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
2. Overview
• Battery Basics
• 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
• Conclusion
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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.
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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
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11. Introduction to BMS
• An electric vehicle generally contains the following major components: an electric motor, a motor controller,
a traction battery, 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 make decisions on charge and discharge rates on the basis of load demands, cell voltage, current, and
temperature measurements, and estimated battery SOC, capacity, impedance, etc. BMS is a part of
complex and fast-acting power management system.
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12. History of BMS
• On 7th January 2013, a Boeing 787 flight was parked for maintenance, 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.
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13. Why we 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.
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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.
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16. BMS architecture
• A modular battery pack suggests a hierarchical master – slave BMS
design.
• There is normally a single “master” unit for each pack.
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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.
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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.
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19. Battery pack – Voltage sensing
• Why we consider cell voltage?
1. Indicator of relative balance of cells.
2. Input to most SOC and SOH estimation algorithms.
3. 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).
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20. Battery pack – Current sensing
• Why battery pack electrical current measurements are required?
1. To monitor battery-pack safety. To log abuse conditions.
2. By most state-of-charge and state-of-health algorithms.
• There are two basic methods to measure electrical current:
1. Using a resistive shunt.
2. Using a Hall-effect mechanism.
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21. Battery pack – Temperature sensing
• Why battery pack temperature measurements are required?
1. Battery cell operational characteristics and cell degradation rates are very strong functions of
temperature.
2. 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.
1. Negative-temperature-coefficient (NTC) thermistors.
2. Positive-temperature-coefficient (PTC) thermistors.
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22. Battery pack – Isolation sensing
• Why isolation sensing is required?
1. 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.
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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, closing an internal switch that connects
its main terminals.
• If both contactors were closed simultaneously,
enormous current would flow instantly and blowing
a fuse So, a third “pre-charge” contactor is used.
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24. BMS communications interface
• Control Area Network (CAN) bus is industry ISO standard for on-board vehicle communications.
• Two-wire serial bus designed to network intelligent sensors and actuators; can operate at two rates:
1. High speed (e.g., 1M Baud): Used for critical operations such as engine management, vehicle
stability, motion control.
2. 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.
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25. BMS communications interface
• The Integrated Circuit (I2C) Bus was a low speed bus originally 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 initiate 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 Systems (SBS) with the limited objectives of interconnecting Smart Batteries which
have built in intelligence, with their associated chargers.
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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.
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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)
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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.
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29. Methods to find SOC
• Direct measurement: this method uses physical battery properties, 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 conditions.
• Hybrid methods: combining any two methods to form a hybrid models of each SOC estimation.
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30. Methods to find SOC
• Direct measurement:
1. Open circuit voltage method Terminal voltage method Impedance method
2. Impedance spectroscopy method
• Book-keeping estimation:
1. Coulomb counting method
2. Modified Coulomb counting method
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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
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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 first cell reaches the voltage which represents 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.
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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.
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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 0 percent DOD. A battery at 80 percent SOC is at 20
percent DOD.
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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 presentation. Due to varying situations in real-world
applications, a standard solution was not wanted. Based on the specific situation, different strategies
should be applied to improve and optimize the performance of BMS in future EV and HEV.
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