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Installing the Dual Battery System Part 2

Written by Jared on July 14, 2011

Part two of our series on upgrading our Blue's electrics to include a dual battery system. Below I'll go over how we handled the wiring and how the whole system works once all the pieces are together.

What we used:

  • Intelligent Solenoid by National Luna
  • 29 feet of 1 AWG and 20 feet of 10 AWG wire from Wire and Supply
  • 50 amp, 100 amp & 150 amp ANL fuses
  • 3x e2 ANL fuse holders
  • 10x 1 AWG lugs
  • 4x battery terminals
  • Blade fuse holder
  • 15 amp blade fuse
  • Rubber grommets
  • Mom's Teflon cutting board

Wiring Diagram

Dual battery wiring diagram.

Before we could purchase any hardware we came up with this wiring diagram. It was a huge help keeping us organized, especially given the amount of wire and variety of fuses we'd need.

Using the diagram we made a list of each cable we'd need, its length, and how it would be terminated - 3/8" lug, 1/2" lug or battery terminal. Then we spent a good deal of time researching the gauge of wire we would need given the distance from the battery and the power inverter.

The longer and thinner the wire, the more the voltage drops. If the voltage drops too far, the inverter or fridge will shut down automatically. We erred on the side of caution and bought the biggest wire we could find that we knew would fit and wouldn't break the bank.

We opted to have a professional cut and terminate the wires since we lacked the tools. A local battery shop was able to help, and sold us lugs and battery terminals for less than we could find online.


Every current-carrying wire that comes off of the battery must have a fuse of the appropriate amperage rating. The instructions that came with our solenoid (the doohicky that connects the two batteries) stated that we'd need a 50 amp fuse from the starter battery and a 100 amp fuse from the auxiliary battery.

We chose the power inverter fuse by applying Ohm's Law to calculate the maximum current (amps) the inverter would draw from the battery. Our 800W inverter can use up to 1200W of power from a 12V battery. That gives us 12V / 1200W = 100A. We chose the next fuse size up from 100A to play it safe, which is 150A.

Now to the installation...

4Runner stock fuse box.

This is the stock fuse box. The lid pops off to reveal not a lot of empty space. Not wanting to mess with the harness wiring and the 18 gajillion wires running out from under that box we opted to buy stand-alone fuse holders that we could mount wherever there was space.

ANL fuses mounted on top of fuse box.

Coincidentally the only space to mount our fuses was on top of the fuse box. Pictured above is a piece of Teflon cutting board that is bolted onto the lid of the stock fuse box. On top of that we mounted our ANL fuse holders, fuses and attached the cables. It's a bit of a pain to get inside the fuse box, but space is tight and the fuses had to be mounted as close to the batteries as possible.

Battery and Solenoid Wiring

The next step was to wire up the batteries to the fuses and the fuses to the solenoid. The solenoid is the battery connection, it engages once the engine has been running for five minutes, and when the starter battery voltage is a safe level. This prevents the auxiliary battery from draining the starting battery and allows the alternator to charge both once the starter is topped off.

Dual batteries with wires.

This picture shows the wiring connected to the dual batteries, running to the fuse box.

Dual battery wiring.

Here is the (almost) finished dual battery install. All that's missing is the large cable running to the back seat for the power inverter.

connected solenoid

Here's a shot of the solenoid with wires attached below, as you can see it's cramped and there's no room for taking pictures.

After half a days work the wiring is finally finished! We started the car up, timed five minutes and heard the solenoid click on. For the first time our two batteries were connected. Beer, anyone?

Up next Blue gets a lock box, a fridge and much improved rear storage.

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