A typical 24 volt vehicle uses two 12 volt series connected batteries. Over time, the two batteries can get out of balance; meaning that their respective charge levels are different. When this happens, the alternator is unable to restore the charge levels equally. After repeated cycling, the situation gets worse until the weaker battery eventually fails.

A common reason for imbalance is "stack tapping" where 12 volt loads are connected to the centre point of the pair of batteries. Less common reasons for imbalance include manufacturing differences, natural differential ageing, or the simple act of changing out one of the pair. The stack tapping effect is particularly marked and rapidly leads to failure of the 12 volt battery being cycled.

Nevertheless, stack tapping can be very convenient, particularly if the 12 volt load demands an infrequent, but high current. To solve the stack tapping problem you can:

• Periodically swap the batteries (time consuming and not always effective).
• Use overnight AC charging using a balancing 24V charger, or two 12 volt chargers (one across each battery); effective only if carried out regularly.
• Use an on-board DC DC battery equaliser (this will also solve the natural ageing effect).