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Battery management system

Battery management system

A Battery Management System (BMS) is any electronicdevice 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.
A BMS may monitor the state of the battery as represented byvarious items, such as:
Voltage: total voltage, voltage of periodic taps, or voltages ofindividual cells
Temperature: average temperature, air intake temperature, airoutput temperature, or temperatures of individual cells
State Of Charge(SOC) or Depth Of Discharge (DOD): to indicate thecharge level of the battery
State Of Health(SOH), a variously-defined measurement of theoverall 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, usingcommunication links such as:
CAN Bus(typical of automotive environments)[2]
Direct wiring
DC-BUS- Serial UART communication over power-line
Serial communications
Wireless communications
A BMS may protect its battery by preventing it from operatingoutside its safe operating area, such as:
Over-voltage (during charging)
Under-voltage (during discharging), especially important for LeadAcid and Li-Ion cells
Over-pressure (NiMH batteries))
The BMS may prevent operation outside the battery's safe operatingarea by:
Including an internal switch (such as a relay or solid statedevice) which is opened if the battery is operated outside its safeoperating area[3]
Requesting the devices to which the battery is connected to reduceor even terminate using the battery.
Actively controlling the environment, such as through heaters, fansor even air conditioning
In order to maximize the battery's capacity, and to preventlocalized under-charging or over-charging, the BMS may activelyensure that all the cells that compose the battery are kept at thesame State Of Charge, through balancing[4] It may do so by:
Wasting energy from the most charged cells, such as by connectingthem to a load (such as through passive regulators)
Shuffling energy from the most charged cells to the least chargedones (balancers)
Reducing the charging current to a sufficiently low level that willnot damage fully charged cells, while less charged cells maycontinue to charge
BMS technology range in complexity and performance
Simple passive regulators across cells bypass charging current whentheir cell's voltage reached a certain level to achievebalancing
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, andprotects the battery
BMS topologies mostly fall in 3 categories:
Centralized: a single controller is connected to the battery cellsthrough a multitude of wires
Distributed: a BMS board is installed at each cell, with just asingle communication cable between the battery and a controller
Modular: few controllers, each handing a certain number of cells,communicate with each other
Centralized BMSs are most economical, least expandable, and areplagued by a multitude of wires. Distributed BMSs are the mostexpensive, simplest to install, and offer the cleanest assembly.Modular BMSs offer a compromise of the features and problems of theother two topologies.

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