The Battery Management System (BMS) for power lithium-ion batteries is a crucial component of the power lithium-ion battery system in electric vehicles. On one hand, it detects, collects, and preliminarily calculates real-time battery status parameters, and controls the on/off state of the power supply circuit based on comparisons between detected values and permissible values. On the other hand, it reports the collected key data to the vehicle controller and receives instructions from the controller, coordinating with other systems on the vehicle. The requirements for the management system often differ depending on the type of lithium-ion battery cell.
A typical lithium-ion battery management system (BMS) topology is mainly divided into two parts: a master control module and a slave control module. Specifically, it consists of a central processing unit (master control module), a data acquisition module, a data detection module, a display unit module, and control components (fuse devices, relays), etc. Typically, the connection between data communication modules is achieved using internal CAN bus technology.
The following discussion focuses on the design scheme of a power lithium battery management system to achieve overcharge protection, over-discharge protection, overcurrent protection, and equalization charging functions for the lithium battery power pack.
1.1 Overcharge protection
For lithium-ion batteries, the maximum voltage of a single cell after charging must not exceed a specified value; otherwise, the electrolyte in the battery will decompose, leading to increased temperature and gas production, thus shortening the battery's lifespan and potentially causing an explosion in severe cases. Therefore, the protection circuit must ensure that overcharging is never prevented and must monitor the terminal voltage of each cell in the battery pack. When the battery voltage exceeds the set value, the overcharge protection function is activated, and the protection circuit cuts off the charging circuit, stopping charging. Protection stops when the battery voltage returns to the permissible voltage and the overcharge lockout mode is released. Different lithium-ion battery materials have different specified protection and release voltage values.
1.2 Over-discharge protection
Over-discharging lithium-ion batteries can shorten their lifespan, and the damage is often irreversible. To prevent over-discharge, over-discharge protection is activated when the battery voltage falls below its over-discharge voltage detection point, stopping the discharge process and allowing the battery to enter a low quiescent current standby mode. Parameter settings are similar to those for the over-discharge voltage detection point. Charging protection is also included.
1.3 Overcurrent/Short Circuit Protection
Lithium-ion batteries have a limited maximum discharge current. Excessive discharge current can cause irreversible damage to the lithium battery and affect its lifespan. Short-circuit protection is essentially an extension of overcurrent protection. If a large current discharge occurs due to an external short circuit or other reasons, the discharge should be stopped immediately; otherwise, it may seriously damage the lithium battery itself and external equipment.
1.4 Battery power balance
Power lithium-ion batteries typically require several, dozens, or even hundreds of cells in series. During production, from the initial film preparation to the finished product, the battery must undergo numerous processes. Even with rigorous inspection procedures ensuring that each battery in a power bank has the same voltage, resistance, and capacity, after a period of use, the internal resistance, voltage, capacity, and other parameters of the battery will fluctuate, resulting in inconsistencies in its state and differences in one way or another. When the battery pack is fully charged or discharged, this difference is reflected in the different voltages between the series-connected battery cells. In this situation, during the charging process, battery cells with excessively high voltage will trigger the overcharge protection of the battery pack prematurely, while during the discharging process, battery cells with low voltage will trigger the over-discharge protection of the battery pack, thus significantly reducing the overall capacity of the battery pack.
