Battery Management Systems
A battery management system is any electronic system or device that manages a rechargeable battery pack by protecting it from operating outside its safe operating area, monitoring its state, calculating and logging secondary data, regulating its environment and balancing. A BMS incorporates all the functions of a battery monitor, a smart charger and a battery balancer. Read about battery balancers here, and battery monitors here.
There are three main objectives common to all Battery Management Systems
They protect the cells or the battery from damage
The help prolong the life of the battery
They maintain the battery in the state in which it can satisfy the functional requirements of the application for which it was specified.
To achieve these objectives the BMS may incorporate one or more of the following functions.
Battery Protection: They protect the battery from out of its specified operating conditions. This is the fundamental work of a BMS. Operating a battery outside of its specified design limits will inevitably lead to failure of the battery and of course, financial loses. This is particularly true for high voltage and high power automotive batteries which must operate in hostile environments and which at the same time are subject to abuse by the user.
Charge Control: They control the charging parameters to prevent damage.
Output Management: They control the current drain on the battery by designing power saving techniques into the applications circuitry and thus prolong the time between battery charges.
State of Charge Determination: Many applications require a knowledge of the State of Charge (SOC) of the battery or of the individual cells in the battery pack. This may simply be for providing the user with an indication of the capacity left in the battery, or it could be needed to ensure optimum control of the charging process.
State of Health Determination: The State of Health (SOH) is a measure of a battery's capability to deliver its specified output. This is helpful in assessing the readiness of emergency power equipment and whether maintenance actions are needed.
Battery Balancing: When multiple batteries are connected together, small differences in in the conditions of the different batteries become magnified with each charge and discharge cycle. The weaker batteries become over-stressed during charging, causing them to become even weaker, until they eventually fail and cause a premature failure of the whole battery pack. Battery balancing is a way of compensating for these weaker batteries in the pack by equalizing the charge on all the batteries in the pack, thus extending the life and increasing the efficiency of the pack. Read more here.
Information Logging: The BMS monitors and stores the battery's history. This is needed in order to estimate the State of Health of the battery, but also to determine whether it has been subject to abuse. Parameters such as number of cycles, maximum and minimum voltages and temperatures and maximum charging and discharging currents can be recorded for subsequent evaluation. This can be an important tool in assessing warranty claims in case of damage.
BMS also allows the possibility to record the manufacturer's type designation and the cell chemistry, serial number to enable traceability in case of cell failures.
Communication: Most BMS systems incorporate some form of communications between the battery and the charger or test equipment. Some have links to other systems interfacing with the battery for monitoring its condition or its history. Communications interfaces are also needed to allow the user access to the battery for modifying the BMS control parameters or for diagnostics and test.
While laptops come with an inbuilt BMS, a BMS for inverter batteries has to be acquired separately. Most BMS are sold as 'smart chargers'.