Battery management systems are a central component of every lithium-ion battery. They protect the battery, make optimum use of the capacity and offer some comfort functions.
A battery management system has the following functions:
When referring to the safety functions of the BMS, one distinguishes between first-level protection and second-level protection.
First-level protection is a recoverable (reversible) protective device. It monitors the voltages of the individual cells and of the battery pack, the loading and unloading current, as well as the temperature. If one of the parameters is outside of the limit values, the BMS prevents any further charging or discharging.
In contrast to the first-level protection, the second-level protection is an irreversible form of protection. Once triggered, the battery pack can no longer be used by the customer. An example of second-level protection is a fuse.
Cell balance is used in battery packs with more than one cell connected in series to keep the cells and the states of charge in a balanced condition.
In particular, states of charge which arise due to different temperatures and internal resistances should be balanced by the cell balance. It prevents the battery pack trom shutting down prematurely due to under or excess voltage of a cell, whereby the weakest cell determines the performance of the entire system.
In order to determine the capacity of the battery pack, a BMS has a gas gauge system.
The gas gauge system
A bus system is available to the battery pack for reading the data. The most common bus systems are I2C, SMBus, CAN, LIN or RS232, whereby all other buses are also possible.
In low-power systems, it can make sense to integrate the charging unit for the battery pack into the BMS. This has the advantage that the charger in the event of a cell change can also be adjusted, thus always ensuring backwards compatibility to older systems.
In systems in which the battery is the central element, system functions can be transferred to the battery. This means a second form of intelligence (micro controller) in the system can be saved. For example, motor control and battery protection circuitry are unified in the battery. This does away with the control-related micro controller and the double configuration of the power FETs (FET = Field-Effect-Transistors).