Leisure battery installation and Split Charge circuit.
Part 4- The relay
In order to connect the leisure battery to the charging circuit, we need a relay. A relay is simply a small box containing an electro-magnet and a switch.
When there is no power connected to the electro-magnet, the switch remains open. But as soon as the power is connected, the electro-magnet pulls the switch on.
How does this help us?
Well, one of the pre-requisites of a leisure battery installation is that the leisure battery needs to be connected to the charging circuit when the engine is running to make sure there is enough charge for the days camping, but to disconnect the leisure battery when the engine stops so that all your nice 12v equipment is powered purely from the leisure battery and is not draining your vehicle's main battery.
So, if we take a feed from the alternator charging circuit that goes live when the alternator is charging, and connect it to the electro-magnet of the relay, we can use the relay's switch to connect and disconnect the leisure battery from the charging circuit.