Design and Development of a Teaching Tool for Lithium-ion...

Session ETD-535

Proceedings of the 2015 Conference for Industry and Education Collaboration Copyright ©2015,

American Society for Engineering Education

Design and Development of a Teaching Tool for Lithium-ion

Battery Management System

Y. Gene Liao, David T. Fu Engineering Technology, Electric-Drive Vehicle Engineering

Wayne State University Detroit, Michigan

Abstract A Battery Management System (BMS) in the electric-drive vehicle is a control unit between the battery pack and the vehicle and plays a vital role in improving battery performance and optimizing vehicle operation in a safe and reliable manner. It is challenge to teach and train students in BMS for series-connected lithium-ion battery cells in classroom and laboratory environment due to safety and time consuming. In typical testing of an industry-scale battery pack, several cycling tests (charging and discharging processes) are essential in order to monitor the battery State of Charge (SOC). It could also take many hours even days to complete the battery cell balancing procedure. This paper presents the development of an interactive and computer-controlled unit that serves as a teaching-aid for the BMS. The developed teaching tool emulates the battery terminal voltage for up to 12 serially connected cells. Each cell’s voltage can be manually adjusted to simulate SOC changes. By manually changing the cell voltage on the fly, the battery cells’ overcharge, over-discharge and balancing conditions can be emulated and the results can be displayed for student to observe. The integrated system is computerized for measurement and control hence it is comprehensive and visualized for student doing the laboratory experiments. The developed BMS teaching tool not only enhances the advanced energy storage training and education, but also inspires students’ interest in the green movement of transportation and renewable energy. Keywords: battery management system; electric energy storage; energy storage; lithium-ion battery pack; renewable energy; vehicle electrification 1. Introduction Electrochemical batteries are the primary option for energy storage in electric-drive vehicles, uninterruptible power supplies, and renewable energy systems. Rechargeable batteries for those

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used for traction purposes are generally more complex system which consists of hardware and software. As the market share of Electric Vehicles (EVs) and Hybrid Electric Vehicles (HEVs) is kept increasing, safety and reliability are the top concerns of drivers1, 2. Both of the safety and reliability are subject to not only the electrochemistry technology but also the management system of the battery. A crucial role is played by the Battery Management System (BMS), whose main goal is ensuring that the battery is operated within safe limits and achieves optimum performance over its life and over a wide range of operating and environmental conditions3, 4. The primary functions of the battery pack are to store electrical energy from the grid charging (for EV and Plug-in HEV) or produced by the vehicle (for HEV via the internal combustion engine, or during regenerative braking). The battery pack then provides electrical energy for use by the vehicle electric-mode propulsion or for HEV during acceleration and peak power demands. The pack needs to do so in a manner that is safe, reliable, and efficient. This includes protecting the vehicle from voltage surges or drop-outs, preventing harmful conditions, minimizing operational stresses, such as excessive temperature, over-discharging or overcharging that can shorten the life of the battery cells. Therefore, a BMS, as the connector between the battery and the vehicle, plays a vital role in improving battery performance and optimizing vehicle operation in a safe and reliable manner. In view of the rapid growth of the EV and HEV market, it is essential to develop a comprehensive BMS course materials and laboratory for the electric-drive vehicles and advanced energy storage training and education. However, it is challenge to teach and train students in BMS for series-connected lithium-ion battery cells in classroom and laboratory environment due to safety and time consuming. In typical testing of an industry-scale battery pack, several cycling tests (charging and discharging processes) are essential in order to monitor the battery State of Charge (SOC). It could also take many hours even days to complete the battery cell balancing procedure. Besides current and time, battery cell voltage measurement is one of the important parameters for developing controls algorithm for the BMS. The motivation of this project is to develop an interactive and computer-controlled unit that serves as a teaching-aid for BMS. The developed teaching tool emulates the battery terminal voltage for up to 12 serially connected cells. Each cell’s voltage can be manually adjusted to simulate SOC changes. By connecting this multi-channel battery emulator to a Linear Technology LTC6802 BMS demonstration board5, a safe and quick teaching environment can be realized. By manually changing the cell voltage on the fly, the battery cells’ overcharge, over-discharge, and balancing conditions can be emulated and the results can be displayed on the LTC6802 demonstration software for student to observe. The integrated system is computerized for measurement and control hence it is comprehensive and visualized for student doing the laboratory experiments. The Department of Engineering Technology at Wayne State University has offered undergraduate degree in Electric Transportation Technology (ETT) and Undergraduate Certificate in Advanced

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Energy Storage. Several courses have been developed and delivered in battery related topics, such as ETT4150 (Fundamentals of Hybrid and Electric Vehicles), ETT4310 (Energy Storage Systems for Hybrid and Electric Vehicles), ETT4410 (Introduction to Advanced Energy Storages), and ETT4510 (Power Management and Applications of Energy Storages). The developed BMS teaching tool not only enhances the advanced energy storage training and education, but also inspires students’ interest in the green movement of transportation and renewable energy. 2. Overview of Battery Management System The power and energy capabilities of a battery pack greatly depend on operating conditions: charge/discharge current rate, state of charge, load characteristics and temperature. Furthermore, it is important to avoid the circumstances that could damage the battery or trigger off safety issues for the surrounding environment. Especially for battery chemistries like the lithium-based ones connected in a high-power context. Due to volatility, flammability and entropy changes, a lithium-based battery could be ignited if overcharged. This is a serious concern, especially in EV and HEV applications, because an explosion could cause a fatal accident. Moreover, over-discharge causes reduction of cell capacity due to irreversible chemical reactions. A BMS needs to monitor and control the battery based on the safety circuitry incorporated within the battery packs. Whenever any abnormal conditions (such as over-voltage or overheating) are detected, the BMS will issue a warning and execute the preset correction procedures. In addition to these functions, the BMS also monitors the system temperature to provide a better power consumption scheme, and communicates with individual components (engine, generator, or traction motor). Similar to the engine control unit in a fossil fuel car, a gauge meter should be provided by the BMS in EVs and HEVs. The BMS indicators should show the state of the safety, usage, performance, and endurance of the battery. A comprehensive BMS should include the following functions: Data acquisition Safety protection Ability to determine and predict the state of the battery Ability to control battery charging and discharging Cell balancing Thermal management Delivery of battery status and authentication to a user interface Communication with all battery components Prolonged battery life The BMS does not directly control the electrical usage of the battery since a master controller is responsible for determining energy management to propel the vehicle. The BMS provides accurate data on the state of the battery for the master controller algorithm to run properly, safely and reliably. The activity of a BMS can be typically classified into six functions6, 7.

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(1) Measuring: Measurable characteristics of the battery cells and pack are quantified. Typical measured quantities are voltages, currents, and temperatures, either at the single cell or at the battery pack level.

(2) Calculating: The measurement data is processed to determine additional indirectly measurable information. These include the calculated quantities like the SOC, State-Of-Power (SOP), and State-Of-Health (SOH). The first two indicate the remaining energy capacity and the instantaneous power availability, respectively. The SOH quantifies the performance deviation from original condition due to aging.

(3) Monitoring: Some of the measured or calculated characteristics are checked against pre-defined thresholds. A warning is issued in case the measurement violates the critical threshold.

(4) Communicating: The information from monitoring, measuring, and calculating functions is provided to other subsystems or devices in a useful manner.

(5) Control: The BMS may have direct control on critical aspects of the battery pack, such as interrupting current during charge or discharge, or altering the thermal management system.

(6) Balancing: Cells are usually matched during the manufacturing of a battery modules and pack. Over time, an imbalance in the SOC may develop between cells and reduce the overall capacity of the pack. This can be due to slight differences in self-discharge, temperature, or many other factors, between cells. In practice, the cells will be unbalanced, and one cell will be the first to be fully charged and then be overcharged. Once a cell is fully charged, it cannot take more current while the other cells in series still get their needed charge. Continue charging the fully charged cells will rapidly increase the voltages of those cells and possibly to dangerous levels. Implementing a strategy that equalizes the SOC of the cells allows the pack to operate safer and longer.

3. Establishment of a BMS Teaching Tool Unit The BMS of an electric-drive vehicle battery pack comprises hardware and software to monitor pack status and optimize performance. One of its important functions is to execute algorithms that continuously estimate battery SOC, SOH, and available power. The objective of the lab is to provide students with hands-on experience on industry standard multi-cell lithium battery measurements and control functions. The lab utilizes commercially available hardware and software in BMS. The scope of the lab development, as illustrated in Fig. 1, includes integration of software and hardware, hardware setup and interface, and designs of experiments. There are 3 circuit cards for the lab setup as shown in Fig. 2. First, a battery module consists of 12 cells (18650 size) are serially connected which provides 12 cell voltages and two temperature sensors data. The other two circuit cards are from Lear Technology. The main circuit is the Cell Voltage Measurement Board (DC1393B) which provides interface to the battery module. The USB Interface Board (DC590B) receives the commands from computer and communicates with the Cell Voltage Measurement Board8, 9. To lower the overall battery module voltage and protect

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cells during short condition, a 1Amp fuse is installed in the connections between cell 6 and 7. The fuse serves as a manual disconnect and it needs to be plugged in for normal operation. In the event of a short, the fuse will blow to prevent over-discharge for the battery cells.

Figure 1. The design scope of the lab

Figure 2. The developed teaching unit for BMS 3.1 Software Integration

USB interfaceboard

Stacked cell voltage measurement board

Fuse

12 18650- Lithium battery cells

Battery cell voltage and temperature emulation circuits

Battery Management System (BMS)

Active load emulator

Inverter

Current sensor

Isolation fault monitor

The scope of lab development

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From the graphic user interface on PC, the student can perform several battery management functions, such as reading individual or all cell voltage, identifying circuit faulty conditions, changing voltage thresholds, and discharging individual cells for cell balancing purpose. The student has the opportunity to key-in the commands to the computer and control each function. In a real BMS operation, these functions will be operated by a pre-programmed microprocessor and data can be transmitted to other controllers. The graphic user interface provides the following eight operating functions as illustrated in Fig. 3. (1) Set board address (2) Read configuration (3) Program cell voltage monitoring thresholds (4) Write configuration (5) Read cell voltages (6) Read temperatures (7) Discharge cells (8) Read fault flags and warning messages

Figure 3. Indication of eight operating functions 3.2 Hardware Interface Setup This section describes the hardware specifications and the installation procedure. (1) Connect Computer to DC590B USB Interface Board (2) Connect a ribbon cable from DC590 to the Main connector of DC1393B

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(3) Set jumpers on DC1393B to the default positions indicated in Table 1 (4) Install a DC590 driver before running the DC1393 GUI. To do this, install the Quick Eval

Software10. After installation, close the DC590 driver program and launch the GUI control program. When the DC590 board recognizes the String ID code from the DC1393B board, the program will display the control screen. The program will not operate unless the DC590 is connected to the computer as well as the DC1393B board.

(5) Connect the cells which are to be monitored to the cells connector J1. This connector is in two pieces. The setscrew piece can be unplugged to make it easier to attach a wiring harness from the number 4 to 12 cells battery stack. The LTC6802-2 is intended to measure from the number 4 to 12 individual cells for a total stack voltage of 10V to 60V. If fewer than 12 cells are to be monitored, the bottom cell of the stack should always be connected as Cell-1 between terminals J1-5 (cell positive contact) and J1-4 (cell negative contact). Terminals J1-4 and J1-1 are the ground reference points for the battery cell stack and the DC1393B board.

(6) Apply power. Inserting the setscrew piece into connector J1 will apply power to the board from the battery cell stack.

Table 1. Default indicator on DC1393B

4. Conclusions The developed teaching-aid emulates the battery terminal voltage for up to 12 serially connected cells. The integrated system is computerized for measurement and control hence it is comprehensive and visualized for student doing the laboratory experiments. The developed teaching-aid serves as hands-on experience workstations for students with multidisciplinary backgrounds who are enrolled in the advanced energy storage related courses. The teaching-aid would augment the students’ knowledge on electric energy storage technologies and enhance our training and education programs. We also hope the hands-on experience would inspire students’ interest in the green movement of renewable energy.

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References 1. D. Doughty and A. Pesaran, Vehicle battery safety roadmap guidance, subcontract report NREL/SR-5400-54404

U.S. Department of Energy, Office of Energy Efficiency & Renewable Energy, October 2012.

2. G. Zhang, Study on methods of electric vehicle safety test, Proceedings of the 13th International Conference on

Man-Machine-Environment System, 259, 2014, pp 133-142

3. Y. Xing, E. Ma, K. Tsui and M. Pecht, Battery management systems in electric and hybrid vehicle, Energies,

4, 2011, pp. 1840-1857.

4. J. Chatzakis, K. Kalaitzakis, N. Voulgaris, and S. Manias, Designing a new generalized battery management

system, IEEE Transactions on Industrial Electronics, 50/5, October 2003.

5. Linear Technology, LTC6802-1 Multicell Battery Stack Monitor data sheet.

http://www.linear.com/designtools/software/, Accessed 15 August 2014.

6. D. Andrea, Battery management systems for large lithium battery packs, Artech House, Inc., Norwood, MA,

2011.

7. G. Plett, R. Billings and M. Klein, Desktop and HIL validation of hybrid-electric-vehicle

battery-management-system algorithms, SAE International, 07AE-304, 2007.

8. Linear Technology, DC1393B - Demo Board for: LTC6802-2 - Multicell Addressable Battery Stack Monitor

Users Guide, 2009, http://www.linear.com/designtools/software/, Accessed 15 August 2014.

9. P. Rako, Battery-stack-monitor ICs scrutinize the cells, edn.com, Jan. 20, 2011,

http://www.edn.com/electronics-products/other/4363937/Battery-stack-monitor-ICs-scrutinize-the-cells,

Accessed 15 August 2014.

10. Quick Eval Software. http://www.linear.com/designtools/software/, Accessed 15 August 2014.

Biography Y. GENE LIAO is currently Director of the Electric Transportation Technology program and Professor at Wayne

State University. He received a M.S. in mechanical engineering from Columbia University, and a doctor of

engineering from University of Michigan, Ann Arbor. He has 15 years of industrial practices in the automotive sector

prior to becoming a faculty member. Dr. Liao has research and teaching interests in the areas of hybrid vehicle

powertrains and advanced energy storage systems.

DAVID T. FU received his M.S. from University of Detroit-Mercy and B.S. from Lawrence Technological University,

both in Electrical Engineering. He currently works with General Motors Corporation and is an adjunct faculty in WSU.

David's areas of expertise include vehicle electronics, battery system, and electric propulsion integration.

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Appendix This section describes the operation process in the computer Graphic User Interface (GUI). Each step is also indicated in Fig. 4.

Figure 4. Operating the Control Screen (1) Set Board Address

A feature of the LTC6802-2 is that each device/board has a unique 4-bit address. Communication between the GUI and a board will occur only when the address set on the board (jumpers J2 through J5) matches the address set in the GUI for that board, as shown in Fig. 5. The default board and address on power up is Board #1 and address 0x80. The actual address byte for each board is 8-bits long, but only the 4 LSBs are programmable. To enable communication with the DC1393B, set the jumpers to the address on the GUI or change the GUI to match the board (using the Set Hex Address box in the lower left corner of the GUI).

(2) Read Configuration Click the command button labeled Read Config. If all is properly connected and operating, the startup default configuration of the LTC6802-2 (standby mode) will be read from the board. The Hex codes for the six bytes of configuration setting will appear in the Configuration Registers section in the boxes labeled Configuration Read From LTC6802-2.

7

1

6 3 4

2

2

5, 8

6, 8

9

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Figure 5. Setup of measure board

The initial configuration bytes should be 0xE0 for con-figuration register 0 (CNFRG0) and 0x00 for the other five bytes. In addition the LTC6802-2 calculates a Packet Error Code, PEC is appended to the data stream each time it sends out data. For the six bytes sent by this command and received by the GUI, the control program calculates a PEC in the same manner. This byte is compared with the appended byte to check that data transmission was properly executed. Both PEC bytes are displayed in the top section labeled Packet Error Code Check and they should match. The oval located at the top of the color-coded status panel will turn green if the PEC bytes match. Data transmission errors will produce red warning indications if the PEC bytes do not match.

(3) Program the Cell Monitoring Voltage Thresholds In the labeled Set Voltage Limits, click on the boxes and enter voltage values for the over-voltage and under-voltage thresholds required for the cells being monitored. The voltage value entered will be rounded to the actual value used by the LTC6802-2 and displayed in the box. The voltage ranges for these thresholds is 0V to 6.12V. The value of over-voltage threshold is always greater than that of under-voltage. These monitor thresholds can be applied globally to each cell in the system or customized for the cells connected to an individual board by clicking the desired button. Individual board for programming can be selected by the tabs in multiple board systems.

(4) Select an Operating Mode

The power-on operating mode for the LTC6802-2 is standby. The GUI initialized to this state serves as a reminder that the device must be configured before starting operation. This function can done by selecting one of the seven Comparator Duty Cycle (CDC) options from the scroll box in the Set I/O Mode section at the bottom of the screen.

(5) Write Configuration

Nothing is changed in the LTC6802-2 until the Write Configuration command is executed. Clicking the Write Config command button does this. When the command is sent, the six Hex

LTC6802-2

J2-J5

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bytes shown in the Configuration Registers section in the boxes (labeled Configuration Written To LTC6802-2) will become bold type. Student can note the exact hex values required by the LTC6802-2 for specific conditions in these boxes to facilitate their control program development. Clicking the Read Config button can provide confirmation that the configuration change was actually made. The six bytes read-back, sent-out, and the PEC/CRC check bytes should be a match (green PEC oval above the stack status display).

(6) Read Temperature

The LTC6802-2 has three ADC channels dedicated to temperature measure. The temperature indications are for the internal temperature of the LTC6802-2 and two externally connected thermistors. The display returns a voltage measurement. The internal temperature sensor produces a voltage based on a rate of 8mV/°C relative to absolute zero. To convert the voltage reading to degrees Centigrade, divide the voltage by 8mV then subtract 273°C. For an example, 25°C is a reading of 2.384V. For external temperature measurements, connect thermistors across cells connector terminals J1-3 to J1-1 and J1-2 to J1-1. Alternatively thermistors can be connected to the board as resistors R35 and R40 located near connector J1. A thermistor with a 25°C value of 10KΩ will produce a half scale voltage reading at 25°C. Any thermistor value may be used; however the scaled voltage measurement may require changing the values of resistors R29 and R30 on the DC1393B circuit board. To take a temperature reading, simply click the Start Temp command button to make the LTC6802-2 ADC conversion. Then click the Read Temp command button to download the data from the board and display the voltage readings of temperature.

(7) Read Cell Voltages

One essential function of the LTC6802-2 is to measure and report the voltage of each cell. This function is accomplished by two command buttons in the control screen. First click on the Start Cell Volt button. This commands an A/D conversion of all 12-cell voltages in the time configured from the selected Operating Mode (Set CDC option) in the Set I/O Mode box. All 12 cell voltages are converted every time regardless of the number of cells actually connected to the LTC6802-2. Unused cell connections, which should be shorted together, will return voltage readings of 0.000 volts. The actual cell voltage measurements are not displayed until the Read Cell Volt command button is clicked.

(8) Read Flags

When any cell in a stack exceeds the programmed over or under voltage threshold limit, one of two flag bits is set in an internal register for that cell to serve as a warning. This is important feedback for battery charging algorithms to know simply when to start or stop charging. A

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simple click of the Read Flag command button will indicate the status of these warning flags at any time. The Hex code for the three flag bytes appears in the Flag Reg section of the control panel at the top of the screen. One of the configuration options is to mask these flags from the register bytes that are read from the LTC6802-2. This feature can be used to prevent or allow these flags to interrupt the flow of the control algorithm. A check box is provided for each cell in a stack to select the mask interrupts option for that cell. To implement the masking requires checking the box and then writing the new configuration with a Write Config button push.

(9) Discharge Cells

One major feature of the LTC6802-1 is the ability to remove charge from individual cell. This can help to distribute the cell charge evenly over a stack of batteries. The DC1393B contains a P channel MOSFET in series with a 15Ω resistor across each cell connection. When enabled, a cell is shorted and charge is pulled from the cell with energy dissipated in the switch and resistor. A check box is provided for each cell to be discharged. Checking this box and then writing the new configuration with a Write Config button push will short the cell. The discharge transistors are automatically turned off momentarily while the A/D converter is measuring the cell voltage. This prevents any voltage drop errors caused by the discharge current flowing through the cell inter-connection wiring. An accurate indication of the true SOC of the cells is then obtained. The command set and GUI offer an option of keeping the discharge transistors on while measuring the cell voltages. This is done by holding the Start Cell - a DCC command button. A blue indicator is illuminated when this command is in effect. A lower voltage reading due to the discharging condition is the actual cell voltage contribution to the stack.

(10) Continuous Operation For convenience, the control panel allows for continuous operation of the DC1393B board. The command button labeled Start Continuous Read Cells can be clicked and the board control is placed in a continuous loop to execute the following commands automatically in the sequence: Start cell voltage, Read cell voltage, Start temp. Read temp, and Read flags.

While running in the continuous operation, the configuration can arbitrarily be change. All values are updated continually. Simply changing a configuration item (Discharge cells for example) and clicking the Write Config button will implement the new configuration and return to continuous operation. A green box in the lower right hand corner indicates that the system is running continuously. A red box means that the system is stopped and waiting for a new command to be sent.

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