battery management system (BMS)

A battery management system is the electronic brain and bodyguard of a multi-cell pack. In a 100-cell EV battery the cells are never perfectly identical — one always charges a hair faster than its neighbours — and a Li-ion cell that climbs above ~4.25 V or falls below ~2.5 V degrades or ignites. The BMS watches every cell's voltage, the pack's current, and a scattering of temperatures, then steps in: it cuts off charging when the weakest cell is full, blocks discharge when one cell runs dry, and tripping a contactor if anything overheats.

Its most underrated job is cell balancing. Because a series string can only charge until its highest cell is full, a single overfilled cell would strand capacity in all the others; the BMS bleeds off (passive balancing) or shuttles (active balancing) charge to keep cells in step, squeezing out usable range. It also estimates state of charge and state of health, talks to the charger over a bus like CAN, and logs faults — making it the difference between a safe pack and a fire.