Electrical Question - Fuses

Generally you select the fuse to protect wiring and/or connected devices. In case of a battery combiner.

If you have 25mm2 cable a 100A fuse will do BUT consider this:

For the sake of the example:

You have connected an inverter with 40mm2 to your second battery (or a winch for that matter) and you draw 140A from the second battery. You start the engine to support the second battery, the redarc will engage and connect the two batteries and your fuse will go instantly because the the flow is too high. Your 100A fuse also protects the redarc. I do not know from the top of my head how much the redarc takes but I think it was around 100A or was it 200 ?? dont remember (I don't use such a device).

What I am saying is that you first have to calculate your maximum load and then choose the cable and the fuse and if your switch (redarc or anything else) does not support the max current you have to make sure to reduce the load. This is bit difficult with a winch though, so for winch connections from both batteries you use 40mm2 or more depending on the winch (well for an electric one of course)...

So a 100A fuse is ok for a 25mm2 cable but is the cable ok for your load ??

I use 35mm2 cable for an 1800W inverter with a 225A fuse and am at the upper limits of that cable but have only a cable length of 30cm.. 40 or 50mm2 is pretty expensive at the moment (with high copper prices) and very clumsy to use.

So if you have only a few low current devices connected like fridges etc. the 100A should be ok for standard batteries.

A large AGM depleated to 50% will take higher surge when connected to the main battery and will also blow your fuse.

good luck
gmd

I wouldnt fuse between the two batteries, buy some shrink insulation and double insulate the cable and forget about a fuse.Mine is the same setup as yours Redarc and all and no fuse

If its a Diesel cruiser you are splitting the batteries on you also have to extend the alternator input cable across to the L/H side battery otherwise you will only be charging the "house" battery.
I use 2 gauge cable between batteries and 8g for the alternator.

Comments:
#1 If that circuit is to be used for cranking/jump starting etc, then its best to have no fuse because of the very high current. If you go the no fuse option, you have to be absolutely certain that there is no way that cable can short to earth. If it were to short, then you'll end up with a fire under the bonnet and a melted battery. Take a look at a 100series TD motor with twin cranking batteries - the cable deliberately is routed away from any metal objects - Toyota run the cable with a heap of insulation over the plastic cowl and use plastic retainers.

#2 If its going to be used purely for charging the aux battery then its best to have short circuit protection. Best short circuit protection is to use fusible links mounted at the battery posts. Rotronics supply them for their kits. Vehicle manufacturers use them . Fuses usually have a higher resistance at high currents, and can corrode with time. The Rotronics links look to be about 12 gauge.

If you are using a Redarc isolator it has provision for a button on the dash that when u hold it in it links the two batteries together for starting.
Or as I have done you can put a relay in that does it automatically.
Dont know about the 105 but on the 100 series I had to lengthen the alternator cable as the r/h battery becomes the second battery.
all the engine electrics come off the l/h one