Battery balancing is a way of improving the capacity and longevity of a battery pack. Battery balancing is necessary for power systems that has more than one battery or batteries with differences in their output capacities or age or state of charge. Battery balancing is a way of accounting for these differences to maximize the life and efficiency of the system.
When multiple batteries are connected together, small differences in the conditions of the different batteries become magnified with each charge and discharge cycle. The weaker batteries become over-stressed during charging, causing them to become even weaker, until they eventually fail and cause a premature failure of the whole battery pack. Battery balancing is a way of compensating for these weaker batteries in the pack by equalizing the charge on all the batteries in the pack, thus extending the life and increasing the efficiency of the pack.
There are always slight differences in the state of charge, self-discharge rate, capacity, impedance, and temperature characteristics of batteries. This is true even if the cells are the same model, same capacities and same manufacturer.
For instance, a battery balance connected across two 12V batteries will turn on to compare the voltage between the two batteries when the charge voltage of 24V increases to more than 27V. It will then draw a current of up to 1A from the battery with the highest voltage. The resulting charge current differential will ensure that all batteries will converge to the same state of charge.
Balancing can be active or passive. In passive balancing, energy is drawn from the most charged cell and dissipated as heat, usually through resistors.
In active balancing, energy is drawn from the most charged cell and transferred to the least charged cells, usually through capacitor-based, inductor-based or DC-DC converters.
Battery balancing can be performed by balancers or by DC-DC converter circuits, 'battery regulators' and by some types of intelligent battery chargers. A separate battery balancer is not a standard requirement anyway.