The BMS is used to level the voltage of the
batteries connected in series so that all the
batteries charge at the same value of 2.5Volts, because having more elements in series they
will never charge at the same voltage if a
balancer is not used.
All 6-piece packs must have a BMS. When the voltage of each cell exceeds 2.5V it intervenes and the circuit will bring them back to the voltage of 2.5V. It does not replace the battery charger which
must necessarily be used for charging, where the car alternator is not enough. The charging voltage for a 6 cell battery pack should not exceed 15V, otherwise you risk burning the BMS.