Accumulator Review
RESS Requirements
 High power, low energy
 FSAE rules
 DMOC requirements
Considered Technologies
 Mechanical
 Capacitive
 Chemical (battery)
Review of Storage Technologies
 DC/DC converter issue
 Cost
 Complexity/Reliability
 Performance trade offs
Design
 Preliminary Design
o Justification
 Final Design
o Changes
RESS Requirements:
“The RESS must be a lightweight, low energy, high power, efficient, source/sink for
DC electrical power.”
The premise of this project is to design and implement a rechargeable energy storage
system (RESS) for the electrical needs of the AERO GreenSpeed hybrid electric race car. The
car is roughly the same size as any other SAE formula car (basically an oversized go-cart) and
will weigh roughly 800lbs. It will be driven by a Honda CRF250 dirt bike engine (ICE) and an
Azure Dynamics AC25 electric 3-phase induction motor. The motor will be controlled by the
Azure Dynamics DMOC, which is a digital motor controller coupled with a three phase inverter.
The DMOC can be connected to the AC24 using either Y or Delta wiring configurations,
the effects of this are apparent in the peak torque curve on the motor and the voltage/current
requirements from the DC source which feeds the DMOC. Using Y configuration the peak
torque is higher as is the peak power drawn (by the AC24) and the DC source must be able to
supply 336 volts at 150 amps in order for the maximum power to be attained. Using Delta
configuration the peak torque is lower, as is peak power and the DC source need only supply 156
volts at 250 amps for maximum power. For both Delta and Y, 450 volts is the maximum
allowable voltage on the DC terminals of the DMOC.
The rules of the hybrid competition include a formula that equates the size of the
accumulator to some predetermined volume of gasoline fuel. This calculated volume of gas will
be deducted from the total amount of fuel allotted to each team at the beginning of the race. So it
would make sense not to use a larger accumulator than absolutely necessary so there is no
electrical energy still unused by the time the race is over. It also makes sense from a physics
standpoint to use a small accumulator because the smaller it is the less it’s going to weigh and
thus the faster the car can accelerate/turn. It also states in the rules that formula hybrid race cars
will be given 20% less fuel for a given race track then would normally be used by a conventional
formula race car.
From a conceptual standpoint the purpose of creating a hybrid drive system is to make
something that is somehow better than conventional drive system. Consumers of hybrid vehicles
are primarily concerned with fuel economy. With a hybrid this can be done in a few ways, using
an electric motor in conjunction with a gas motor means that: a smaller gas motor can be used to
achieve the same rate of acceleration, the gas motor can be turned off when it is not needed, the
gas motor can be run at it’s most efficient rpm, and most importantly the electric motor can be
used to generate/store energy from braking (regenerative braking). All of these ideas in some
way or another require a means of storing energy on a temporary or semi-long term basis.
Considering the energy conversions that may be involve in doing this it is important to remind
ourselves that whatever means of storing energy we chose it must be efficient at doing so in order
to glean maximum benefit from it.
In an effort to maximize the benefits of a hybrid drive train in our race car the car has
been outfitted with a four wheel drive system so that the electric motor can apply regenerative
braking to all four wheels and generate the maximum amount of energy possible. Regenerative
braking in particular requires that the car have a high power “sink”, basically a means of storing
energy that can accept an inflow of energy at a very high rate. In the case of an electrical energy
storage system this means that not only does it have to be capable of discharging energy at a high
rate (high power source) it also has to be capable of charging at a high rate (high power sink). All
this boils down to the basic requirements that underlie the design process and design choices for
this accumulator.
The RESS must be a lightweight, low energy, high power, efficient, source/sink for DC
electrical power. Sourcing peak powers of either 156Volts @ 250Amps (Delta) or 336Volts @
150Amps (Y config). At this point in the discussion there is no known power requirement as to
what the RESS must sink for power, only that is will be less than or equal to the peak power
discharge.
Considered Technologies:
Maxwell ultracapacitors would be capable of supplying the maximum power demands for
charging and discharging making regenerative braking very feasible. They are also very
efficient but will be heavier than batteries, and are somewhat fragile.
The A123 Systems lithium iron phosphate cell is durable physically and electrically, is light
weight, and has high discharge and charge rates. It may be capable of supporting regenerative
braking.
From the previous section it is obvious that for our race car the storage technology will be
electrical in nature so either ultracapacitors or batteries, however there is research being done in
industry on mechanical energy storage for hybrid cars. These concepts mainly revolve around the
use of flywheels and are attractive because there is no energy loss due to conversion as will be
discussed shortly.
Ultracapacitors are a relatively new technology and like a regular capacitor they store
electric charge directly in an electric field. A regular capacitor has two metal plates separated by
some dielectric (electrically insulating) material and an imbalance of charge on the two plates is
created when a voltage is applied. Later when the two plates are connected together threw some
electrical load the charges move back to a balanced state and in the process they perform work on
the load. An ultracapacitor has a metal plate or electrode that is a material with very high surface
area such as activated carbon which on a microscopic level is extremely. Outside of the coating
there is an electrolyte material (sometimes solid sometimes liquid depending on design) that has
free ions (atoms with a net charge, which are free to move around in the material). So when a
voltage is applied these ions drift towards the coated electrode and fill in all the pores/spaces on
the surface of the coating. The distance between these ions and the electrode is very small
compared to the distance between plates of a regular capacitor and so the total energy that can be
stored in these ultracapacitors is much higher than in a regular capacitor.
The behavior of a capacitor in general is defined by the energy equation E  12 CV 2
(where C is the capacitance in farads). So from this we see that the voltage on terminals of the
capacitor is dependent on the energy that is currently stored in the capacitor, so as energy is
drawn the voltage will decrease.
There are not very many manufacturers for ultracapacitors but the biggest and most
popular of the few is Maxwell. They have a line of ultracapacitors they call “boost caps” that all
have 2.7volts max per cap and capacitance ratings between 650-3000 farads. Enough of these
linked together could provide exactly the amount of energy and voltage needed to power the car
and will undoubtedly have the high power discharge/charge characteristics desired. It should also
be extremely efficient because it is storing the energy directly as it is without any conversion.
Unfortunately they are not as durable physically or electrically and must be managed carefully to
ensure safe operation and reliability.
A more traditional way of storing electrical energy is in a battery. The way a
rechargeable battery works is by using electrical charge to induce a chemical reaction on a
material which raises that material’s potential energy. Later when the chemical reaction is
reversed charge is released. This conversion has losses mainly due to heat. Batteries have much
lower specific power (Watts/kg) than ultracapacitors but have much higher specific energy (Watthrs/kg) so for the same given weight the batteries will have more energy and less power. The
oldest currently used battery technology is lead acid which was immediately precluded from use
in this car because of its weight and slow charging rates. Newer rechargeable chemistries include
nickel cadmium (NiCd) and nickel metal hydride (NiMH). Nickel cadmium is bad for the
environment (cadmium is toxic) and the batteries themselves work well but have one draw back
which is called the “memory effect” which can cause the batteries to lose capacity if not charged
properly. Nickel metal hydride is the standard high performance rechargeable these days, it has
no memory effect and is not as bad for the environment as nickel cadmium. NiMH batteries are
often recycled to reclaim the nickel for use in metal alloys, making their use even more attractive.
Both NiMH and NiCd have decent discharge power and are lighter than lead acid but
they can’t charge fast enough to make regenerative braking feasible. The road doesn’t end here
however; lithium based batteries have made many improvements and now there are two lithium
based chemistries which can source high power – lithium polymer and lithium iron phosphate.
The Li-ion batteries in most cell phones and camcorders are usually of the low power variety and
are chosen for those applications for their light weight, high energy, and form factor (shape/size
options). Lithium polymer (L-poly) batteries have a liquid electrolyte and are often used in
remote controlled airplanes but they are very dangerous if charged improperly and because of this
are not usually used in automobiles. Lithium iron phosphate cells are just as durable physically
and electrically as a NiMH cell but are lighter weight and have higher discharge/charge rates.
Unfortunately a company called A123 Systems is the only manufacturer of these cells and they
are rather hard to get a hold of in large quantities. If they can be acquired the A123 Systems
lithium iron phosphate cell may be capable of supporting regenerative braking.
Review of Storage Technologies:
“Due to the complexity involved in making a safe and reliable ultracapacitor storage system
and given the fact that a working design must be implemented in a relatively short time span
(before May 08) ultracapacitors will be set aside for development on next years car.”
“It would seem that despite our best intentions using A123 Batteries for regenerative braking
will not be very successful even though they do provide a light weight high power source.”
The DMOC will continue to power the AC24 to the best of its ability given a DC input
power as long as the voltage on its DC terminals is between 100v-400v. Imagining a scenario
where DMOC is drawing a constant power over time. Ultracapacitors connected directly to the
DMOC would be decreasing in voltage exponentially and once that voltage drops below 100v the
DMOC/AC24 will no longer function. This means that whatever energy is still contained in the
capacitors at or below 100 volts is never used and is an unneeded fuel penalty. This also means
that in order even attain a peak voltage in the ultracapacitor bank the accumulator must have a
minimum of 40 ultracaps in series which not only takes up a lot of space but it also weights a lot.
Also depending on the configuration of the DMOC (Y or Delta) the voltage desired for peak
power is a fixed value so although it may be able to supply enough energy for the race and
provide an excellent sink for regenerative braking it would be unlikely to supply peak output
power on demand. This is not due to a limitation of the caps but rather the DMOC.
The other way to connect a bank of ultracapacitors (ultracaps) to the DMOC would be to
use a DC/DC converter between the ultracaps and the DMOC. The purpose of the DC/DC
converter would be to keep the voltage on the terminals of the DMOC always at the peak power
voltage (whether or not peak current was being drawn). The benefit of this would be that fewer
ultracaps could be used thus saving on weight and peak power would be available on demand
whenever the driver of the car needs it (given that there is still energy in the accumulator). The
disadvantage to this is the introduction of a loss to the system and the added complexity/cost of
designing and building a high power bi-directional DC/DC converter. Another thing to consider
is the weight of the converter, depending on their design a DC/DC converter can weight a
considerable amount due to the use of a large inductor. Azure Dynamics strongly recommends
against the use of a DC/DC converter in this way and will terminate all warranties on the DMOC
if this design option is chosen. Due to the complexity involved in making a safe and reliable
ultracapacitor storage system and given the fact that a working design must be implemented in a
relatively short time span (before May 08) ultracaps will be set aside for development on next
years car.
Another effect of high current loads on both ultracaps and batteries is a sudden drop in
voltage due to their internal resistance. What this means for capacitors is that if a constant current
is drawn from a cap till it reaches zero voltage on it’s terminals it will still have energy stored in it
but not enough to overcome it’s internal resistance given the current demanded. What this
means for batteries is that the voltage on the terminals of the battery changes with current even
though a battery is a constant voltage source. So for example if the internal impedance of each
battery is 0.01 ohms the nominal voltage (unloaded mode average voltage) of each battery is 3.3
volts the bank total (all cells in one series string) will be 616 volts if it is designed to meet the
peak power requirement of the DMOC in Y configuration (336v@150A). This nominal bank
voltage is obviously much higher than the maximum DMOC voltage of 450 and it requires
187cells in one series string and is technically impossible using the A123 cells because they can’t
source more than 120amps per cell/string. The solution to this voltage drop problem is to reduce
the amount of current being drawn threw each cell/string. This is done by putting multiple
cells/strings in parallel. So if three strings are used the nominal voltage is 396 volts using a total
of 360 cells – this is an acceptable although heavy design. However if the same process is run on
the Delta configuration two strings would yield 251 volts nominal with a max current of 125
amps per string and would use only 152 cells total. Three strings would yield 207 volts nominal
with a max current of 83 amps per string and would use 189 cells total.
After looking threw the numbers it would seem that Y configuration of the DMOC is
impractical if considering batteries as the storage medium. In Delta configuration the two string
design could work but it would be pushing the limits of what the batteries can handle and would
not be as reliable as the three string design. Looking deeper at the three string design we can
determine what sort of charging capabilities it will have. The recommended charging current is
10A up 3.6v constant current constant voltage which should yield a 15 minute charge. This
works out to about a 6 kW charging rate. The cars regenerative torque is about equal to its driving
torque we can make an assumption that the max regen power is comparable to the max driving
power given the rpm. This would put our max regen rate at about 39 kW which is considerably
higher than what the batteries can handle. So it would seem that despite our best intentions using
A123 Batteries for regenerative braking will not be very successful even though they do provide a
very light weight high power source.
Preliminary Design:
BATTERIES
CURRENT SENSORS
BALANCE MAINTENANCE
+
STRING 1
STRING 2
STRING 3
+
+
+12VDC
CHARGE COUNTER
12V_EN
CAN
VOLTAGE TRANSDUCER
GFI
12V_EN
RELAY
The current design of the RESS utilizes three strings of A123 cells, 63 cells in each
string, all connected in parallel. The DMOC will be connected in Delta configuration with the
AC24 and the battery’s state of charge (SOC) will be monitored by a charge counter. The charge
counter will also monitor the voltage on the accumulator and if the voltage is above or at
maximum the regenerative function of the DMOC will be disabled and if the voltage is below the
minimum the drive function of the DMOC will be disabled. The charge counter will display the
SOC to the driver and will monitor the current going into/out of each string to ensure it doesn’t
exceed the batteries limitations and to notice any sizable differences in the currents in each string
with respect to one another. Differences in these currents can indicate that the cells are going out
of balance with each other. “Balance” in reference to a battery bank is the SOC of each cell in
the bank compared to every other cell in the bank, the easiest way of measuring the balance of a
bank is to compare the differential voltages of each cell. The reason cells go out of balance with
each other is minuet differences in internal resistance in each of the cells causing charging and
discharging to occur at different rates relative to each other. The battery bank in the car will be
balance charged before the race and will be balance charged again after the race. Any charging or
regenerative braking that occurs in the three days during the period of the race will be done
without any balance precautions although differential voltage measurements will be taken to see
if anything is drastically wrong with the balance. The cells will be sourced from DeWalt 36Volt
Packs which have been purchased at about 100 dollars per pack (which equates to 10 dollars per
cell). DeWalt chargers will be used to do the balance charging.
The enclosure and configuration of the accumulator has several stipulations listed in the
formula hybrid rules. The hot terminal of the accumulator must be connected to a ground fault
detector that is grounded on the frame of the car. The accumulator must also have a relay that
will disconnect the terminals of the accumulator from the rest of the car when not receiving a
signal from the low voltage system. The accumulator must also be fused in case of a short circuit.
The wires must be SAE rated and the enclosure of the battery bank must be water resistant to
prevent ground faults during rain. The batteries are going to produce between 4000-20000 BTUs
of heat so this enclosure must also be able to dissipate this without allowing any of the cells to get
over 60°C. All the parts excluding the structural materials for the enclosure have been picked out
or already ordered.
Final Design:
Some changes were made to the preliminary design – the battery strings now have only 60 cells
due to size constraints and the charge counter idea was discounted due to a lack of development
time. The DMOC will now be used to read the terminal voltage of the pack and to set max
currents/voltages. The DMOC is not made for battery management so to ensure that
current/voltage limits are not exceeded the values will be set very conservatively. Over temp
situations will be handled by passive components called thermal cutoff switches (not shown in the
schematic above) that will be run in series with the kill switches and ground fault detector (GF).
The DC/DC converter has been added to sustain charge on a small 12 volt motorcycle battery
which is the low voltage power source for the hybrid drive controller and DMOC.