Critique my auxiliary wiring plan...


Active member
Hi all, I’m working on adding auxiliary wiring/power circuits to my LR3. At this point, I don’t want a dual battery system, so I’m planning on running a portable battery box with a deep cycle marine battery when needed. Here’s the basic plan, hopefully it’s legible.


I have an 80 amp fuse on the battery terminal, with 8 gauge wire running to a Blue sea fuse block under the dash. These fuses will be hot constantly. Running out of the fuse block will be another length of 8 gauge wire going back to a continuous duty solenoid which will be connected when the ignition is on, a 40A circuit breaker, and a 50A rated plug at the rear of the vehicle for maintaining trailer batteries. I realize it won’t fully charge a battery, I have a NOCO genius charger for that, it’s just to help out on longer trips.

At this point, I’m not running a fridge, but I’m going to wire for it so it can be plug and play. I have an ARB fridge socket, which will be powered off the fuse block and 10 gauge wire. I’d like to use this Blue Sea 3-way switch with an ignition triggered relay.

This would allow me to have the fridge socket turned off (bottom position) when no fridge is connected, hot when the ignition is on (middle position) for daily use, and be able to turn it on momentarily (top position) when the vehicle is off. I was thinking this would come in handy at camp after the vehicle has been off awhile and you’ve been in and out of the fridge making dinner. Once everything is put away, turn it on and let it run until it cools off again to sit overnight. Obviously you have to make sure you turn it off again, I also carry a NOCO lithium jump starter.

Thoughts so far?

The second half of this is a portable battery box, carried only when needed. It will be simple, basically just the battery with heavy leads wired to an Anderson connector. The lid will have another ARB fridge plug, 12V socket, USB sockets, and another 50A rated plug for the battery charger or adding some charge via the plug on the back of the car and alternator. I didn’t draw a circuit breaker/fuse on the positive lead, but it will have something there.


This would be brought along on longer trips where the vehicle might be parked for longer, or we want to run the fridge all night. It can be inside the vehicle with the fridge, and plugged into a battery charger if we are in a state park campground with electricity. It could also be used in other vehicles not wired for the fridge, or brought into a hotel room and recharged.

I have a few questions, the first is about safety when charging it inside the vehicle. I won’t be sleeping inside it, so charging would be via the alternator while driving, or plugged in on the charger with no one in it. Theoretically, noxious gasses can be produced, but I have a pair of deep cycle lead acid batteries in my cargo trailer that sits on the charger closed up, and I have never noticed any odor or corrosion/evidence that harmful gasses are being produced. Obviously, I could use an AGM or even lithium battery, but it’s quite a bit more expensive.

The second is if I built a jumper lead from the ARB fridge plug in the vehicle to the input plug on the box, would it pass enough current to keep up with the demand of the fridge running off the box through the 10 gauge wiring? Obviously, the rear plug will be able to send more, but it would be nice to not have a heavy wire running across the cargo area sometimes.

Finally, do the grounds from the rear 50A vehicle plug or fridge plugs need to be run back to the battery negative, or is a good chassis ground sufficient?

Any other advice, pitfalls, things I haven’t considered are appreciated.
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Well-known member
Although I have not built a system like this, I have been studying the ones I see here, my thoughts:

I think you'd want the solenoid under the hood or at least as close to the vehicle battery as possible not in the back.

Do you need 80 amps in the cab? Why not run the 8 gauge back to the back of the vehicle (not sure what an LR3 is) keep the trailer power circuit from a cab circuit.

You will need to run a negative cable to the back as well. You are running 8 gauge for a reason you should match the ground to it. I am not as sure about running the fridge off a ground from the chasis but it would be too easy not to pull a 10 gauge double wire to the fridge's location.

I'd be leary about running a fridge off my vehicle's main battery unless you carry a jump start battery. If you have a trailer and plan on building an auxiliary battery, why not plan to run your future fridge off of either of those?


Well-known member
No danger w/the battery inside as long as its in a box and you regulate charging to it and just dont hook it up to a big dumb shop charger.

Turn off the LVD on the ARB fridge and put an external LVD near the battery.. and you can get away with a bit of voltage drop on long runs.

Chassis grounds are fine for light loads, for any heavy loads you probably need to upgrade your chassis/engine ground straps and perhaps add some redundancy.. ideally you should ground at battery but sometimes thats not reasonable.



Active member
Thanks for the replies. I don’t need 80 amps in the cab, but I’m using one large fuse on the battery terminal to protect the wiring to my ARB compressor, Hella lights, interior fuse block, and the trailer wiring. All of them have individual appropriately rated fuses, this is just to protect the heavy wiring in the event of a hard short (cut). The way the wire needs to run to the trailer plug, it’s going right by the fuse block anyways, so I’m basically just tapping the fuse block into it.

I’m curious about whether where the continuous duty relay is mounted matters, I don’t know why it would. Putting it in the rear was mostly for ease of access in the event of a failure or troubleshooting, and putting it under the hood wouldn’t make my fuse panel hot with the ignition off, which is what I’m going for.

If I get a fridge, it makes sense to me to put it inside the vehicle rather than the trailer. Being able to get drinks while driving, and having sandwich stuff with you when out running trails and the trailer is back at camp are a few reasons.

With the 3-way switch and the fridge hooked to the vehicle battery, the default would only be running when the ignition is on, unless you intentionally bypassed it with the switch, which I might want to do from time to time. In the event that I forget to switch it back off, I do carry a jump starter pack. This would be for day trips generally, when camping and planning on running the fridge overnight, I would bring the auxiliary battery box, and power the fridge from it, that’s what it’s primarily intended for.

I was hoping with the 8 gauge wire and then 10 gauge wire for the fridge circuit, voltage drop wouldn’t be an issue. 10 gauge is what the ARB harness uses, and I’ll have a shorter run of it than that. Sound like running an 8 gauge ground back to the battery is worth doing, I figured it would be a good idea but it’s worth asking. The biggest electrical loads I’ve run in the past are large amplifiers, which do fine with a solid chassis ground like a seat bolt.


Well-known member
The reason you bring an 8 gauge wire to the back of the vehicle is not because you need it for an 80a circuit. You bring a larger wire back because of the distance and the resultant voltage drop. If you have a short run like one to the cab a lesser gauge like 10 gauge might suffice to run the fridge, lights etc. It is possible 8 gauge to the back of the vehicle might be not big enough, fine for running a fridge but might limit trailer battery charging via voltage drop.

I am not sure what a continuous duty solenoid is but most of the solenoid battery cut offs I see designed don't close the circuit until the truck battery has reached certain voltage, thus needing to be close to the battery to get input from the battery.