capacity (Ah)
(16)
Minimum Power (Watts) = battery voltage (V) × battery pack capacity (Ah)
(17)
After fully charging the battery and allowing its cells to balance, connect both the battery and a
voltmeter across the resistor to begin discharging the battery. The resistor will heat up considerably as it
dissipates all the energy as heat, so keep it cool with a fan and keep it away from any flammable items.
Once the battery is connected, monitor the battery’s voltage from the voltmeter at regular time
intervals and plot them with respect to time to produce a load analysis graph of that battery.
The load analysis graph will show not only the actual measured capacity of the battery in Ah (assuming a
1C discharge current) at the beginning, but it is also the discharge curve and hence will indicate how
much the battery’s voltage will change from its fully charged state to its depleted state. Load analysis
graphs in Figures 18 and 19 are shown for the 4 batteries that were considered for this project. Take
note of the exceptionally flat discharge curve of the lithium-phosphate battery in Figure 19, which
makes it the clear best choice for an unregulated, high power motor system.
Electronics Batteries
14.4V LiNiMnCoO2 - 1C Rate
14.8V LiMnNi - .925C Rate
13V Cutoff
18.0
16.0
14.0
Voltage
12.0
10.0
8.0
6.0
4.0
2.0
0.0
0
10
20
30
40
50
Time (min)
Figure 18. Load Analysis of Electronics Batteries
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60
70
Motor Batteries
25.2V LiNiMnCoO2 - 1.16C Rate
25.6V LiFePO4 - 1.28C Rate
22V Cutoff
30.0
25.0
Voltage
20.0
15.0
10.0
5.0
0.0
0
10
20
30
40
50
60
Time (min)
Figure 19. Load Analysis of Motor Batteries
3 Things to discuss with teammates
The two most important things to discuss with your teammates are the approximate voltage level of the
system and the current limit. The voltage level needs to be specified, or at least estimated, very early on.
For example, if you have a large system, you will probably need 24 V, for a smaller one, 12 V might be
okay. You should also get an estimate on the magnitude and duration of the current in your system so
that you can design to accommodate it.
An important thing to keep in mind is the stall current of any motors you use. The stall current of a
motor is a fixed value; changing the voltage into the motor will not affect its stall current. Therefore, the
stall current is a fixed parameter you need to design around. Your motor design teammates should
communicate to you the stall currents of the motors used.
4 Board Components List
A list of the board components and their references is posted with the guide on the website.
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5 PCB Artist Power Board Layout
This figure is shows how the board is physically laid out and just serves to better visualize the board.
Figure 20. Power Board Layout
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