The  traditional split charging system is a way of connecting a second "leisure" battery in a motorhome so that it charges when the engine is running but is disconnected from the vehicle system when the engine is turned off. This is done to ensure that should you fully discharge your leisure battery that you do not at the same time discharge your engine starter battery.

The device that connects  the leisure battery for charging is a relay which is an  electromagnetic switch. 

All modern vehicles have alternators for charging. The alternator has 3 basic connections

1)  The frame connection that connects via its fixings to the chassis of the vehicle. Likewise does the negative poles of both the starter and the leisure batteries.

2) The D+ terminal. This terminal is normally connected to one side of the ignition warning lamp in the vehicle, the other side of the warning lamp being connected to the starter battery positive terminal via the ignition key switch.  When the key switch is initially turned ON before starting the engine some small amps will flow through the key switch, through the bulb and via the D+ terminal of the alternator through the  slip rings and rotor winding so that the alternator can initially excite. At this time the voltage on the D+ terminal is close to zero. As soon as the engine starts and the alternator starts to charge the voltage at the D+ terminal instantly goes to 12 or more volts. In a split charging system we use the voltage at D+ to energise a relay.

The relay contacts then connect the output voltage of the alternator at B+ to the leisure battery positive terminal.

In most cases  this circuit is duplicated in a motor home that has a fridge for its 12 volt mobile operation.

The size of some of  the wiring of the leisure battery is quite important.  The wires associated with the COIL of the relay on terminals  85 and 86 is not critical, 0.75 mm sq is quite sufficient for mechanical strength. However the wiring associated with the main charging path to terminals 30 and 87 needs to be a thick as one can sensibly use. 10mm sq is not unusual, in some applications where the battery is some way away from the alternator even thicker. Similarly for the leisure battery negative cable.

As always, use quality fuses chosen to protect the size of cables you are using.














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