The purpose of having a dual battery system was so I could have a dedicated battery just for running “house” electronics such as a fridge, rice cooker, hot water kettle, personal office electronics, etc. I didn’t want to risk killing the starter battery and being unable to start the truck. I also wanted to prevent repeatedly deeply discharging the starter battery as this can permanently damage the starter battery.
There were several objectives I wanted to accomplish in the setup:
1) Connect the auxiliary battery in parallel so that I could charge it off the truck’s alternator while the truck was running.
2) Isolate the starter battery from the auxiliary battery when the truck was not running, thus preventing the starter battery from being drained.
3) Install fuses to prevent fires.
4) Convert direct current (DC) to alternating current (AC) to run house electronics.
5) Add solar panels to charge the house battery when camped for an extended period of time.
Connect the auxiliary battery
Since there was no space under the hood (without moving existing wiring around) for the second battery, I had to place it in the bed of the truck. I’m using an Absorbent Glass Mat (AGM) deep cycle battery for the auxiliary battery. AGM batteries are sealed and do not require ventilation and are made to be deeply discharged unlike starter batteries.
Wires were run from under the hood to the back of the truck following the under body of the truck and coming up through the drain holes in the truck bed. The wiring for the battery/isolator circuit was 4 AWG which according to three different wire sizing charts is the correct gauge to use. As a general rule, it’s better to err on the side of larger wire.
The circuitry
Isolate the batteries
To isolate the batteries, I chose a digital voltage sensitive relay (VSR)** which was mounted under the hood near the starter battery. The VSR automatically links both batteries in parallel when the engine is running and separates them when the engine is off based off of a voltage reading. An added benefit of the VSR is that it allows a second battery charging source (in my case solar panels) to charge the main battery as well. The VSR I’m using does not have a manual switch to override the VSR to use the auxiliary battery as a jump start in case the starter battery is dead. Should the starter battery die, I do have an Antigravity XP-1 to jump start the vehicle.
**The actual VSR I'm using I purchased from Boatersland http://www.boatersland.com/bep710-140a.html
The VSR is in mounted on a plate in the back corner
Install Fuses
In order to prevent fires and wire meltdowns, I chose 100 amp ANL fuses and E-Block fuse covers for the two red wires between the two batteries and the VSR. The E-Block fuse covers are clear which allow you to see if the fuse has blown. The fuses were installed as close to the battery positive leads as possible because I wanted to minimize the chance of the wires melting and starting a fire. Fuses are an absolute must since most modern cars basically run off their internal electronics and a short could blow your car's computer chips to bits.
Convert DC to AC power
The energy stored in the auxiliary battery is 12v direct current (DC) which is nice until you want to run things that require 110v alternating current (AC or wall outlet power). I’m using a 1000w pure sine wave inverter with 2000w surge capacity to convert the DC to AC power. A pure sine wave inverter delivers AC power which is the closest to what the electrical company sends to your home. The inverter is wired directly to the battery using 4 AWG wire and a 100 amp circuit breaker. I’m pretty sure the inverter already has a fuse built into it, but I’d rather reset a circuit breaker in case I draw too much power. The inverter has two wall outlets and two USB plug-ins, which is sufficient.
Even though the inverter will allow me to have AC power, I have also wired additional 12v outlets because converting DC to AC power wastes power. It is most efficient to use electronics which take 12v instead of 110v. I will be running things like the fridge and small fans off 12v power.
View from the front
Solar panels
For my solar setup, I have a plug-and-play kit from Overland Solar. The kit comes with folding 60w solar panels, a charge controller, and about 30ft of wire which can hook right onto the battery terminals. While I have done quite a bit on solar setups and solar technologies, there were only so many things I wanted to attempt a DIY solution. This was not a task I wanted to undertake. The Overland Solar kit fits my needs as they are portable which means they can be manually moved to track the sun (solar panels don’t work in the shade). The panels can also be easily stowed away when not in use.
Overland Solar solar panels soaking in the Santa Fe sun!
That concludes the three part "The Build" series. Stay tuned for an in depth discussion on the theory and science of each component of a dual battery system.
