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6.UAP Final Report: Prismatic Cell Cycler

Student : Praveen Subramani, SB Candidate in Course 6-1: Electrical Science & EngineeringFaculty Supervisor : William J. Mitchell, Professor of Architecture and Media Arts & Sciences,MIT Media Lab/MIT Design Laboratory

Co-supervisor: Raul-David Poblano, Doctoral Candidate, Smart Cities Group, MIT Media Lab

Introduction & Prior Research

The Smart Cities Group at the MIT Media Lab is pioneering the future of Mobility on Demand systems to revolutionize urban transportation by transforming the way people move aroundcities. These systems consist of fleets of lightweight, energy-efficient electric vehicles that arestrategically distributed throughout the city. Similar to the bicycle-sharing systems that are

present in many European cities, users can walk up to any of the electrical charging stations,swipe a membership card, and pick up a vehicle. They can then drive or ride the vehicle to theirdesired destination and drop off the vehicle at a different station. This one-way rental system

eliminates the need for wasteful return trips that are often unnecessary and allows for point-to- point movement that facilitates access to public transportation networks. These Mobility on Demand systems create a model for highly sustainable urban mobility by maximizing publicaccess to a centrally maintained fleet of zero-emission vehicles.

Many challenges exist in designing, developing, and implementing this system, many of themrelated to the design of the electrical grid infrastructure and battery packs. One of the essentialfeatures of a vehicle in a shared-use system is the ability to rapidly recharge the battery pack (in10-15 minutes) to enable quick turnover in vehicle rental and allow for a lower capacity packonboard the vehicle itself. Rapidly recharging battery systems require a cell chemistry withsufficient energy density and low internal resistance to prevent overheating during charging,

electricity sources that can supply large amounts of current, battery pack designs that can handlethese high currents and heat, and high power chargers. Battery cells from A123 Systems, anMIT-spinoff developing nanophosphate-based lithium ion cells, are commercially available andcan be rapidly recharged. Until very recently, the major cell produced by A123 Systems andutilized for high power applications was a small, cylindrical cell with a 2.3 Amp-hour capacityand 8 m ! of internal resistance. These cells have been extensively tested and characterized byA123 Systems and the MIT Electric Vehicle Team (EVT), and battery packs made from thecylindrical cells have been constructed. However, a new cell known as the HD Prismatic (with a20 Amp-hr capacity at 3.3V) will be available soon and presents an improved cell configurationfor automotive applications due to its higher energy density and streamlined form factor.

While the HD Prismatic cells are very appealing in terms of their energy density and physicalsize, cycle life tests of rapid charging have not yet been conducted to determine the feasibility ofrepeated use in automotive applications. The cycle life test is an important characterization of arapidly recharging cell which provides information on how many times the cell can be fullycharged and discharged at rapid rates without losing a significant amount of its capacity. Forexample, a cell cycling system and high power charger were constructed for the cylindrical cellsand demonstrated a capacity loss of ~10% over 1500 cycles as shown in Figure 1 .


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Figure 1: Cell Cycling Results for A123 26650 (Cylindrical Cell)showing approximately a 10% degradation in cycle life after 1500 cycles.

The purpose of this project was to design and construct a cell cycler and high power charging

system for the HD Prismatic cell. Using a high-current source fixed at approximately 4V, thecycler charges the 20 amp-hour cell with 80A of current for a full-charge in 15 minutes.

Schematic


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Design Process

Design and construction of the cell cycler involve a variety of design constraints. Since the cellwill be rapidly charged and discharged 1500-2000 times, thermal management is an importantconcern. To address this, a polycarbonate enclosure for the cycler with fans mounted on one end

and vents on the other was constructed (see enclosure section). Furthermore, the MOSFET and power resistor used for the cycler were mounted to aluminum heatsinks with fins to dissipateexcess heat (see thermal management section).

The basic architecture of the cell cycler is to provide two current paths, one for charging of thecell and the other for draining of the cell. This was done with two contactors, essentially powerrelays, to switch control the active current loop. During the charging cycle, the contactor on theleft side is closed and the contactor on the right side is open to block the right side current loop.During the drain cycle, the left contactor is opened and the right switch closes to create a closedloop between the cell and the power resistor. All circuit connections in the main current loopswere expected to handle up to a maximum of 100A, so hefty 4-gage wire with copper lugs were

used for the high current connections.

Enclosure

After completing a rough sketch of the design including relative sizing of components, the firstmajor step of constructing the cell cycler was the fabrication of a durable enclosure. Anenclosure is required primarily for safety in case the cell explodes or overheats, but it also usefulfor keeping many of the components together in a contained environment. Polycarbonate wasselected as the material for the enclosure due to its optical transparency, resistance to shatter andimpact, and thermal resistance capabilities. Since polycarbonate is fairly expensive, the enclosurewas designed to house only components that generate significant heat and could potentially

overheat or explode.

The rectangular enclosure was created as a digital CAD model using DS SolidWorks with thefollowing dimensions:

Height: 6 inchesWidth: 15 inchesLength: 18 inches

Each of the six faces was created individually as a part and the design was verified in CAD bycreating an assembly to make sure all parts fit together as intended, as shown in Figure 2 . The

enclosure was designed to accommodate active air-cooling in the form of two 120mm square-mount fans. Thus two circular holes were created on one end and air inlets were created on theopposing end to allow for consistent, roughly laminar airflow through the enclosure.


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Enclosure Top View Enclosure Bottom View

Enclosure Inlet View Enclosure Lateral View

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Charge Cycle

In order to extend the life of the cell and ensure for realistic simulation, the prismatic cell should be charged with constant current for as long as possible before leveling off to constant voltage.Given the high amount of constant current necessary for this (80 amperes), purchasing adedicated constant current supply was prohibitively expensive. The only supply that wasavailable for this project with sufficient current output was a 4V, 250 A supply. However,charging the cell with 250A would be both very harmful to the cells cycle life and an inaccuraterepresentation of the rapid charging of vehicular battery packs. Since the current output of thevoltage supply could not be explicitly controlled, a MOSFET controlled with a feedback loopwas utilized to keep the cell in the constant current regime for as long as possible. The MOSFETacts as a switch, which spends more time in an open state when the current is too low and moretime in the closed state when the current is too high. This feedback loop was implemented usinga Hall effect current sensor, which measures the voltage difference across an electrical conductortransverse to an electric current in the conductor and the magnetic field perpendicular to thecurrent. The output from the current sensor was fed into an LM311 voltage comparator, with theother terminal attached to a variable resistance. The output of the comparator was fed to the gateof the MOSFET through a passive, first order low-pass RC filter. The variable resistance wastweaked to ensure that the resulting feedback loop would apply high voltage to the gate when thecurrent was too low and low voltage to the gate when the current was too high. The RC timeconstant of the output filter was designed to make these transitions smooth and ensure stabilityfor the feedback system, such that the current remained roughly constant. Since the sampling rate


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of the comparator was 200 kHz (or a sampling period of 5 " s), the RC time constant was sized to be significantly larger than this sampling rate. A 4.7 " F capacitor and 270 ! resistor were usedfor a time constant of about 1.3 milliseconds:

" = RC = 4.7 F # 270 $ = 1.27 ms

A 10 k ! pulldown resistor was also placed between the gate and the source of the MOSFET to pull the gate down to the ground and close the current loop through the cell in the event offeedback-loop failure.

The cell itself must be managed carefully for use in high-current rapid charging. When highlevels of current are pumped into the Prismatic, the pouch cell can bulge and eventually explode,so the cell must be held under high pressure. To address the potential of bulging during rapidcharging, the cell was placed in a cell flattener, designed by the EVT. The cell flattener consistsof two metal plates with a thermally insulating layer that are held together with interlockingscrews. To prepare the cell for cycling, the cell is placed in the cell flattener and a large amount

of pressure is applied to the flattener while the screws are tightened to maintain the pressurelock.

Drain Cycle

The drain cycle was activated through the opening of the charge contactor and the closing of thedrain contactor. The resulting closed-loop path connected the fully charged cell to the drainresistor, a 250W rated 0.05 ! power resistor. In addition, a 100A fuse was placed in thedischarge current loop for safety. Under ideal circumstances, this closed loop was expected togenerate a current of 66A:

I =" V

R=

3.3 V

0.05 #= 66 A

In reality, this current was slightly less due to other resistances in the circuit such as the internalresistance of the cell and resistances of the connectors. In fact, even minor resistances affect thecurrent quite significantly. For example, even adding 8 m ! of internal resistance for the battery(the spec for the 26650 cell) resulted in nearly a 10A drop in current:

I =" V

R=

3.3 V

0.05 # + 0.008 #= 56.9 A

Initial measurements of the drain cycle yielded a current of 55-56A, indicating some other minorresistive losses in the connectors, or perhaps a higher internal resistance of the cell (A123 has notreleased internal resistance specifications for the prismatic cell). Using 55.5 A as an approximateaverage value for drain current, the discharge time for the cell was calculated as:

t drain =cell capacitydrain current

=

20 Amp " hours55.5 Amps

= 0.36 hours = 21.6 min

This drain cycle was timed in conjunction with the ~15 min charge cycle to produce a fullcharge/discharge cycle of about 37 minutes. The power dissipated in the resistor is also an


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important concern, because excess power is dissipated as heat, which can cause componentfailure if over the rated amounts. The power dissipated in the resistor is given by:

Pdiss

= I 2 R = (55.5 A ) 2 " (0.05 # ) = 154 W

Since the resistor was rated for 250W with proper heatsinking (discussed in the thermalmanagement section), this power dissipation was acceptable.

Thermal Management

In addition to the enclosure with active air-cooling, the two primary power components theMOSFET and the power resistor required additional thermal management since they areresponsible for the primary heat dissipation in the circuit. Thermal circuits can be used to modelthe heat dissipation, where heat sources are modeled as current sources, temperatures aremodeled as potentials, and thermal resistances are modeled as resistors, as in Figure 3 . Thethermal resistance between the device junction and the case, ! JC, is present in either scenario, but

the addition of the heatsink substitutes the very large case-to-air resistance with two muchsmaller resistances (between the case and the heatsink and the heatsink and air).

Heat Source : MOSFET or Power Resistor [W]T J: Junction Temperature [C]T C: Case Temperature [C]T S: Heatsink Temperature [C]T A: Ambient Temperature [C]

! JC: Junction-to-Case Resistance [C/W]! CA: Case-to-Air Resistance [C/W]! CS: Case-to-Heatsink Resistance [C/W]! SA: Heatsink-to-Case Resistance [C/W]

Figure 3: Thermal Circuit Models for Power Dissipating Devices With and Without Heatsinks (Source: http://www.altera.com/support/devices/power/thermal/pow-thermal.html)

The two power dissipating devices were mounted to aluminum heatsinks with fins to increasesurface area using thermally conductive paste. In combination with the active cooling, thisheatsinking provided sufficient thermal management so that these two components could operate

without being damaged by heat.


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Control

The control system was implemented using an Arduino Duemilanove ( Figure 4 ), a single-boardmicrocontroller interfaced through USB and the Arduino software suite. Using the Arduino

board allows for implementation of high-level programming on a board-mounted microcontroller

with analog and digital processing capabilities. Furthermore, the Arduino board has a built involtage regulator which provided a regulated 5V output from the 12V DC power supply. While12V was the dominant supply voltage for the entire system, (including the fans, the contactors,and the MOSFET-gate comparator), 5V was needed for certain applications such as the currentsensor and the feedback potentiometer. Thus the Arduino functioned both as a microcontroller

prototyping platform and as a 5V supply for the low voltage components.

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The process of opening and closing the contactors both for safety shutoff and for switching between charging and draining was also an important feature of the control design. Since thecontactors require 12V across their terminals to close, the Arduino digital logic pins with only5V output are not sufficient to switch the contactors. Thus, an NPN bipolar junction transistor

circuit was created to address this problem. The base of the transistor was wired to the Arduinodigital logic pin through a 1 k ! resistor to prevent excess current draw at the base terminal andthe +12V lead of the contactor was hardwired to the 12V rail of the DC power supply. Theemitter of the BJT was wired directly to ground and the collector was wired to the ground lead ofthe contactor. Thus, when a 5V digital high was applied from the Arduino digital output, thissupplied sufficient current into the base of the transistor to send the device into the ForwardActive Region, effectively pulling the collector to ground and closing the 12V circuit across thecontactor pins. This strategy is illustrated in Figure 5 .

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System Photographs

The enclosure setup showing the three heat-generating primary components (PrismaticCell, MOSFET, and Drain Resistor) that were placed within the enclosure with active air-

cooling for safety and thermal management.

Angled view of the test setup looking into the enclosure and showing the active airflowpaths along heat dissipating devices.


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Close-up view of the current source used to rapidly charge the prismatic cell.

Top view of the full setup, showing components within the enclosure as well as the controland safety systems including the contactors, the control board, the feedback system, andthe fuses.


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Next Steps & Future Improvements

The next major research goal in the continuation of this project is to run the cell cycler forapproximately 1000-1500 cycles and measure the degradation of state of charge as a function ofcycle number. Given the approximately 40 minute full charge/discharge cycle, 1000 cycles will

take approximately 28 days to complete and 1500 cycles will take approximately 42 days tocomplete. Prior to running for the full cycle life test, a 2-3 day, actively monitored trial will berun to ensure that the data collection and safety shutoff systems are running properly.

There is also the potential for additional improvement of the data logging and monitoringsystem. Currently, host data is logged on a host computer through the serial input monitor of theArduino. However, this requires a dedicated computer physically connected to the Arduino boardduring the cycle tests. Since the tests will run for a month or more, it will be useful to have theArduino interface with a serial data logging board that writes the measured data into RAM suchas a microSD card, so that a host machine is not required. Furthermore, advanced safetynotifications can be implemented, such as interfacing the Arduino board with a network-enabled

board so that the monitoring system can transmit emergency messages through e-mail or SMS ifany of the temperature, current, or voltage sensors output anomalous values. In any setup withsuch high amounts of current and power, safety is of paramount concern, so it will be beneficialto spend the extra time required to implement improved safety notifications prior to runningextended cycle life tests.

References

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Acknowledgements

Two groups and several individuals were instrumental in the realization of this project. Thefunding, workspace, and equipment for this project were provided by the MIT Smart Citiesgroup and the MIT Electric Vehicle Team (MIT EVT). Many thanks to the project supervisorand co-supervisor, Professor Bill Mitchell and Raul-David Retro Poblano of the Smart Citiesgroup, for their guidance and support of the project. Sincere thanks to Shane Colton and LennonRodgers of the EVT for assistance in the design and troubleshooting process and for theiroutstanding advice on power electronics system architecture. A special thank you to threestudents from the Mechanical Engineering department: Anna Haas for assistance withmechanical design and modeling of the enclosure, Nick Pennycooke for water jet cutting of theenclosure components, and James White for assistance with Arduino prototyping.

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