WEEK 8 eMOBILITY

Battery Management system control (BMS)

BMS is an electronic system that manages a rechargeable battery to ensure it operates safely and
efficiently. BMS is designed to monitor the parameters associated with the battery pack and
its individual cells, apply the collected data to eliminate safety risks and optimize the battery
performance.

The Importance of Battery Management System

BMS provides the following functionality:
1. Voltage, Current, and Temperature control and measurement
2. SoC and SoH assessment
3. Detection of fault
4. Passive cell balancing
5. Data storage

Block diagram of BMS

Working of BMS
 The battery management system tracks the status of each cell in the battery pack. Determining
the SoC (State of Charge) and SoH (State of Health) helps estimate the amount of current needed
for a safe charge and discharge operation without harming the battery.
 The current limits act as a cut-off and prevent the battery from overcharging. This safeguards the
cell voltages of the battery pack from high or low fluctuations, which immunes the battery life.
 The BMS consistently tracks the charge and discharge activities for the battery pack and monitors
cell voltages. This data is useful in deciding if the battery is drained, sustaining passive cell
balancing.
 The CAN (Controller Area Network) bus is the reliable unit for internal communications. It
transfers the information to the CMU (Central Monitoring Unit) or the sub-controller unit.
 The sub-controller unit quickly checks the temperature and voltage signals and sends data to the
CAN bus. The BCU (Battery Control Unit) obtains the signals from the CAN bus and responds by
transmitting back the control signals required in battery pack managing and modeling.

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Components of a Battery Management System (BMS)

Mainly there are 6 components of battery management system.
1. Battery cell monitor
2. Cutoff FETs
3. Monitoring of Temperature
4. Cell voltage balance
5. BMS Algorithms
6. Real-Time Clock (RTC)

1. Battery cell monitor

A battery cell monitor primarily monitors the voltages for battery systems.
 It monitor individual cell voltage. When the voltage of the first cell reaches the voltage limit, the
charging automatically trips. It indicates that the battery charging limit has been reached.
 If the battery pack has a lesser charge than the average cell, then the least charged cell will reach
the limit first, and the rest of the cells will be left partially charged.

2. Cutoff FETs
FET driver is accountable for connection and isolation between load and charger of the battery pack.
The behavior prediction is done through voltage, current measurements, and real-time detection
circuitry.

3. Monitoring of Temperature
With the increase in product requirements, the batteries have been on a constant surge in delivering
currents at fixed voltages. The continuous operation processes may cause a fire or explosion.
 We can identify whether battery charging or discharging is desirable using temperature
measurements.
 Temperature sensors monitor the energy storage system or cell grouping for compact portable
applications helps reduce the temperature inaccuracies and improves the overall measurement
system.

4. Cell voltage balance

It is crucial to determine the health of the battery pack. That is why cell voltage monitoring is done
to ensure that the cells are in a proper running condition for attaining a long battery life.
 The battery life is significantly affected while performing battery operations beyond the voltage
range. This reduces the life of a cell, which may even make it unfit for use.

5. BMS Algorithms
To make quick and effective decisions in real-time based on the information received. For this
purpose, a microcontroller for battery management system is needed to collect, organize and assess
the information from the sensing circuitry.

6. Real-Time Clock
Allowing the user to know the battery pack’s behavior before any alarming event, the real-time clock
acts as a black box system for time-stamping and memory storage.

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Primary functions of BMS

1. Safety:
 The BMS continuously monitors parameters such as temperature, voltage and current in and out
of the pack to ensure it is being operated in safe conditions the entire time.
 BMS is responsible for thermal management of the battery and monitors its temperature
continuously. If required, BMS can adjust cooling and trigger other safety mechanisms to cease
operations and minimize the risk.
 Overcharging of lithium-ion cells can also lead to thermal runaway and potentially an explosion.
BMS continuously monitors the voltage of the pack as well as individual battery cells and controls
the supply of the current to avoid overcharging.
 Sensing electrical isolation – The BMS also checks that the vehicle chassis is completely isolated
from the high voltage battery pack at all times to prevent the user from getting an electric shock.
2. Performance optimization:
 BMS is responsible for optimizing the performance of the battery pack.
 It controls the recharging of the battery pack by energy generated through regenerative
breaking.
 It also performs cell balancing by draining excess energy from cells that are more charged than
others, through active or passive balancing techniques.
3. Health monitoring and diagnostics:
 The BMS uses the collected data points (temperature, voltage, current etc.) to estimate the State
of Charge and State of Health (SoH) of the battery pack.
 The SoC referes to availability in the battery and determines how far the vehicle can go before
needing to recharge.
 The SoH measures the current condition of the battery as compared to its original capacity and
indicates the battery’s stability for the application. Both SoC and SoH are presented as
percentages.
4. Communication:
 The BMS is responsible for communicating with the other ECUs in the vehicle. It relays the
necessary data about the battery percentage to the motor controller to ensure the smooth
running of the vehicle.

Benefits of Battery Management System:

1. It monitors the aging and charging status as well as the depth of discharge of the battery module
2. It control the charging cycle smartly and optimized in regard to speed, thermal management or
over charging.
3. The BMS protects the battery from over charging, deep discharging, over loading, under
temperature, over temperature and short circuit.
4. The BMS ensures that all the cells in the battery are at the same State of Charge (SoC) making
the battery to run at the full capacity.
5. The BMS transfers the extra charge to a hig charge cell to the lowest charge cell, helping in
increasing the battery life by up to 25%.
6. The BMS provides thermal management to the battery, safeguarding it against over and under
temperature.

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Battery management design considerations (Service life, efficiency, safety, operational parameters)
1. BMS Safety Considerations
 BMS includes battery cells, power electronic equipment, controller and monitoring units, and
energy management units. Therefore, any abnormality or accident can cause a BMS-related
accident. It is critical to take appropriate precautions as a rule for every BMS component.
Indeed, BMS safety is essential for both external and internal equipment of BMS. The external
safety procedures, along with technical safety measures, are necessary to ensure complete
BMS safety. However, it should be noted that procedural safety measures are more important
than technical safety measures.
 Today, rechargeable batteries use a combination of energy storage systems, such as the
flywheel and supercapacitor. Therefore, BMS safety is essential not only for the stand-alone
battery pack but also for combined energy storage systems.
2. BMS Parameters Considerations
 Two well-known safety strategies are available in the lithium battery: current interrupt devices
(CID) and positive temperature coefficient (PTC).
 The PTC protects batteries from an external short circuit. In abnormal cases, the PTC will
heat itself, increase its resistance, and block the excess current.
 The CID prevents current flow in an abnormal condition, which may cause gas generation.
It is highly recommended to implement these techniques for ensuring BMS reliability.
 A new solvent with a higher flash point than the existing solvent is recommended to use in
batteries for fire resistance. As battery is a part of the BMS, the development of a new solvent
will reduce the probability of a BMS fire.
 A gas sensor is capable of tracking volatile organic compounds (VOC) from leaked electrolytes.
Hence, gas sensors can be a cost-efficient way to enhance the safety of BMS.

3. Considerations for BMS Installations (Service life and efficiency)

 The equipment rating and marking instruction must be strictly followed. Before the battery is
put into operation, the normal case, worse case, and abuse case conditions of the battery must
be evaluated.
 The way BMS control unit interacts with humans should be checked for each unit of the BMS.
If any modification or replacement is needed for part of the unit, an extensive investigation
must be carried out to evaluate whether the existing unit is compatible with the proposed
change.
 Since the manufacturer and design fault is one of the most dominant causes for BMS failure,
third-party verification is recommended to ensure safety.
 If an accident occurs with a battery bank, it is recommended to remove and replace all of the
battery bank and to avoid using a battery that has had contact with fire, no matter how
minimal the contact.
 Different types of batteries containing liquid electrolytes should not be combined for any
extended use. If two male ends become connected to each other and they come into contact
with flammable material, the impact will cause a fire explosion.
 Every battery must be charged by a specifically rated charger; otherwise, there is a possibility
of overheating, which will damage the battery.
 It is recommended not to place that battery storage systems in high-temperature
environments.
 Safety reviews should be conducted regularly for each BMS unit and be recorded in safety
review reports to assess the changes and required modifications of the BMS unit.
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Battery pack module

A battery pack is a set of any no of identical batteries or individual battery cells.

Types of battery packs:

There are two basic types of battery packs:
1. Primary batteries: are disposable, non-rechargeable devices. They must be replaced once their
energy supply is depleted.
2. Secondary or rechargeable batteries: contain active materials that can be regenerated. When
the energy produced by these batteries drops below optimum efficiency, they may be
recharged according to various methods, depending on the battery construction.
Secondary batteries are useful in applications where frequently replacing disposable batteries is
more costly, such as in electric vehicles, handheld power tools and automobile starters.

Battery pack procedure

1. Cell Sorting. Voltage, Capacity and Internal Impedance are mached. Group cells with similer
operating parameters.
2. Module Assembly. Assemble modules. Attach BMS. Test Module.
3. Pack Assembly. Stack modules in series and parallel. Place pack in enclosure. Attach master BMS.
4. Final Testing.

Battery pack configurations

A single cell is not sufficient for some devices. To achieve desired voltage, the cells are connected in
series to add the voltage of cells. To achieve the desired capacity, the cells are connected in parallel
to get high capacity by adding ampere-hour (Ah).
Sometimes battery packs are used in both configurations together to get desired voltage and high
capacity
1. Series configuration
The series configuration is used where the voltage of a single cell is not sufficient. The series
configuration is achieved by connecting the positive of a cell to the negative of another cell, as
shown in the image below. The four lithium-ion cells of 3.6 V connected in series will give you 14.4
V, and this configuration is called 4S because four cells are connected in series.

2. Parallel configuration
The cells are connected in parallel to fulfill higher current capacity requirements if the device needs
a higher current but there is not enough space available for the battery. That device can use the
parallel configuration to fit high-current capability in a small space.
The four-cell configuration in parallel is called P4, and three cells connected in a parallel
configuration are called P3. The image below shows a P4 configuration. The voltage in the pack
remains the same, but the current capacity (Ah) is increased.

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3. Series – parallel configuration

In this configuration, the cells are connected in both series and parallel. The series-parallel
configuration can give a desired voltage and capacity in the smallest possible size. You can see two
3.6 V 3400mAh cells connected in parallel in the image below, which doubles the current capacity
from 3400mAh to 6800mAh. Because these parallel packs are connected in series, the voltage also
doubles from 3.6V to 7.2 V. The total power of this pack is now 48.96Wh. This configuration is called
2SP2.

Criteria for battery selection pack

1. Battery Type: Primary, secondary, reserve or fuel cell system.
2. Battery Voltage: Nominal or operating voltage, maximum/minimum voltage limits, discharge
profile, voltage delay, start-up time.
3. Load Current & Profile: Constant current, constant resistance, or constant power; value of
load current, constant or variable load current.
4. Duty Cycle: Continuous or intermittent, schedule if cycle is intermittent.
5. Temperature Requirements: Operational temperature range.
6. Service Life: Length of time over which operation is required.
7. Physical Requirements: Size, shape, weight limitations.
8. Shelf Life: Allowable storage time.
9. Charge-Discharge Cycle: Discharge profile and charging efficiency.
10. Environmental Conditions: Atmospheric conditions including pressure and humidity, shock,
vibration, spin, acceleration environment compatibility.
11. Safety & Reliability: Permissible failure rates.
12. Maintenance: Ease of battery maintenance and replacement.
13. Cost: Initial and operating costs.

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Battery working temperature

 EV Batteries have specific operating ranges, which are critical for the battery life and performance.
They are designed to operate at ambient temperature, which is between 20°C and 25°C. A better
control over the battery temperature improves their performance and life.
o During operation, they can withstand temperature between -30°C and 50°C
o During recharges, they can withstand temperatures between 0°C and 50°C
 If temperatures are too cold, such as 0°C, it can result in a loss of capacity due to the chemical
reactions inside the battery slowing down due to the low temperature.
 Very low temperatures can produce a reduction in the energy and power capabilities of batteries.
 High ambient temperatures, however, can contribute to a high internal temperature of the
battery — which can also decrease performance and power capabilities.
 If conditions are too hot, it can result in hazards such as fire and explosion.
 In addition to this issue, high ambient temperatures produce further complications and risks. If
the temperature reaches a critical point, thermal runaway can be triggered in the battery.

Temperature list for all types of batteries

Specification Lead Acid Ni-Cd Ni-MH Lithium ion

Nominal Voltage 2 1.25 1.25 3.6
Operating Temperature -20 to 600C -40 to 600C -20 to 600C -20 to 600C

Different types of electrolytes and additives used in batteries.

Types of electrolytes used in batteries
1. Calcium
2. Chlorine
3. Sodium
4. Potassium
5. Magnesium
6. Phosporous
7. H2SO4

Additives used in batteries:

1. Vinylene carbonate (VC)
2. LiBOB
3. Metal ions
4. Sodium or potassium salts
5. Vinyl ethylene sulfite (VES)
6. Prop-1-ene
7. Sultone
8. Sulfur trioxide
9. Phosphoric ester
10. Tris (trimethylsilyl) phosphite
11. Furanone

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Causes of Battery Explosion

1. Normal Operation, Overcharging and Faulty Systems
 Under normal operating circumstances, it is possible for a flooded lead acid battery to
maintain a hydrogen and oxygen concentration above the level where an ignition source may
cause an explosion.
 Overcharging as a result of faulty vehicle charging systems can produce more of these gasses
and as such can increase the risk of explosion.
 Overcharging can also increase the rate of grid corrosion breakdown of the internal battery
plate and separators leading to the possibility of short circuit and explosion.

2. External Sources of Ignition

Primary sources of ignition such as static sparks, naked flames, cigarettes and sparks caused by
metal objects touching or shorting the battery terminals, loose battery connections and corroded
cables can ignite the flammable gasses built up in a battery.

3. Engine Starting
Starting the engine places a load on the battery that can trigger an explosion when there is an
underlying problem. This is more likely when a battery is near its end of life. Both internal plate
corrosion or a manufacturing fault increases the risk of a short circuit especially when the
electrolyte level is low and the potential short is in the gas space.

4. Manufacturing Faults
Defects or faults in the manufacturing process can cause a battery to short circuit. For example
if the internal terminal post is not correctly fused to the external terminal lead, arcing can occur.
Such a fault is detected by a complete absence of voltage with intermittent spikes up to normal
voltage levels. This is a dangerous situation as just physically moving the battery can cause a
short circuit.

5. End of Life
Batteries nearing their end of life will exhibit increased signs of grid corrosion and degradation
of active material on the battery plates. This can gather in the plate separators leading to a
possibility of short circuits between the battery plates. Blocked vent plugs can also cause a short
circuit as the battery cell expands under pressure.

6. Poorly Maintained Batteries

 Batteries which have been left in a poorly maintained state for extended periods of time can
lead to an increased possibility of explosion.
 If electrolyte levels are allowed to fall exposing the top of the battery plates, they will corrode
faster than the section below causing growth, the possibility of plate contact and an increased
risk of a short circuit occurring.
 Regular battery care and maintenance can help reduce the risk of a battery exploding.

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Factors affecting battery performance

1. Electrode-Electrode Materials and thickness also play a considerable role in battery performance.
At a cell level, voltage, efficiency, coulombic efficiency, and energy efficiency are three
characteristics parameters typically used to evaluate a battery performance.
2. Packing of cells : Chemistry in batteries affects voltage energy density, recharge ability, self-
discharge, cycle life, and safety
3. Safety: Temperature has a significant impact on battery safety. Internal short circuit, external
short circuit, overcharging, excessively fast charging, overloading and external heat transfer can
cause the temperature to increase, leading to gas generation or thermal runway, combustion and
explosion.
4. Cost aging: However, battery ageing is an irreversible process caused by various factors,
including battery charge cycle, overcharging and tickle charging and inappropriate temperature.

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