BMS Protection Functions for Lithium Battery Pack

We all know lithium battery must with BMS to achieve the protection function. What detail protection function it can do? How does it work? See Bonnen engineer summary which typically applied in EV battery pack as below.

The battery management system protects the battery by request to reduce the working current, allowing the load control intelligent module to adjust the output, or cutting off the charging and discharging path, so as to prevent the battery from exceeding the permitted use conditions. The protection function is usually implemented in the following situations.

Overvoltage Protection

The voltage of a single cell in the battery pack exceeds the allowable voltage. According to the purpose of protection, the battery is only allowed to discharge and the charging relay is disconnected. Generally, the BMS will set some warning voltages within the allowable voltage. When the battery reaches this voltage, the BMS will issue a request to reduce the charging current. What needs to be clear is that overvoltage protection and overcharge protection are two different things. If overvoltage protection is effectively implemented, the battery will not be overcharged. If overcharge protection is effective, safety accidents such as thermal runaway can be avoided

Undervoltage Protection

Overcharge can cause damage and safety problems to the battery, accounting for more than 60% of safety accidents, and over-discharge can also cause damage to the battery. Under-voltage protection also sets some voltage values, below which BMS requires reducing the electric current or cutting off the discharge path.

Overheat Protection

The principle of overheating protection is to try to keep the battery below 45℃ to avoid rapid aging. But when the ambient temperature is high in summer, the temperature rise during normal use will exceed 45°C, then there will be a temperature value that prevents excessive damage on the spot. If the two protection temperatures are exceeded, and the battery must be used, and the temperature of the battery rises normally, set a safety protection temperature. If the temperature is higher than this temperature, safety accidents are likely to occur. Of course, this safe temperature cannot be exceeded, and I would rather not use a battery than let a safety accident happen.

Low-temperature Protection 

At low temperatures, the capacity decreases, the activity decreases, and the usable charge-discharge rate decreases, so the battery’s charge-discharge rate must be protected. If the temperature drops to a lower temperature, the battery cannot be charged and discharged normally, and it cannot support the normal functions of the electrical equipment. Forced use will cause the battery to be damaged on the spot. At this time, the charging and discharging circuit will be cut off and the battery will be prohibited.

Over-current Protection

Excessive current will undoubtedly cause irreversible damage to the battery. Experiments show that the discharge rate is increased to 3C, and the cycle of life is reduced to 1/3 compared to 1C. Overcurrent protection will set 3 current values. When the output current reaches the first current value, the BMS will request no more current. If the request is not met, the current continues to increase and reaches the second current value. BMS will request to reduce the use of currently. However, if all these requests are not met, the current continues to increase to the third protection current value, and the BMS will actively cut off the corresponding discharge or charging path.

It should be noted that when BMS sends a request, it needs to allow sufficient response time for subsequent processing intelligence. What needs more attention is that the BMS’s current overcurrent protection strategy must be coordinated with the vehicle controller and the load intelligent controller. Uncoordinated strategies will produce undesirable phenomena in which they work normally but fail frequently.

Some of the phenomena of excessive current are caused by short circuits. The identification of short circuits is that after cutting off the protection of the path, the power can no longer be turned on. As long as the power is turned on, excessive current will occur. At this time, each power branch can be inspected jointly with the high-voltage distribution box of the vehicle controller to determine which load circuit has a short circuit.

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