Fusing the split charge system

The drawing above is a very basic description of a split charge system based on using a switch, this picture is here to show you where to insert the inline fuses in any split charge system that you make up, the wire running between the batteries will be live all the time, so if that wire was to ever touch earth for any reason (known as a short circuit). the fuse will blow, and disconnect the power from flowing down the wire, without the fuse in the circuit, the wire will have to melt before the power can be disconnected, or in rare cases if the wire is thick enough, the battery could explode.

Another reason to put the fuses in the wires apart from short circuit protection, is overload protection, a classic example is if a cell in the leisure battery were to die.. when they die they short out internally, and thus effectively take 2 volts out of the battery, so the 12 volt battery is now 10 volts, your pumping 12 volts into the battery to charge it, and the battery can only take 10 volts, something will have to give, usually it's the alternator that blows something, but with a fuse in the circuit the 25p fuse will blow instead of the £100 alternator,

Another possibility mentioned a lot one some web sites, is if you flattened the leisure battery overnight, woke up in the morning and needed to run a high powered load like an inverter for some reason, most people would go and start the engine, knowing that in doing so the leisure battery will be connected to the starter battery and share the alternators output, thus re-charging the alternator, now as the alternator is a generator of electricity, it can run electrical loads when the battery can't, so you'd think you can then run the inverter even tho the leisure battery is dead, the alternator that's now feeding power into the battery will supply the inverter, it might do, but only if you have fitted a large enough cable between the 2 batteries to handle the inverter load (could be as high as 250 amps if it's a really big inverter)

If you have fitted a big enough cable, and if you've also fitted a big enough switch that connects the batteries together when the engine is running, then your ok, but not many people run 400 amp wire between the 2 batteries just for the split charge system (remember our old enemy voltage drop, so 200 amp cable won't supple 200 amps safely unless the leisure battery lives right next to the starter battery)

So if your like most people, and have 20 or 30 amps wire between the 2 batteries, then with the fuses inline, they will pop and save the cable and or switch from melting when it tries to pass 250 amps or so down it that the inverter demands.