Electric vehicles (EVs) are becoming the new alternative to internal combustion engines (ICs). EVs cause less pollution to the environment, have low operating costs, and have better driving experiences.
As of June 2024, more than 8.32 lakh electric vehicles have been sold in India this year alone. An Electric Vehicle that has a battery to power its driving motor is made of hundreds and thousands of cells based on vehicle size.
So, a controlling unit called Battery Management System (BMS) is required for these cells to operate in safe regions.
What is a Battery Management System (BMS)?
BMS is an electronic control circuit that monitors and regulates the charging and discharge of the battery of an electric vehicle. BMS consists of sensors for measuring battery current, voltage, and temperature. Sensors can be installed across cells in Centralised architecture (only one BMS for entire cells) and Distributed architecture (several BMS required as it is directly installed at the cells site).
For more details: Battery Management System (BMS) for Electric Vehicle
Why is the Battery Management System (BMS) required?
BMS in the electric vehicle is the brain of the battery pack. It takes the current, voltage, and temperature temperature data from the sensors (slave boards) placed at the cell site to monitor and control the cell’s safe operating area. BMS is required for the following reasons:
State Estimations
Based on the sensor data received, BMS estimates the battery states like State of Charge (SOC), State of Health (SOH), and State of Power (SOP). These state estimations are required to get information on how much charge, and power is available in the battery to use. There are many methods to estimate these states but the Coulomb Counting with Voltage (OCV) method is most commonly used by EV manufacturers.
Cell Balancing
BMS performs cell balancing which means keeping all cells within certain limits of voltage and SOC. Thus Cell balancing ensures optimal use of the battery pack. It can be active cell balancing and passive cell balancing.
Active Cell Balancing is based on an inductor or capacitor where a charge is transferred from high-charged cells to low-charge cells. Therefore, it does not dissipate electric charge as heat energy which further requires less cooling mechanism. However active cell balancing is complex and costly to implement.
Passive Cell Balancing uses a resistor to discharge the more charged cells’ energy to bring them back to other lower-charged cells. And hence energy is dissipated as heat which requires a complex cooling mechanism. However, this method is less complex and easy to implement. 
Thermal Management
Heat is generated in the battery pack of an EV because of internal resistance and chemical reactions involved inside the cell. The heat generated needs to be removed from the battery to the safe operation of the EV and to avoid thermal run and fast degradation of the battery pack.
Thermal Management can also be done actively and passively. Li-ion battery packs work optimally in the temperature range of 15℃ to 35℃ so BMS starts the thermal management mechanism if temperature exceeds this limit. It is generally recommended that thermal management does not exceed 20% of the weight of the whole battery pack.
Based on the cell chemistry, and battery pack complexities the following types of cooling strategies are used in Battery Thermal Management Systems (BTMS): Air-based, Liquid-based, Nanofluid-based, Phase change material (PCM)-based, Thermoelectric-based, and Hybrid BTMS which integrates passive and active methods to improve the passive systems’ heat transfer process and to allow their working processes to be actively controlled.
Fault Detection and Protection:
EV batteries have a very dynamic electrical load (motor) that is connected which gives a high probability of faults occurring like overcurrent, deep-discharge, short circuit, cell unbalancing, and so on. So a safe operating area (SOA) is defined in the BMS at the cell as well as the battery level for voltage, current, and temperature. If any of these limits are violated then BMS turns off the contactor (switch) and thus disconnects the battery pack from the rest of the electronic components. This way BMS protects the battery pack and other connected electronic components.
Communication
The BMS usually has to communicate with the Vehicle Control Unit (VCU) to provide status information and receive instructions and parameters. Based on the battery charge status, VCU determines the motor performance (torque) and other accessories like multimedia, and cabin temperature control. Usually the Controller Area Network (CAN) interface is used to transmit messages between VCU and BMS.
Data and Event Logging
BMS also records any mishappening like overcharging, deep discharge, balancing failure, and garbage values in battery state estimation. This helps to develop more robust adaptive algorithms in the future. This also helps in providing warranty security and developing new business models.
Hardware requirement in BMS
Electric vehicle battery pack of thousands of cells connected in series-parallel configuration based on the requirement of current, voltage, and capacity of the battery pack. If cells are connected in series then the voltage is added while if cells are connected in parallel then the current and capacity of the battery pack are.
To use these all cells in safe optimal use, BMS requires the following hardware components:
Sensors
BMS uses a current sensor, voltage sensor, temperature sensor (thermistor), and pressure sensor (Piezoresistive Sensor). These sensors collect the cell level data (battery pack level data in the case of the current sensor) and send it to the slave board (analog front end, AFE).
- Usually, one voltage sensor is used for each cell block (cells connected in parallel, total number depends on the total number of cells in parallel)
- Only one current sensor is used to measure the entire battery pack current, not used at the cell level.
- Random numbers of thermistors are placed in the battery pack at different locations.
Contactor (Relay)
A contactor is a high-current handling electromechanical switch that controls the flow of current between the battery pack and the external circuit (load). Usually, three numbers of contactors are used in the BMS, Charge Negative, Charge Positive, and Precharge Contactor. The precharge contactor protects the electrical load from high inrush current.
- The contactor can handle high currents.
- Eliminates the problem of spark during switching.
Insulation Monitoring Device (IMD)
IMD continuously monitors the insulation resistance between the battery’s high voltage terminals and ground (chassis of EV). It injects a small, safe AC into the system and measures the resulting voltage drop. Based on this measurement, the IMD calculates the insulation resistance. If the insulation resistance falls below a predetermined safe threshold, the BMS triggers alarms and takes protective actions like Disconnecting the battery.
- Early Detection of Faults and hence enhances safety.
- Checks leakage current and hence reduces the loss of charge
Fuses
In BMS, fuses play a critical role in protecting the system from potential damage caused by overcurrent conditions. A fuse is a sacrificial device that has a thin strip of metal that melts and breaks the circuit when the current exceeds the safe limit.
- Fuse is simple to use and is cost-effective.
- Requires minimal maintenance.
- Fuse provides reliable Overcurrent Protection.
Communication Interface
CAN (Controller Area Network) and SPI (Serial Peripheral Interface) are communication protocols used within BMS. CAN is a robust, multi-master communication protocol enabling data exchange between various electronic control units (ECUs) within a system, including the BMS. It includes CAN Transceiver and CAN Bus (twisted wire).
SPI is a simpler, lower-cost communication protocol typically used for short-distance communication between the BMS microcontroller and slave boards. SPI has hardware for SPI Interface and SPI Bus (four Wires). 
- CAN offers Reliable Data Transmission
- Priority-Based Messaging and is robust to noise
- SPI is cost effective and message transmission is fast.
Master and Slave board
Master and Slave boards are electronic printed circuit boards (PCBs). The master board consists of a Microcontroller, Memory, Power Supply Circuitry, Safety Control Circuit, and Input/Output (I/O) Interfaces.
Slave boards are generally simpler PCBs compared to the master board. They primarily focus on data acquisition. It has mainly Voltage and Temperature Sensing Circuits, Communication Interface (SPI), and Cell Balancing Circuitry.
| Master board | Slave board |
 |  |
- This architecture has scalability, reduces cost of BMS
- Improved Fault Tolerance, and provide redundancy to BMS
Working on Battery Management System
The slave board of BMS collects the cell data (voltage, temperature) using the sensors. The slave board has AFE which processes the received data to the master board using the SPI communication channel. If the voltage and temperature of cells are in the safe operating area (SOA) then the contactors are turned ON and current starts flowing to the motor (load) in case of discharging or to the battery in case of charging.
During charging current flows inside of the battery pack to store the charge while voltage and temperatures are continuously monitored. If there is more deviation of voltage (40mV) between cell blocks (cell connected in parallel) then the cell balancing starts. Also if the cell gets heat while charging then coolant starts flowing to extract heat. 
Also during discharging current flows out of the battery pack to power the electrical load connected to it.
Presently, most automobile companies use passive cell balancing so no cell balancing is done by BMS but thermal management is done if cell temperature rises above the predefined limit (35℃). Also if the cell reaches minimum cut-off voltage then the contactor is turned off to prevent the cell from deep discharge.
In this way, BMS monitors, controls, and diagnostics the EV battery pack.
