BATTERY MANAGEMENT SYSTEM (BMS) IN ELECTRIC VEHICLES

Introduction
Why we need BMS?
General function of BMS
Block diagram of BMS
BMS architecture
Battery pack – Voltage, Current, Temperature and Isolation sensing
HV contactor control
BMS communications interface
Estimation of energy and power and SOC
Methods to find SOC
Cell Balancing
Relationship between SOC and DOD

1. BATTERY MANAGEMENT SYSTEM (BMS) IN ELECTRIC VEHICLES Prepared by BHAGAVATHY P 1

2. Overview Battery Basics Introduction Why w e need BMS? General function of BMS Block diagram of BMS BMS architecture Battery pack – Voltage, Current, Temperature and Isolation sensing HV contactor control BMS communications interface Estimation of energy and power and SOC Methods to find SOC Cell Balancing Relationship between SOC and DOD Conclusion 2

3. BATTERY BASICS 3

4. Difference Between Cell, Module & Pack Battery cell: Unit of a battery that exerts electric energy by charging and discharging. Made by inserting anode, cathode, separator and electrolyte into a aluminum case. Battery module: Connecting a number of cells in parallel or series is called battery module. Battery Pack: Composed of battery module connected in series and parallel. 4

5. INTRODUCTION 5

6. Difference Between Lithium and Other Batteries 6

7. DIFFERENCE BETWEEN LITHIUM AND OTHER BATTERIES Lead based cell – 2.1V/cell Nickel based cell – 1.2V/cell Lithium based cell – 3.8V/cell 7

8. Types of Lithium Batteries LCO - Lithium Cobalt Oxide LMO - Lithium Manganese Oxide NMC - Lithium Nickel Manganese Cobalt Oxide LFP - Lithium Iron Phosphate NCA - Lithium Nickel Cobalt Aluminum Oxide LTO - Lithium Titanate Oxide 8

9. C – Rate Battery current handling capability It is a constant current charge or discharge rate, which the battery can sustain for one hour For eg : 12V,20Ah battery 20A can be deliver at one hour or 2A for 10hrs 1C rate - 20A for 1hr 2C rate - 40A for 30min 3C rate - 60A for 15min 0.5C rate - 10A for 2hrs 0.1C rate - 2A for 10hrs 9

10. CELL FORMATS OF LITHIUM BATTERIES 10

11. Introduction to BMS An electric vehicle generally contains the following major com ponents: an electric motor, a motor controller, a traction bat tery, a battery management system, a wiring system, a vehicle body and a frame. The battery management system is one of the most important components, especially when using lithium batteries. The lithium cell operating voltage, current and temperature must be maintained within the “Safe Operation Area” (SOA) at all times . To maintain the safe operation of these batteries, they require a protective device to be built into each pack is called battery management system (BMS). BMS ma k e decisions on ch a rge a nd disch a rge rates on the basis of load demands, cell voltage, current, and temperature mea surements, and estimated battery SOC, capacity, impedance, etc. BMS is a part of complex and fast-acting power manag e ment system. 11

12. History of BMS On 7th January 2013 , a Boeing 787 flight was parked for main tenance, during that time a mechanic noticed flames and smoke coming from the Auxiliary power unit (Lithium battery Pack) of the flight. On 16th January 2013 another battery failure occurred in a 787 flight operated by All Nippon Airways which caused an emergency landing at the Japanese airport. After a series of joint investigation by the US and Japanese , the Lithium battery Pack of B-787 went through a CT scan and revealed that one of the eight Li-ion cell was damaged causing a short circuit which triggered a thermal runaway with fire . This incident could have been easily avoided if the Battery management system of the Li-ion battery pack was designed to detect/prevent short circuits. 12

13. Why w e need BMS? Detects unsafe operating conditions and responds. Protects cells of battery from damage in abuse and failure cases. Prolongs life of battery. Maintains battery in a state. 13

14. General function of BMS Sensing and high-voltage control Measure voltage, current, temperature, control contactor, pre-charge; ground-fault detection, thermal management. Protection Over-charge, over-discharge, over-current, short circuit, extreme temperatures. Interface Range estimation, communications, data recording, reporting. Performance management State of charge (SOC) estimation, power limit computation, balance and equalize cells. Diagnostics Abuse detection, state of health (SOH) estimation, state of life (SOL) estimation. 14

15. Block Diagram 15

16. BMS architecture A modular battery pack suggests a hierarchical master – slave BMS design. There is normally a single “master” unit for each pack. 16

17. BMS master role Control contactors that connect battery to load. Monitor pack current, isolation. Communicate with BMS slaves. Control thermal-management. Communicate with host application controller. 17

18. BMS slave role Measure voltage of every cell within the module. Measure temperatures. Balance the energy stored in every cell within the module. Communicate this information to the master. 18

19. Battery pack – Voltage sensing Why w e consider cell voltage? Indicator of relative balance of cells. Input to most SOC and SOH estimation algorithms . Safety: overcharging a lithium-ion cell can lead to “thermal runaway,” so we cannot skip measuring any voltages. Voltage is measured using an analog to digital converter(ADC). A direct-conversion or flash ADC. Successive approximation. Delta-sigma. Special chipsets are made to aid high-voltage BMS design. Multiple vendors make chipsets (e.g., Analog Devices, Maxim, Texas Instruments). 19

20. Battery pack – Current sensing Why battery pack electrical current measurements are required? To monitor battery-pack safety. To log abuse conditions. By most state-of-charge and state-of-health algorithms. There are two basic methods to measure electrical current: Using a resistive shunt. Using a Hall-effect mechanism. 20

21. Battery pack – Temperature sensing Why battery pack temperature measurements are required? Battery cell operational characteristics and cell degradation rates are very strong functions of temperature. Unexpected temperature changes can indicate cell failure or impending safety concern. There are two methods to measure temperature : Using a thermocouple and using a thermistor. Thermistor has two types. Negative-temperature-coefficient (NTC) thermistors. Positive-temperature-coefficient (PTC) thermistors. 21

22. Battery pack – Isolation sensing Why isolation sensing is required? Isolation sensing detects presence of a ground fault. Primary concern is safety . In a vehicle application, we must maintain isolation between high-voltage battery pack and chassis of the vehicle. FMVSS says isolation is sufficient if less than 2mA of current will flow when connecting chassis ground to either the positive or negative terminal of the battery pack via a direct short. 22

23. HV contactor control Disconnecting or connecting a battery pack at both terminals requires high-current capable relays or “contactors”. A low-voltage/low-current signal activates the contactor, clos ing an internal switch that connects its main terminals. If b oth contact o rs w ere closed simultaneousl y , en o rmous current would flow instantly and blowing a fuse So, a third “pre-charge” contactor is used. 23

24. BMS communications interface Control Area Network (CAN) bus is industry ISO standard for on-board vehicle communications. T w o-wire serial bus designed to ne t w o rk intelligent sens o rs and actuators; can operate at two rates: High speed (e.g., 1M Baud ): Used for critical operations such as engine management, vehicle stability, motion control. Low speed (e.g., 100 kBaud ): Simple switching and control of lighting, windows, mirror adjustments, and instrument displays etc. The LIN Bus is another automotive communications standard, similar to the CAN Bus. It is a single wire Local Interconnect Network operating at 20 KBaud with low cost IC solutions. The FlexRay Bus can support fast responding dynamic control systems rather than just the simpler sensors and actuators per- mitted with the CAN Bus. The FlexRay data payload per frame is 20 times greater than the CAN Bus. 24

25. BMS communications interface The Integrated Circuit (I2C) Bus was a low speed bus origi nally designed for use between internal modules within a system rather than for external communications. It is a bidirectional, half duplex, two wire synchronous bus. It runs with data rates up to 3.4 Mbits /s and is suitable for Master - Slave applica - tions . Multiple slaves are possible but only a master can initi ate a data transfer. Typically used for internal communications within embedded systems such as a BMS. The SMBus (System Management Bus) is a two wire, 100 KHz , serial bus designed for use with low power Smart Battery Sys tems (SBS) with the limited objectives of interconnecting Smart Batteries which have built in intelligence, with their associated chargers. 25

26. Estimation of energy and power Cannot directly measure the available energy or available power Therefore, must estimate SOC, SOH . To estimate energy, we must know all cell states-of-charge and charge capacities. To estimate power, we must know all cell states-of-charge and resistances. Available inputs include all cell voltages, pack current, and temperatures of cells or modules. 26

27. State of Charge (SOC) The SOC of a battery is defined as the ratio of its current capacity Q(t) to the nominal capacity Q(n).The nominal capacity is given by the manufacturer and represents the maximum amount of charge that can be stored in the battery. The SOC can be defined as follows: SOC changes only due to passage of current , either charging or discharging the cell due to external circuitry, or due to self- discharge within the cell. SOC ( t ) = Q ( t ) Q ( n ) 27

28. Why SOC is important? Prevent overcharge or discharge. Improve the battery life. Protect battery. Improves the battery performance. For cell balancing applications, it is only necessary to know the SOC of any cell relative to the other cells in the battery chain. 28

29. Methods to find SOC Direct measurement: this method uses physical battery prop erties, such as the voltage and impedance of the battery. Book-keeping estimation: this method uses discharging current as the input and integrates the discharging current over time to calculate the SOC. Adaptive systems: the adaptive systems are self designing and can automatically adjust the SOC for different discharging con ditions. Hybrid methods: combining any two methods to form a hybrid models of each SOC estimation. 29

30. Methods to find SOC Direct measurement: Open circuit voltage method Terminal voltage method Impedance method Impedance spectroscopy method Book-keeping estimation: Coulomb counting method Modified Coulomb counting method 30

31. Methods to find SOC Adaptive systems: BP neural network RBF neural network Support vector machine Fuzzy neural network Kalman filter Hybrid systems: Coulomb counting and EMF combination Coulomb counting and Kalman filter combination Per-unit system and EKF combination 31

32. Cell Balancing Cell Balancing scheme to prevent individual cells from becoming over stressed. These systems monitor the voltage across each cell in the chain. Active cell balancing methods remove charge from one or more high cells and deliver the charge to one or more low cells. Dissipative techniques find the cells with the highest charge in the pack, indicated by the higher cell voltage, and remove excess energy through a bypass resistor until the voltage or charge matches the voltage on the weaker cells is known as passive balancing . Charge limiting is a crude way of protecting the battery from the effects of cell imbalances is to simply switch off the charger when the fi r s t cell reaches the voltage which re p resents its fully charged state (4.2 Volts for most Lithium cells) and to disconnect the battery when the lowest cell voltage reaches its cut off point of 2 Volts during discharging. 32

33. Cell Balancing Lossless balancing is a superior way of cell balancing by means of software control. All of these balancing techniques depend on being able to determine the state of charge of the individual cells in the chain. More precise methods use coulomb counting and take account of the temperature and age of the cell as well as the cell voltage. 33

34. Relationship between SOC and DOD A battery’s depth of discharge (DoD) indicates the percentage of the battery that has been discharged relative to the overall capacity of the battery. Depth of Discharge (DOD) is the fraction or percentage of the capacity which has been removed from the fully charged battery. Conversely, the State of Charge (SOC) is the fraction or percentage of the capacity is still available in the battery. A battery that is at 100 percent SOC is at percent DOD. A battery at 80 percent SOC is at 20 percent DOD. 34

35. Conclusion As batteries are the core energy sources in EVs and HEVs, their performance greatly impacts the salability of EVs. Therefore, manufacturers are seeking for breakthroughs in both battery technology and BMS. The major concerns of BMS were discussed in this presenta tion. Due to varying situations in real-world applications, a standard solution was not wanted. Based on the specific sit uation, different strategies should be applied to improve and optimize the performance of BMS in future EV and HEV. 35

36. THANK YOU 36