Li-Ion BMS

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Battery Management Systems

A BMS monitors and protects cells in a battery

The primary job of a BMS is to protect the battery (by preventing operation of any cell outside its safe operating area)

A secondary job may be to maximize the battery capacity (by balancing the battery's SOC).

Battery Management Systems (BMS) are used to manage a battery pack, such as by:
• Monitoring its state
• Calculating secondary data
• Reporting those data, protecting it
• Controlling its environment
• 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
• Current: current in or out of the battery
• Temperature: overall pack temperature, air intake or exhaust temperature, or individual cell temperatures
• Environmental conditions: e.g.: air flow in air cooled batteries
Additionally, a BMS may calculate values based on the above items, such as:
• State Of Charge (SOC) or Depth Of Discharge (DOD): to indicate the charge level of the battery
• Sate Of Health (SOH), a variously-defined measurement of the overall condition of the battery
• Maximum charge current as a Charge Current Limit (CCL)
• Maximum discharge current as a Discharge Current Limit (DCL)
• Resistance: dynamic resistance for entire pack or individual cells
• 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 or under-temperature
• Over-pressure (typical of NiMH batteries))
The BMS may prevent operation outside the battery's safe operating area by:
• Requesting the devices to which the battery is connected to reduce its use of the battery or even terminate it
• 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
• 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 through a dummy load (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 dissipative ("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 Cell 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 other
• Centralized 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 assembly
• Modular BMSs offer a compromise of the features and problems of the other two topologies

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