Battery Management System

A Battery Management System (BMS) is any electronic device that manages a rechargeable battery (cell or battery pack), such as by monitoring its state, calculating secondary data, reporting those data, protecting it, 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, air intake temperature, air output 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
* Air flow: for air cooled batteries
* Current: current in or out of the battery

Additionally, a BMS may 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

A BMS may report all the above data to an external device, using communication links such as:
* Can Bus (typical of automotive environments)
* Direct wiring
* Serial communications
* Wireless communications

A BMS may protect its battery by preventing it from operating outside its safe operating area, such as:
* Over-current
* Over-voltage (during charging)
* Under-voltage (during discharging), especially important for Lead Acid and Li-Ion cells
* Over-temperature
* Under-temperature
* Over-pressure (NiMH batteries))

The BMS may prevent operation outside the battery's safe operating area by:
* Including an internal switch (such as a relay or solid state device) with is opened if the battery is operated outside its safe operating area
* Requesting the devices to which the battery is connected to reduce its use of the battery or even terminate it
* Actively controlling the environment, such as through heaters, fans or even air conditioning

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. 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

BMS technology range in complexity and performance
* Simple passive regulators across cells bypass charging current when their cell's voltage reached a certain level, to achieve balancing
* Active regulators intelligently turn on a load when appropriate, again to achieve balancing
* A full BMS reports the state of the battery to a display, and protects the battery

BMS topologies mostly fall in 3 categories:
* Centralized: a single controller is connected to the battery cells through a multitude of wires
* Distributed: a BMS board is installed at each cell, with just a single communication cable between the battery and a controller
* Modular: few controllers, each handing a certain number of cells, communicate with each otherCentralized BMSs are most economical, least expandable, and are plagued by a multitude of wires (spaghetti)Distributed BMSs are the most expensive, simplest to install, and offer the cleanest assemblyModular BMSs offer a compromise of the features and problems of the other two topologies.

See also

* Battery balancer
* Battery monitoring
* Battery charger

External links

* [http://www.cameronsoftware.com/ev/EV_BatteryManagementSystems.html Battery Management Systems]
* [http://www.nrel.gov/vehiclesandfuels/energystorage/pdfs/3a_2002_01_1918.pdf A Modular Battery Management System for HEVs]
* [http://www.metricmind.com/ac_honda/bms.htm The battery management system]
* [http://enviromotors.com/wiki/index.php/Sparrow/BMS Sparrow BMS]
* [http://liionbms.com/php/aboutbms.php info on BMS] for Li-Ion batteries
* [http://btechinc.com/btech-products.shtml info on BMS] for Lead-Acid batteries
* [http://www.claytonpower.com/products/bms/ Battery Management Systems - Cell balancing]