A Battery Management System (BMS) is any electronic circuit that manages a rechargeable battery (cell or battery pack), such as by monitoring its state, calculating secondary data, reporting that data, protecting it, controlling its environment, and/or balancing it.

Functions of Battery Management System (BMS)

Battery Monitoring

Battery Management System monitors the state of the battery as represented by various items, such as:

  • Battery Voltage: total voltage, voltage of periodic taps, or voltages of individual cells
  • Battery Temperature: average temperature, air intake temperature, air output temperature, or temperatures of individual cells
  • Battery State Of Charge (SOC) or Depth Of Discharge (DOD): to indicate the charge level of the battery
  • Battery State Of Health (SOH), a variously-defined measurement of the overall condition of the battery
  • Air flow to Battery: for air cooled batteries
  • Battery Current: current in or out of the battery

Battery Computations

Battery Management System calculate values based on the above items, such as:

  • Maximum charge current as a Charge Current Limit (CCL)
  • Maximum discharge current as a Discharge Current Limit (DCL)
  • Total energy delivered since manufacture
  • Total operating time since manufacture

Battery Communication

Battery Management System report all the above data to an external device, using communication links such as:

  • CAN Bus (typical of automotive environments)
  • Direct wiring
  • DC-BUS – Serial UART communication over power-line
  • Serial communications
  • Wireless communications

Battery Protection

Battery Management System protects its battery by preventing it from operating outside its safe operating area, such as:

  • Over-current protection
  • Over-voltage (during charging) protection
  • Under-voltage (during discharging), especially important for Lead Acid and Li-Ion cells
  • Over-temperature of battery
  • Under-temperature of battery
  • Over-pressure (NiMH batteries)

Also, Battery Management System prevents operation outside the battery’s safe operating area by:

  • Including an internal switch (such as a relay or solid state device) which is opened if the battery is operated outside its safe operating area
  • Requesting the devices to which the battery is connected to reduce or even terminate using the battery.
  • Actively controlling the environment, such as through heaters, fans or even air conditioning

Battery Optimization

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. It may do so by:

  • Wasting energy from the most charged cells, such as by connecting them to a load (such as through passive regulators)
  • Shuffling energy from the most charged cells to the least charged ones (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