All batteries of a type are not created equal….each has a slightly different capacity, especially when new. The best way to make sure that each battery is charged to its maximum would be to charge them individually. Unfortunately, they are connected in series to power the motor and therefore must be charged in series. Consequently, one battery will reach full charge before the rest and as the charger treats the whole series chain as one entity, it is inevitable that the first batteries to reach full charge will be overcharged (boiling) before the charger cuts back the current. To avoid this we equip each battery with a regulator that monitors the state of charge and bypasses the current when full charge is reached on any one battery. All regulators communicate with the charger and thus the charger “knows” when the whole string has reached full charge.