In order to choose the best BMS for your lithium battery, you will need to know a little bit about the functions that a BMS provides. Lithium-ion batteries do not require a BMS to operate. With that being said, a lithium-ion battery pack should neverbe used without a BMS. The BMS is what prevents your battery cells from being drained or charged too much. Another important role of the BMS is to. Lithium-ion battery packs are composed of many lithium-ion cells in a complex series and parallel arrangement. Many cells are needed when. Well, that is actually a rather broad question with no single answer. When it comes to picking the best BMS, the brand is not super. When someone refers to the 'size' of a BMS, they are generally referring to the maximum amount of current the BMS can handle. You need to make sure to get a BMS that can support the amount of power that is required by your load. In fact, it's a good practice to add.