Dual Battery and Electrical System for our Toyota 4Runner

We’re aggressively upgrading the truck before our adventure season kicks off at the end of May during Memorial Day Weekend.

Originally, we were going to be starting our season in April, but things fell through and pushed us back to May.

But no time lost, we’ve been using this time to get the truck finished up.

Our next big milestone has been upgrading, organizing, and preparing our Dual Battery and Electrical Systems.

Dual Battery System

I’ve received this question a few times now so I thought I’d explain it a bit better.

Why do you want or need a dual battery system? This question is two-fold.

First and foremost, having two batteries means I virtually can never experience a dead battery.

Secondly, having a completely isolated electrical system for our accessories means no major changes to the factory systems and allowed us to put a much, much larger battery for longer-term off power use.

How Does a Dual Battery System Work?

Under the hood of our 4Runner, we have a group 35, 55 amp hour starter battery connected to the factory vehicle systems and an automatic charge relay.

We also have a group 31, 100 amp hour auxiliary battery that’s on the other end of the automatic charge relay and then is connected to all of our accessories.

An automatic charge relay is an electronic device that detects the voltage coming into the ACR.

When the vehicle is turned on, and the alternator outputs voltage that increases to 14+ volts (charging status) the ACR automatically starts a countdown clock to active in two minutes. This allows the starter battery a chance to recharge some juice first, before activating the ACR.

When the ACR is activated, the voltage flows from the alternator to the starter battery, through the ACR, then into the auxiliary battery, thus charging the auxiliary battery.

When the truck is turned off, the reverse happens. The ACR detects a drop in voltage and automatically turns off, isolating the two batteries. So when the fridge or camping accessories are running for 3-7 days without new power, we never run the risk of killing our primary starter battery and can always start the truck.

The other safety net is if for whatever reason we ever left the lights on, or something happens to the starter battery, we can manually activate the ACR, and start the vehicle from the auxiliary battery.

Electrical System

On top of having a dual battery system, we decided we wanted our accessories to be completely separate from the factory electrical systems.

The perks to this are that if we ever want to remove or upgrade the accessories we’ve added, it doesn’t affect the factory system in any way.

So, we set out to find a solution that was simple, clean, and robust.

We ended up having to create and wire our own Relay and Fuse Block instead of just purchasing a ready made system like the SPOD.

It was a fun project to learn how to wire, and it’s always best to know how everything works for when it inevitably breaks.

In the end, we got exactly what we wanted, a waterproof relay and fuse block with custom switches that fit perfectly inside our 4Runners factory button positions.

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6 comments on “Dual Battery and Electrical System for our Toyota 4Runner

    • Mike Revelle on

      Hi Steve, Great question. I did use their supplied wiring harness. I did not however cut any factory cables like they say to do. I instead used a bolt, nut, and shrink wrap to connect the wiring hardness to the factory ground cable. Just incase I ever wanted to undo it.

  1. Faizan on

    Hi Mike – thank you for sharing your experience with this setup. To move the auxiliary batter to the stock batter position, were you able to do this with the OGE kit or was there anything else custom or special from OGE required? Thanks!

    • Mike Revelle on

      OGE provided everything I needed. The only thing I did not do was cut the factory cables. I instead used a bolt, nut, and shrink wrap to connect the wiring hardness to the factory ground cable. Just incase I ever wanted to undo it.

  2. Steve Ronsko on

    Hi Mike,
    I finally was able to get the batteries for this conversion, even went with the 100 amp hour battery.
    I noticed you mentioned you “did not cut the factory cables like they say to do.” Is “they” OGE? In my instructions there was no mention of cutting cables. In fact they way I have mine wired it looks like I have connected the batteries in parallel without isolating them.

    • Mike Revelle on

      Hi Steve, sorry for the late reply.

      In my instructions they wanted me to cut the factory ground wire and splice it into the new ground wire OGE provided. I instead used a bolt and nut with a terminal and bolted them together and wrapped it with electrical tape. Doing this has saved my butt twice already when getting my truck services, Toyota wouldn’t touch it unless I “removed” the battery system. Which I was able to easily do by undoing that bolt/nut.

      Im glad the instructions no longer require the cut. I have a Blue Sea Automatic Charge Relay in between the batteries, that isolates them when I want, but otherwise they are wired parallel.

      hope that helps!

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