FRIDGES

Auxiliary Batteries are used  to operate appliances while you are away from mains 240v power, and so you don’t run the risk of compromising you vehicles main starting battery (jump or push starting your vehicle is not much fun and best avoided don’t you think?)

So lets look at what we need to build a system that will deliver the best performance for our hard earned dollars.

Please note,
this is not a comprehensive step by step guide to installing a dual battery system, however it should give you a very good insight to what is needed and why, and how to go about sizing and fitting it all up.

What’s needed.

Well I guess a good place to start would be an auxiliary battery, then we need an efficient way of charging this battery using the cars alternator, and most importantly a way of isolating it from your starting battery so that it is not compromised, to do this we use a battery isolator, and then all that’s left is some cable to connect it all up, simple eh, well it is simple as long as you use the right bits, and put it together the right way……

Auxiliary Batteries.

Basically you should be thinking about how big a battery can you fit, and less about how small and cheap a battery you can get away with, the larger the battery the less percentage wise you will be draining it, and the longer it will last both in power usage and battery life, with the extra advantage of more capacity when needed.

Hot Tip …Batteries love being fully charged, they don’t like being discharged at all, they like it when you only take them down to 50% of their capacity and then top them back up soon after, they will lose capacity and die quickly when left discharged for long lengths of time.

Deciding on what size and type of Auxiliary Batteries to use is really a whole other story, please read my article on AGM (Absorbed Glass Matt) batteries www.fridge-and-solar.net/agm.htm

The Battery Isolator.

The battery isolator is a device that allows the cars alternator to recharge the auxiliary battery while protecting the cars starting battery from discharging, if these batteries are simply linked together you run the very real risk of draining them both to a level that you can not start the engine, and there are many other reasons why you should not do it this way.

So hence we have the battery isolator, there are many different methods used to isolate the batteries, most use some form of solenoid, this is an electronic switch that can handle large (charging) currents, much more than standard switches can, and they are operated by applying a small amount of power to them, this is turn actuates a solenoid that links the batteries together so that large currents and be passed between the batteries.

There are many ways to operate these battery isolators, some much better that others.

The cheapest way is that you manually turn them on and off, I call these idiot switches, because sooner or later some idiot (and I’ve been there) will forget to turn the switch on (no charge goes into the aux battery) or off at the right time (and then the starting battery gets drained as well).

The Redarc Smart Start battery isolator and how it works.

When you start your engine the Redarc works by first letting your starter battery recover it’s charge, it lets it achieve 13.2 volts, at this point that battery has about as much charge in it as it's going to get, once the Redarc sees this 13.2 volts it closes the contacts on the solenoid, this links both batteries together for charging purposes.

The Redarc Smart Start battery isolator is rated at 100Amps constant duty with a surge capacity of 400Amps, it has silver contacts unlike some of the cheaper ones that have copper (silver is much better), the Redarc also has spike protection so it won’t upset the delicate electronics on newer vehicles, the Redarc is on SPECIAL and is priced at $139.50.

Wiring…this is without doubt where many people get it wrong and end up with a system that may work, BUT barely, and nowhere near as well as it could and should, doing it right can double the useable capacity of your auxiliary batteries over and above what many people normally have, and help them charge much faster and last longer too.

Battery placement.

A very important thing to consider is where you are going to place your batteries, standard open wet cell batteries give off hydrogen gas when they are charging and discharging, this is a very explosive gas, remember the Hindenburg, there is no problem if you are putting the auxiliary battery under the bonnet of your vehicle, or mounting it somewhere on the chassis with good ventilation to the open air.

If you need to place batteries inside vehicles, then AGM batteries should be seriously considered here as they are totally sealed, AGM batteries if placed in an enclosure only need to be vented to atmosphere not necessarily vented externally.

Putting AGM batteries under the car bonnet is fine as far as safety goes, the only consideration here is that AGM’s and Gel batteries should be kept away from the extreme temperatures of turbochargers, and if they must be placed close to the turbocharger then heat shields should be used.

How to wire it all up.

Now this is much simpler than most people think, it is not complex, and as long as you take care almost anyone can do it.

When running the cables between the batteries you should take great care with where you place the cable, making sure that it won’t rub or be cut through, also be careful to keep it clear of sharp corners etc, and if the auxiliary battery is going to be placed anywhere other than under the bonnet, you should seriously consider using a protective covering like split convoluting tubing and secure it well with clips or cable ties at well spaced intervals.
The cable should also be protected against short circuit, and I have found the most economical way to protect the cable is with Mega fuses of 100A, an all too common mistake made by many people and even some auto sparkies is that they only fuse at one battery (normally the starting battery), but the cable needs to be protected from both ends, i.e. a fuse at both batteries, after all it is being feed from both batteries.

All cable lugs used with the 6 B&S cable are far too big to be crimped with standard crimping tools, so they should be crimped by an auto electrician, or you can solder them if you have a gas torch, they do take a lot of heat to solder on properly.

As you can see in the picture of the the Redarc battery isolator, it has two wires coming from it, one blue and one black, and it has two terminals for attaching heavy cable, now the blue wire is rarely used, so lets forget about that one for the time being, the black one is simply an earth that should be secured under one of the screws that you mount the Redarc to the vehicle with.

Now with the terminals, the one with the small black box attached to it is where you connect the cable from the start battery positive terminal, and the other terminal gets connected to the cable that goes to the  positive on the auxiliary battery, it's that simple, next the earth/negative cable simply gets connected to the negative terminal of both batteries, see easy.............yes I know there is some monkey work laying in the cable, sometimes crawling under the car to secure wires etc if you can't run them inside the car, but still simple :-)

**Now the blue wire coming from the Redarc, most people don’t need to use this wire, is for back-starting, and/or putting a indicator light on the dash of your car, but you really need to talk to me before doing either, as I said most people don’t need it.

Below are some examples of how an auxiliary battery set-up may be done, but there are many many other combinations that can be done to suit almost all needs.