Thread: Cable size for connecting main and auxiliary batteries

I'd suggest drawing up a list of what you are likely to run OFF the battery, as well as what current is going to go into it to charge it.
For example, that 10A wiring you have is essentially a fuse and is going to vapourize instantly if it needs to draw any significant current out of it (a winch for example). That 10A circuit breaker will probably melt too!

If you were connecting battery to battery and they are beside each other, I'd throw it out there as an uneducated guess that you'd need at least 3 gauge (25mm^2). If you have a longer cable run, and you want minimal voltage drop, use a voltage drop calculator to crunch the sums.

Hi Mick, I will try to give you the info you are after but a few more details if you could?

Is the battery in the rear your only auxiliary battery.

Have you selected the auxiliary battery yet?

Are you planning to run electric winch off both batteries?

Thanks for your replies. More info:
The temporary 10 amp is certainly too small for anything permanent. The only thing its currently doing is running some charge from the Traxide to the small battery in the Ark box. I occasionally carry an Engel 40 litre fridge in the back and plug it into the battery box. I deferred making a permanent arrangement until the drawers were installed.
In the future the plan is to hopefully be able to fit an AGM (or maybe GEL) of around 100 amps into a compartment at the rear of the drawers. The cable I'm asking about would solely connect it to the Traxide. No accessories would run off this cable. I want to use a nice highly rated cable so plenty of current can flow from the starting battery through it to the auxiliary battery.
The winch comes off the starting battery and that won't change.
From the auxiliary battery I will want to be able to power the Engel or another fridge, some lights and some devices through USB or other plugs. So, I assume around 10-20 amps should be enough for those uses, depending on the rating of the plugs I fit. We don't regularly use an inverter, but I suppose it might be smart to also fit one lead off the auxiliary battery rated to cope with an inverter, just in case.
In future I may also fit a solar panel to a roof rack and run it down through a regulator to either the starting or auxiliary battery - haven't decided which yet.
I found a post on another forum in which Drivesafe was asked about this and recommended either 30amp or 50 amp wiring and circuit breakers between starting and auxiliary batteries, so I am assuming either would be fine for the Traxide to handle.
So, if I go with 50amp cabling and circuit breakers, will that do a good job? I want to keep it simple and over specify it for safety's sake. So, for example, I'll put circuit breakers at both ends of this cable.
Incidentally, I have still to decide a route for this cable. I suppose it could go out of the battery box, run along underneath and come up into the drawers, although that means drilling another hole in the floor.
Alternatively, it could come out of the battery box, go up the door pillar, along the top of the passenger door channel, down under the trim and to the drawers, but that would mean a longer cable.
What do you think?

The cable size you're going to need (remember accessories will draw through it and the battery charge current also) will need to be reasonably hefty (2 B&S) - winch cable diameter essentially - to ensure you can feed that battery when it gets low.

Yes, a nice thick sucker is planned. Just have to find the right battery first.

Obviously batteries will only accept so many amps of charge - so that depends on the second battery.
If the battery takes (hypothetically) 15 amps, and youve a fridge running (say 4 amps), at start up in the morning youll be drawing 19 amps.

But, if you've a winch running with both batteries connected, then it's going to share the load between the the batteries (if resistances were equal). So if your winch draws 400 amps, it'll be a big cable.

Sent from my SM-G900I using AULRO mobile app

I used 6 B&S to my rear battery. This was advice I got from Tim.

Went to Autobarn yesterday and, for a four-metre (yes, I know that's a long cable) Matson heavy duty cable with soldered connections was quoted - wait for it - $130!
Didn't buy it. Gotta be able to get it cheaper. Might reassess my planned cable route too to make it shorter.