Many of you Guy's like to fit split charge relays, so I have done a diagram of the way it is fitted.
Charging an additional battery can be quite simple, however some people are concerned about flattening their engine starter battery (vehicle battery) as well as their auxiliary battery. Its quite simple to safely keep batteries isolated from one another when the engine is not running, thus preventing accidental draining of the vehicle battery. This is done by using a relay that is activated from the alternator. This simple circuit is commonly known as a 'split charge relay circuit'
I have coloured the main cable red which is most likely the same as your vehicle, however the other vehicle cables will be a variety of colours. The terminal numbers above are the standard numbers found on automotive relays. Terminals 85 & 86 are the coil, which activates the relay. Terminals 30 & 87 are the switch contact terminals.
Basically, all you need to do is identify the cable which runs from the alternator to the warning light on the dash board. Rather than pull your dash to bits, it may be easier to have a look at the cables on the back of the alternator. You'll find one or two large cables which will be the cables which run back to the battery. You will also find one smaller cable which is the charge indicator cable (no doubt someone will come up with some exceptions)
Now you've identified the charge light cable, its a simple mater of connecting it to the one side of the coil on a 12 volt relay (assuming your vehicle is 12 volts), the other side of the coil is then connected to an earth point (which eventually goes back to the negative terminal on the battery).
I have drawn the above with 30 amp fuses in line with the battery link cable (do make sure the cable between the vehicle and auxiliary battery is capable of handling 30 amps, otherwise you can quite easily create a fire hazard). This size is normally sufficient, however if you have a really dead flat second battery they may blow. Don't increase the fuse size unless your cable and relay are rated for higher currents.
The above is reasonably reliable, however some alternators don't like additional loads being applied to its charge indicator circuit, in which case the alternator not only fails to charge, but also fails to activate the split charge relay. In this situation a voltage sensing relay could be used.
VOLTAGE SENSING RELAY
There is another relay available which is more expensive than the above plain relay, however it consists of some electronics to activate a relay to connect the two batteries together.
The relay input is basically connected to a live cable on the vehicle, preferably the main battery feed. The relay output cables are then connected up to your auxiliary battery (no need to run a cable to the alternator charge indicator wire).
The relay operates by sensing the vehicle voltage, as the voltage to the vehicle battery will rise when the alternator is producing power, when the relay senses an increase in voltage, it activates its contact to allow power to the auxiliary battery.
Split Charge Diode Block
There is now another way to split the charging of your batteries. Instead of using relays, its now possible to use a split charge diode block. The first units produced did use actual diodes, whilst the units worked, they were not efficient due to the voltage drop of around 0.8 - 1.2volts, this doesn't sound a lot, but in terms of charging batteries it is quite a drop. To overcome this voltage drop, the modern units use clever circuitry with mosfets, the result is that the voltage drop is now lowered to around 0.04volts
Split charge diode blocks any problems associated with the above relays and provide a good positive supply to both battery banks, whilst maintaining total isolation between them.
The split charge diode blocks are incredibly easy to install, as shown in the following drawing
