Why we need BMS? General function of BMS Block diagram of BMS 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)
Prepared by
Bhagavathy P
Project Associate
IIT-Madras
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2. Overview
1 Introduction
2 Why we need BMS?
3 General function of BMS
4 Block diagram of BMS
5 BMS architecture
6 Battery pack – Voltage sensing
7 Battery pack – Current sensing
8 Battery pack – Temperature sensing
9 Battery pack – Isolation sensing
10 HV contactor control
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3. 11 BMS communications interface
12 Estimation of energy and power
13 State Of Charge (SOC)
14 Why SOC is important?
15 Cell Balancing
16 Methods to find SOC
17 Relationship between SOC and DOD
18 Conclusion
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4. Introduction
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-ion batteries.
The lithium-ion 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 mea-
surements, and estimated battery SOC, capacity, impedance,
etc. BMS is a part of complex and fast-acting power manage-
ment system. 4 / 32

5. 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.
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6. 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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7. Block diagram of BMS
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8. Interfacing BMS with battery
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9. General function of BMS
1 Sensing and high-voltage control
Measure voltage, current, temperature, control contactor,
pre-charge; ground-fault detection, thermal management.
2 Protection
Over-charge, over-discharge, over-current, short circuit,
extreme temperatures.
3 Interface
Range estimation, communications, data recording, reporting.
4 Performance management
State of charge (SOC) estimation, power limit computation,
balance and equalize cells.
5 Diagnostics
Abuse detection, state of health (SOH) estimation, state of life
(SOL) estimation.
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10. 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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11. 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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12. 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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13. Battery pack – Voltage sensing
Why we 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).
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14. 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.
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15. 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.
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16. 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.
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17. 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 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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18. 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:
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 wth 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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19. 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.
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20. 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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21. 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(t) =
Q(t)
Q(n)
(1)
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.
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22. 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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23. Cell Balancing
Cell Balancing scheme to prevent individual cells from becoming
overstressed.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 discon-
nect the battery when the lowest cell voltage reaches its cut off
point of 2 Volts during discharging.
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24. 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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25. 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.
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26. 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
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27. Methods to find SOC
Adaptive systems:
BP neural network
RBF neural network
Support vector machine
Fuzzy neural network
Kalman filter
Adaptive systems:
Coulomb counting and EMF combination
Coulomb counting and Kalman filter combination
Per-unit system and EKF combination
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28. Methods to find SOC
Coulomb Counting method
This method measures the discharging current of a battery and
integrates the discharging current over time in order to estimate
SOC. Coulomb counting method is done to estimate the SOC(t),
which is estimated fromthe discharging current, I(t), and previously
estimated SOC values, SOC(t-1).
SOC is calculated by the following equation:
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29. Methods to find SOC
Kalman filter
It is designed to strip unwanted noise out of a stream of data. It
operates by predicting the new state and its uncertainty, then
correcting this with a new measurement.
It is suitable for systems subject to multiple inputs and is used
extensively in predictive control loops in navigation and target-
ing systems.
With the Kalman Filter the accuracy of the battery SOC pre-
diction model can be improved and accuracies of better than
one percentage are claimed for such systems.
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30. 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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31. 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.
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32. THANK YOU
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