For those less conversant with the workings of boat electrics, split charging is a method of charging both the engine start battery and domestic battery at the same time but preventing both being discharged when the engine is stopped and the domestic electrics are being used. While the very smallest of boats manage without split charging, any vessel with pretensions of comfort needs a proper system to ensure the engine will start the following morning when away from shore power.

One of the myths of most split charge systems is that the engine start battery is ‘prioritised’ within the system so that it receives the initial charge before the auxiliary battery begins charging. If it worked it would be a pointless exercise, as the engine start battery is almost always fully charged, having only to start the engine when required and is the least used battery on board. This ‘Priority’ system is based on charge voltage levels so that the engine battery is charged alone until the voltage rises to a pre-set level where the relay cut-in and connects both batteries for charging. In practice, using modern alternators, the voltage level rises to the cut-in limit almost immediately so in reality there is no ‘prioritisation’.