Ask ten buyers what a BMS does and nine will say "it protects the battery." That is true in the same way that saying a pilot "keeps the plane in the air" is true. It hides everything that matters. A lithium-ion cell has no self-preservation instinct. Push it above roughly 4.25 V and the electrolyte begins to oxidise at the cathode. Pull it below about 2.5 V and the copper current collector starts to dissolve, then re-plates as dendrites on the next charge. Charge it below 0 °