A battery management system (BMS) is any electronic system that manages a rechargeable battery (cell or battery pack), such as by monitoring its state, calculating secondary data, reporting that data, protecting the battery, controlling its environment, and / or balancing it.
A BMS may monitor the state of the battery as represented by various items, such as:
Voltage: total voltage, voltage of periodic taps, or voltages of individual cells
Temperature: average temperature, or temperatures of individual cells
State of charge (SOC) or depth of discharge (DOD): to indicate the charge level of the battery
State of health (SOH), a variously-defined measurement of the overall condition of the battery
Current: current in or out of the battery
A BMS may report all the above data to an external device, using communication links such as:
Serial communications. A CAN bus is one particular implementation of a serial link most commonly used in automotive environments
DC-BUS - Serial communication over power-line
A BMS may protect its battery by preventing it from operating outside its safe operating area, such as:
Over-voltage (during charging)
Under-voltage (during discharging), especially important for lead–acid and Li-ion cells
In order to maximize the battery's capacity, and to prevent localized under-charging or over-charging, the BMS may actively ensure that all the cells that compose the battery are kept at the same State Of Charge, through balancing. The BMS can balance the cells by:
Wasting energy from the most charged cells by connecting them to a load (such as through passive regulators)
Shuffling energy from the most charged cells to the least charged cells (balancers)
Reducing the charging current to a sufficiently low level that will not damage fully charged cells, while less charged cells may continue to charge (does not apply to Lithium chemistry cells)
The requirements for a BMS in mobile applications (such as electric vehicles) and stationary applications (like stand-by UPSs in a server room) are quite different, especially from the space and weight constraint requirements, so the hardware and software implementations must be tailored to the specific use. In the case of electric or hybrid vehicles, the BMS is only a subsystem and cannot work as a standalone device. It must communicate with at least a charger (or charging infrastructure), a load, thermal management and emergency shutdown subsystems. Therefore, in a good vehicle design the BMS is tightly integrated with those subsystems. Some small mobile applications (such as medical equipment carts, motorized wheelchairs, scooters, and fork lifts) often have external charging hardware, however the on-board BMS must still have tight design integration with the external charger.
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