8 - BMS

Battery Management
Systems
BY ARPatel
9th August 2021
Battery Management System for Electric Vehicles
Why do we need battery Management
System ?
 The Lithium-ion batteries have proved to be the battery of
interest for Electric Vehicle manufacturers because of its
high charge density and low weight.
 In EV every cell should be individually monitored for its
safety and efficient operation which requires a special
dedicated system called the Battery Management System
 Also to get the maximum efficiency from a battery pack, we
should completely charge and discharge all the cells at the
same time at the same voltage which again calls in for a
BMS.
Battery Management System design considerations
 The complete considerations depend on the exact end

application in which the BMS will be used
1. Discharging Control:
The primary function of a BMS is to maintain the lithium cells
within the safe operating region
For example a typical Lithium 18650 cell will have an under
voltage rating of around 3V
It is the responsibility of the BMS to make sure that none of
the cells in the pack get discharged below 3V
 2. Charging Control:
 Apart from the discharging the charging process should also be monitored
by the BMS
 Most batteries tend to get damaged or get reduced in lifespan when
charged inappropriately
 For lithium battery charger a 2-stage charger is used
 The first stage is called the Constant Current (CC) during which the
charger outputs a constant current to charge the battery
 When the battery gets nearly full the second stage called the Constant
Voltage (CV) stage is used during which a constant voltage is supplied to
the battery at a very low current
 The BMS should make sure both the voltage and current during charging
does not exceed permeable limits so as to not over charge or fast charge
the batteries
 3. State-of-Health (SOC) Determination:
 The capacity of the battery not only depends on its voltage and current
profile but also on its age and operating temperature also
 The SOH measurement tells us about the age and expected life cycle of
the battery based on its usage history
 This way we can know how much the mileage (distance covered after full
charge) of the EV reduces as the battery ages and also we can know
when the battery pack should be replaced.
4. Cell Balancing:
 Another vital function of a BMS is to maintain cell

balancing. For example, in a pack of 4 cells connected in
series the voltage of all the four cells should always have
equal
 If one cell is less or high voltage than the other it will
affect the entire pack
 5. Thermal Control:
 The life and efficiency of a Lithium battery pack greatly depends on the
operating temperature
 The battery tends to discharge faster in hot climates compared with

normal room temperatures. Adding to this the consumption of high
current would further increase the temperature
 This calls for a Thermal system (mostly oil) in a battery pack
 This thermal system should only be able to decrease the temperature but
should also be able to increase the temperature in cold climates if
needed
 The BMS is responsible for measuring the individual cell temperature and
control the thermal system accordingly to maintain the overall
temperature of the battery pack
 6. Data Logging:
 It is important for the BMS to have a large memory bank since it has to
store a lot of data
 Values like the Sate-of-health SOH can be calculated only if the charging
history of the battery is known
 So the BMS has to track of the charge cycles and charge time of the
battery pack from the date of installation, and interrupt these data when
required
 This also aids in providing after sales service or analyzing a problem with
the EV for the engineers
Building Blocks of BMS
Lithium Ion Battery Comparison
 Lithium is the lightest of all metals

 It has the highest electrochemical potential and provides the largest
specific energy per weight
 Thus, Li-ion batteries have characteristics of high energy and power
density
 They are less affected by memory effect
 These make the Li-ion battery the most likely candidate to be used in
EV application
 Longevity, low self discharge (less than half that of Ni-MH), and rapid
charging and high-load capabilities are among the advantages offered by
Li-ion batteries.
Safety issues and thermal considerations in BMS
 30 min discharge time for 2C and a 20 min discharge time for 3C discharge
 The heat is generated increases the cell temperature to 50°C (for 2C) and
55°C (for 3C) from a 24°C start condition for a single prismatic cell with free
convection boundary condition
 Effect of discharge rate on Li-ion cell Discharge rates of 1C, 2C, and 3C, which
correspond to 20, 40, and 60 A discharge currents, respectively
Safety issues and thermal considerations in BMS
 Higher temperature can result when extrapolated to 100s of prismatic

cells in a battery pack, where there is no free boundary convection, but
only conduction between pouch cells
 This can initiate swelling, thermal runaway, electrolyte fire, and
explosion
 Exposure of Li-ion batteries to subfreezing temperatures drastically
reduces their energy and power
 Therefore, for optimum performance and longevity, the Li-ion battery
should operate within 25°C–40°C
 The temperature variation between battery modules should be less
than 5°C
Battery Charging Methods
1. Constant voltage (CV)
2. Constant current (CC)
3. CC-CV charging
4. Trickle charging
Battery charging methods
1. Constant voltage (CV)

 In this method, the battery is charged at a constant voltage
It is the simplest charging scheme which is suitable for all kinds of
batteries
 The battery charging current is high at the initial stage and gradually
decreases to zero when the battery is fully charged
 Drawback: Requirement of very high power in the early stage of
charging.
Battery Charging Method
2. Constant current (CC)

 In this scheme, the charging voltage applied to the
battery is controlled to maintain a constant current to the
battery
 The SOC will increase linearly versus time for a constant-
current method
3.CC-CV charging
 During the charging process of a battery, normally both
the methods will be used
 At the initial stage, the battery can be precharged at a
low, constant current
 Then, it is switched to charge the battery with constant
current at rated C-rate
 When the battery voltage (or SOC) reaches a certain
threshold point, the charging is changed to CV charge
Battery Management System
 Voltage Balancer Topology (Bi-directional Buck-Boost Converter)
 When R2> R1, T2 serves as a primary switch working in the PWM mode
 When T2 is turned ON, the current in the inductor increases and energy is stored in the inductor
 When T2 is turned OFF, the inductor current begins to decrease linearly and the energy stored will be
released, and the unbalanced power flows to the positive dc load with the help of the voltage
balancer
 With the voltage balancer circuit, it is possible to create a stable unbalance voltage
 The voltage Vx can be set to (2n-1)V/n, where n is any positive number
4. Trickle charging
 Method of charging a fully charged battery at a rate equal
to its self-discharge rate
 It enables the battery to remain at its 100% SOC charged
level if the battery is not discharged using a load
Any Questions
 Thank You

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Battery Management

 The complete considerations depend on the exact end

 Another vital function of a BMS is to maintain cell

 The battery tends to discharge faster in hot climates compared with

 Lithium is the lightest of all metals

 Higher temperature can result when extrapolated to 100s of prismatic

1. Constant voltage (CV)

2. Constant current (CC)

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