Battery management system

A battery management system (BMS) is any electronic system that manages a rechargeable battery (cell or battery pack), such as by protecting the battery from operating outside its Safe Operating Area, monitoring its state, calculating secondary data, reporting that data, controlling its environment, authenticating it and / or balancing it.[1]

A battery pack built together with a battery management system with an external communication data bus is a smart battery pack. A smart battery pack must be charged by a smart battery charger.

Functions

Monitor

A BMS may monitor the state of the battery as represented by various items, such as:

Electric Vehicle Systems: Energy Recovery

Computation

Additionally, a BMS may calculate values based on the above items, such as:

Communication

A BMS may report all the above data to an external device, using communication links such as:

Internally a BMS might use:

via these communication busses:

Protection

A BMS may protect its battery by preventing it from operating outside its safe operating area, such as:

The BMS may prevent operation outside the battery's safe operating area by:

Battery connection to load circuit

A BMS may also feature a precharge system allowing a safe way to connect the battery to different loads and eliminating the excessive inrush currents to load capacitors.

The connection to loads is normally controlled through electromagnetic relays called contactors. The precharge circuit can be either power resistors connected in series with the loads until the capacitors are charged. Alternatively, a switched mode power supply connected in parallel to loads can be used to charge the voltage of the load circuit up to a level close enough to battery voltage in order to allow closing the contactors between battery and load circuit. A BMS may have a circuit that can check whether a relay is already closed before precharging (due to welding for example) to prevent inrush currents to occur.

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 voltage or State of Charge, through balancing. The BMS can balance the cells by:

Topologies

BMS technology varies in complexity and performance:

BMS topologies fall in 3 categories:

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.

Various Battery balancing methods are in use, some of them based on state of charge theory.

See also

External links


References

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