Fuse on Dual Battery Setup?

Marc, you are correct.
A fuse should be at each positive terminal to protect the cable from both sources of power.

I have never put a fuse on cable linking batteries. just use quality double insulated cable or put cable inside conduit.

Absolutely correct IF you are goimng to have any fuses at all between the primary and secondary batteries, you should have them both ends.

NOW here comes the issue........how heavy is the cable and how much current do you expect to pass between the batteries.

Most decent voltage sensitive relays will be good for around 100 amps ( give or take).
There is a whole other argument about this and current limiting, but lets not get into that for now.

So if you have anything other than a DC to DC charger type dual battery controller that limits the charging current.
You need to be looking at a fuse or breaker in the order of 100 - 120 amps.
Persoanlly Id be looking at a circuit breaker and one specifically designed for DC.

AH...NO then if you have wired your dual battery in serioulsy heavy cable and expect to crank or winch using the second battery, fuses and breakers are pretty well pointless.

Satarter motors draw in the vacinity of 150 to 200 amps, winches draw around 400 amps at stall.
That is a lot of current and there are two issues.

Firstly getting a fuse or breaker that will function effectivley and safely at that current and for DC......let me tell you I don't want to be around when a 400 amp fuse blows...even if it is a high rupture cartridge fuse.
BTW a fuse or breaker that will be effective at that current for DC will be large and EXpensive.

Secondly a partial short below the fusing value can be very distructive.....lets asy a 200 amp psrtial short with a 400 amp circuit breaker will still blow holes is metal body panels produce heaps of sparks and very likely start a fire.

The above two reasons are why almost without exception starter circuits are not fused.

SO with ALL battery cables we have to rely primarily on the proper install and mechanical protection of the cable.

cheers

Depends on what you are using your batteries for and whether you connect them it in a professional manner.
Under the bonnet I have two 100 AH marine batteries directly linked – no fuses - no dual battery controller.
They both start the vehicle, run the winch, and do the deep cycle work – fridges are fused and run through a low voltage cut-off relay.

Marc,

Due to limited space under the bonnet, I installed large auxillary battery in the gull wing canopy on the ute. Ran double insulated wire pair (about 8mm diameter not gauge) between the two batteries. Installed 100amp fuses at either end of the positive wire. Used a fuse holder that allowed me to solder the ends of the 8mm diameter wire and then insert them into a barrel connection (no thin wires for current loss). Works well, purchased fuse holders and fuses at Autobarn.
If installing under the bonnet I would still have a fuse between the two batteries - peace of mind, however, there are a variety of views on this.

Cheers,
Greg

New vehicles such as the 100 and 200series TD Landcruisers had twin cranking batteries joined by a thick cable with no fuses. But if you look at the insulation and the routing of the cable (over the plastic cowl and well away from any metal) then you'd feel comfortable that it's unlikely to short circuit. And you'd feel comfortable that in a crash, it might be safe.

But most dual battery installations I've seen need a fuse/CB/fusible link at each battery because the routing and insulation is not that good. If the second battery is to be used for cranking, then you won't want a fuse, but for the usual isolated aux battery, I prefer the safety of a fuse at each battery. I solder a maxifuse into the wiring next to the pos terminal. Seen too many fuseholders rust out in that location.

Having had a battery short circuit and catch fire (when a bonnet came down on an uninsulated pos terminal) I suggest this not be taken lightly.