What is a Battery Management System (BMS)?

BMS means different things to different people. To some it is simply Battery Monitoring, keeping a check on the key operational parameters during charging and discharging such as voltages and currents and the battery internal and ambient temperature. The monitoring circuits would normally provide inputs to protection devices which would generate alarms or disconnect the battery from the load or charger should any of the parameters become out of limits.

For the power or plant engineer responsible for standby power who's battery is the last line of defence against a power blackout or a telecommunications network outage BMS means Battery Management Systems. Such systems encompass not only the monitoring and protection of the battery but also methods for keeping it ready to deliver full power when called upon and methods for prolonging its life. This includes everything from controlling the charging regime to planned maintenance.

For the automotive engineer the Battery Management System is a component of a much more complex fast acting Energy Management System and must interface with other on board systems such as engine management, climate controls, communications and safety systems.

There are thus many varieties of BMS.

How The BMS Works

The Orion Battery Management System (BMS) performs three primary functions:

1. It protects the battery pack from being over-charged (cell voltages going too high) or over-discharged (cell voltages going too low) thereby extending the life of the battery pack. It does this by constantly monitoring every cell in the battery pack and calculating exactly how much current can safely go in (source, charge) and come out (load, discharge) of the battery pack without damaging it. These calculated current limits are then sent to the source (typically a battery charger) and load (motor controller, power inverter, etc), which are responsible for respecting these limits.

2. It calculates the State of Charge (the amount of energy remaining in the battery) by tracking how much energy goes in and out of the battery pack and by monitoring cell voltages. This value can be thought of as a fuel gauge indicating how much battery power is left in the pack.

3. It monitors the health and safety of the battery pack by constantly checking for shorts, loose connections,  breakdowns in wire insulation, and weak or defective battery cells that need to be replaced.

There are also secondary functions that the BMS performs:

1. Balances all the cells in the battery pack by intelligently bleeding off excess energy from cells that are charged more than others. This provides the maximum amount of usable energy (capacity) from the battery pack since the pack is only as strong as the weakest cell.

2. Monitors the temperature of the battery pack and controls a battery fan to regulate the temperature of the pack. Additionally, it constantly monitors the output of the fan to make sure it is working properly.

3. Provides real-time information and values to other devices such as motor controllers, chargers, displays and data loggers using several different methods (CANBUS, analog outputs, and digital outputs).

4. Stores error codes and comprehensive diagnostic information to aide in fixing problems with the battery pack should any issues arise.

The BMS is part of a multi level safety system with the following objectives and safeguards

Intrinsically Safe cell chemistry

Cell technial design audit

Cell Supplier and Production Audit

Technical competence of staff

Process controls (Installed and working)

Cell level (internal) safety devices

Circuit Interrupt Device (CID) Cuts circuit if internal pressure limits exceeded

Shut down separator

Pressure vent

External circuit devices

PTC resistors (Low power applications only)

Fuses

Cell and battery isolation. Electrical and mechanical separation (Contactors and physical separation) to prevent event propagation

BMS Software

Monitoring of all key indicators coupled to control actions. (Cooling, Power disconnect, Load management)

Control actions or switch off in case of out of limits condition

BMS Hardware - Fail safe back-up

Hardware switch off in case of software failure. Set to slightly higher limits

Battery switch off in case low voltage BMS power supply fails

Containment

Robust outer container with controlled venting

Physical barriers between cells

Conclusions

The BMS is needed to measure the state of charge and condition of the Lithium battery due to their charge/discharge characteristics and safety considerations. It performs several key functions to ensure that the pack capacity can be maximised such as cell balancing and the control of chargers.Some battery company may think it’s not necessary to build a bms inside.But actually, it’ s a key part of battery pack. The lithium-ion battery BMS management system is a powerful guarantee for the safe use of the battery, so that the battery is always in a safe and controllable charge and discharge process, which greatly improves the cycle life of the battery during actual use.