Third battery set up - is it possible?

I have done this and the simplest way: 1 got a battery box (easier to tie down the battery) put an isolator in the box, with the battery,( make sure you turn off this switch when you stop the motor) use 6mm wire or better from pos side of the cranking battery, fuse at each end of the wire, 30 amp. When the motor is running I only lose .1 of a volt between main battery and fridge battery. I have a neg wire on second battery hooked to tie down in the back. Hope this is of some help, Ive had this system for 4 or so years without any trouble.
Cheers Bruce.

Seems as you already have your second battery isolated from the cranker, I would do similar to above, only wire the 3rd battery in parallell with the existing aux battery (with fuses both ends etc). I have two aux plugs in the back of my wagon, and have made up a lead that plugs in, and clamps to a third battery, which also has a plug on it. Both the remaining plug in the back, and the plug on the 3rd battery now run of both aux batteries. I see its a wagon in your rig profile, so obviously you don't want a std lead acid battery in the car, use tha appropriate sealed unit. (please note that I am yet to use this setup on a trip, but it worked well in the back yard.)

Although it may sound a little complicated with the way I have explained it, it is versatile as you can remove the 3rd battery in seconds, as I will only need 3 batteries for about 2 weeks a year.

How long to you intend the batteries to last between the car running, and do you have any other power supply you are taking (gennie, solar etc).

Cheers Andrew

I run a 3 battery system in the surf. I use a wetcell Calcium battery in the back storage box. It is stored in a plastic battery box (about $20 from super cheap). Thats the idea of a battery box, to allow you to use a wetcell battery in an environment where you don't want spillage. Calcium-calcium batteries produce little (if no) gassing when charging from your alternator as they do not gas until about 14.9v making the system safe for inside an enclosed space such as a car cabin.

I would recommend using an either identical or similar capactiy battery to your existing deep cycle and running the two in parralell. ie Just hook the third battery directly (fuesd of course, I use a 70amp circut breaker at both ends myself) to the existing deep cycle. You may need to upggrade your wiring to 8g or 4g to get the best results, I use 4g all the way to the front.

The reasons for this are:
1. Cheaper
2. Simpler
3. It will work better.

According to Peukert's Law a 100amp hour deep cycle battery with a 4 amp load (ie a fridge) will discharge faster than two 100amp hour batteries hooked up together. "DEERR" you're probally thinking, it's got twice the storage capacity so of course it will!

What I'm talking about is that the more load you draw from a battery the less efficiant it becomes. Most deep cycle batteries sizes are rated on a particular draw. 20amps seems to be the most common. ie If you have a 100amp hr battery and you draw 20amps continously from it, in theory it should last 5 hours. (100/20). If you draw 1 amp from it, it may last 110 hours and if you draw 100amps from it it may only last half an hour (instead of the expected 1 hour that the rating would lead you to belive).

Therefore by hooking both batteries togerther you now have a 200amphour battery bank instead of two seperate 100amp hour batteries meaning that the current draw you now are taking from the system is 50% less of the total amp hour rating than it was previously giving you more run time.

You can read about Peukert's Law here: www.smartgauge.co.uk/peukert2.html

I found that when I installed the third deep cycle battery I don't just get double the run time out of the fridge, I get substantially more than double. On a hot summers day (40c) I could pull up at camp, run my fridge at -2c, music, lights, water pump all day and night and have bascially nothing left in the 100amp/hr by morning (about 11.0v). By putting the second identical battery into the system I barley even dent the battery bank by doing the same thing (12.1v). The batteries are fairly old and after rest charged normally only sit at 12.5v, maybe 12.6v after a good 3 stage charge at home.

I run a 6 battery system so no reason why you cant run a 3 ,mine is 80amp Fullriver AGM HGL as starter ,2x same as aux under bonnet thru Redarc clone wired in para plus 3x same batts in c/t wired thru anderson plug and 8mm sq + and - effectively in parra to batt no3 under the bonnet,, easy as.

Hi Mike,

Yes you can run a third battery.

This is the same basic setup as my 'Flyer' Universal Dual Battery System. I would recommend a fresh set or wires to the Redarc to optimise charging and wire the fridge to the 3rd battery.

Regards Derek.

Hi Guys,

We have created an online AC or DC calculator to help determine you battery capacity and solar requiements.

You can check it out here:

for AC:

DC solar calculator

for DC:

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Max
Information Manager
Energy Matters - Solar and Battery Sales
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