Does a lithium battery need a management system (BMS)?

Connecting several lithium batteries in series can form a battery pack, which can power various loads and can also be charged normally with a matching charger. Lithium batteries do not need any battery management system (BMS) to discharge. So why are all lithium batteries on the market equipped with BMS?

The answer is: safety and life.

The battery management system BMS (Battery Mangement System) is used to monitor and control the charging and discharging of rechargeable batteries. The most important function of the lithium battery management system BMS is to ensure that the battery remains within the safe operating range and take immediate action when any individual battery begins to exceed the limit. If the BMS monitors that the voltage is too low, it will disconnect the load and if the voltage is too high, it will disconnect the charger. It will also check the voltage or low voltage of each cell in the battery pack - which is often the reason for the lithium battery fires we see in the news. It can even monitor the temperature of the battery and disconnect the battery pack before it gets too hot and catches fire. So, the battery management system BMS is to protect the battery, rather than relying purely on a good charger or correct user operation.

Why don't lead-acid batteries (AGM, GEL, deep cycle, etc.) need a battery management system? The composition of lead-acid batteries is less flammable, and they are much less likely to catch fire if there is a problem with charging or discharging. The main reason has to do with the behavior of the battery after it is fully charged. Lead-acid batteries are also made of cells in series. If one cell is charged slightly more than the others, it will only let current through until the other cells are fully charged, while maintaining a reasonable voltage and waiting for the other cells to catch up. In this way, the lead-acid battery will "self-balance" when charging.

Lithium batteries are different. The positive electrode of a rechargeable lithium battery is mostly lithium-ion material. Its working principle determines that during the charge and discharge process, lithium electrons will run between the positive and negative electrodes again and again. If the voltage of a single cell is allowed to exceed 4.25V (except for high-voltage lithium batteries), the anode microporous structure may collapse. Hard crystalline materials may grow and cause a short circuit, and then the temperature will rise rapidly, and finally cause a fire. When a lithium cell is fully charged, the voltage will suddenly increase and can quickly reach dangerous levels. If the voltage of a cell in a battery pack is higher than that of other cells, this cell will reach the dangerous voltage first during the charging process, while the overall voltage of the battery pack has not yet reached the full value, and the charger will not stop charging. Therefore, the cell that reaches the dangerous voltage first will cause safety risks. Therefore, controlling and monitoring the total voltage of the battery pack is not enough for lithium-based chemicals, and the BMS must check the voltage of each individual cell that makes up the battery pack.

In a narrow sense, the battery management system BMS is used for the protection of large battery packs. The typical use is lithium iron phosphate power batteries. It has overcharge, over-discharge, overcurrent, short circuit, cell balance and other protection functions. Some batteries may also require communication ports and data input and output and other display functions.

In a broad sense, the protection circuit board PCB (Protection Circuit Board), sometimes also called PCM (Protection Circuit Module), is a simple battery management system BMS. It is usually used for small battery packs. It is typically used for digital batteries. Most of the time, it is used for 3.7V or 7.4V battery packs. It has four basic functions: overcharge, over-discharge, overcurrent, and short circuit. Some batteries may also require PTC and NTC.

Therefore, to ensure the safety and long service life of lithium battery packs, a reliable battery management system BMS is indeed required.