Battery management system

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.^[1]^[2]

1 Functions
2 Topologies
3 See also
4 External links
5 References

Functions [edit]

Monitor [edit]

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, coolant intake temperature, coolant 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
Coolant flow: for air or fluid cooled batteries
Current: current in or out of the battery

Electric Vehicle Systems: Energy Recovery [edit]

The BMS will also control the recharging of the battery by redirecting the recovered energy (i.e.- from regenerative braking) back into the battery packs(a pack is typically composed of a few cells).

Computation [edit]

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)
Energy delivered since last charge or charge cycle
Total energy delivered since first use
Total operating time since first use

Communication [edit]

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 ^[3]
Direct wiring
DC-BUS - Serial communication over power-line
Wireless communications

Protection [edit]

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) which is opened if the battery is operated outside its safe operating area^[4]
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, air conditioning or liquid cooling

Optimization [edit]

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.^[5] 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)
Modular charging ^[6]

Topologies [edit]

BMS technology varies in complexity and performance:

Simple passive regulators achieve balancing across batteries or cells by bypassing charging current when the cell's voltage reaches a certain level. The cell voltage is a poor indicator of the cell's SOC (and for certain Lithium chemistries such as LiFePO4 it is no indicator at all), thus, making cell voltages equal using passive regulators does not balance SOC, which is the goal of a BMS. Therefore, such devices, while certainly beneficial, have severe limitations in their effectiveness.
Active regulators intelligently turning on and off a load when appropriate, again to achieve balancing. If only the cell voltage is used as a parameter to enable the active regulators, the same constraints noted above for passive regulators apply.
A complete BMS also reports the state of the battery to a display, and protects the battery.

BMS topologies 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 ^[7]
Modular: a few controllers, each handing a certain number of cells, with communication between the controllers

Centralized BMSs are most economical, least expandable, and are plagued by a multitude of wires. 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.

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.