Individual batteries, even of the same model, can have slight variations in performance. Over time, the charge levels of the batteries can grow more disparate, with the result that some batteries may take longer to charge than others. This imbalance will eventually damage the batteries and greatly shorten their cycle life, as some batteries get overcharged while others lag behind.
In an EV, a battery management system (BMS) monitors the charge level for each battery in your battery pack. The BMS consists of a network of small regulator units, one on each battery. When a particular battery is fully charged, the regulator cuts off the battery from the charging circuit and bypasses it, preventing overcharging while allowing others in the pack to continue charging.
With many battery types, such as lithium ion or nickel cadmium, a BMS is absolutely required, since overcharging can result in a fire. Though optional with flooded lead-acid batteries, battery management systems will help extend battery life while reducing how frequently you will need to water your batteries. Plus, they help keep the battery pack cleaner—given that overcharging leads to excessive gassing of flooded batteries, which causes some electrolyte to escape and coat the battery tops.