People have been using and charging auxiliary batteries in their cars and caravans with varying degrees of success for over a century.
For most of us the days are long gone when all we needed from our auxiliary battery was a small amount of power for a bit of lighting and maybe a radio.

These days we expect much more, we like to run lights, TV’s, Laptops, DVD’s, pumps, fridges, winches, and charge camera batteries and mobile phones etc, some even run microwaves, it is true that some of us like all the luxuries of home when we go ‘camping’.

So over the years our demands on the humble auxiliary battery have increased to the point that we need much more usable energy, and over a longer period of time, and we need fast and safe ways to recharge our batteries.

Many of the old ways used to charge these batteries are now antiquated, and they mostly never did the job very well anyway, neither were they conducive to long battery life.

So lets look at what we need to build a system that will deliver the best performance for our hard earned dollars.
Warning, if you are using lithium auxiliary batteries, then most often a VSR charging system should NOT be used, in this case it is almost always better to use a DC to DC charger.

Please note,
While this is not a comprehensive step by step guide to installing a dual battery system, it should however give you a very good insight as to what is really 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 the auxiliary battery from your starting battery, so that your starting battery is not compromised, and so you will always be able to start you vehicle and not be left stranded.
To do this we use what is called a battery isolator, and then we need 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, now I know money does not grow on trees, however the larger the battery the less percentage wise you will be draining it each day, and the longer it will last for you, both in power usage (battery capacity), and in battery life (longevity), with the extra advantage of some extra capacity available to you if needed.

Hot Tip …Batteries be they lead acid batteries wet cell, and sealed batteries, either AGM or GEL, they just LOVE being fully charged, they do NOT (never) much like being discharged at all, they do LIKE it when you only take them down to about 50% of their capacity, and then top them back up soon as you can (within a day or so).
Batteries will lose capacity if left discharged, and they will die quickly if 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 >>Please click here for my article on AGM batteries<<

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 auxiliary battery) or off at the right time (and then the starting battery gets drained as well).

My choice now is the Enerdrive VSR battery isolator, and I explain below how it works.

These Enerdrive VSR's are priced at around $79.95

When you start your engine the Enerdrive VSR works by first letting your starter battery recover it’s charge from the alternator, it lets the cranking battery achieve 13.3 volts, at this point that battery now has about as much charge in it as it's going to get, now once the  Enerdrive VSR sees this 13.3 volts it closes the contacts on the solenoid, this links both batteries together for charging purposes.

The next thing the Enerdrive VSR is looking for is 12.8 volts, in other words it's likely that you have now shut down your engine and started to draw power from the batteries, so after a short while the voltage across both batteries will slowly fall, and when they get down to 12.8 volts,  the Enerdrive VSR will see this voltage it will open up the solenoid contacts and isolate the batteries, leaving your starting battery for starting, and your auxiliary battery for auxiliarying ;-)

Some of the Enerdrive VSR's major features are............

I love how compact it is, with no ugly exposed cable posts sticking out.

The unit is waterproof, so a bit or even a lot of water is not going to stop the Enerdrive VSR from working.

The Enerdrive VSR battery isolator is rated at 140Amps constant duty.

It also has Surge Protection so it won’t upset the delicate electronics on newer vehicles.

When the VSR is engaged (closed, linking the batteries together) a red LED comes on the top of the Enerdrive VSR .

This VSR consumes no power at all while it is not engaged, i.e. the voltage of both batteries has dropped below 12.8V, and then it consumes only 0.34A when it is engaged.

   The Enerdrive VSR has one more huge advantage in that it is Dual Sensing (bi directional), what this means is that if you have a battery charger, or solar power connected to your auxiliary battery, and that battery gets charged enough to hit the Enerdrive VSR's  set points, then it will allow your charging source to back charge the cranking battery as well, making sure that it's kept fully charged too, but again, if the voltage drops below 12.8 volts, the Enerdrive VSR will again disconnect (isolate) the batteries, so how cool is that!

The Wiring & Cabling.

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 usable capacity of your auxiliary batteries over and above what many people normally have, and it will also help them charge much faster, and in turn they will last longer too.

Fusing & Cable Protection!

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 any fully sealed 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.

Safety Tip   *  Do not install the Mega fuses until the last thing, when you have completely finished all the cable runs and tightened up all other connections first.

  Note: *  Some people can use the chassis for earth return, but my advice is don't, and to always (and it really is best) run the earth/negative cable all the way between the start and auxiliary batteries, for the extra few dollars in cable it really is worth it, and you only have to do it once, so do it right the first time please.

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 convoluted  tubing and secure it well with clips or cable ties at well spaced intervals.

If your auxiliary battery is in a caravan or trailer then the standard trailer lights connectors are way too small for the 6 B&S cable, and can’t even come close to handling the high currents for effective battery charging, so here we use 50A Anderson plugs (Pictured below).

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 if you do have one of the special correct large crimping tools, or can borrow one, as it is essential that you have a very sound crimp joint, or you could have major catastrophic problems.

The Enerdrive VSR battery isolator has two main cable posts underneath it, these are for connecting the main positive  battery link cables to, so here the one with the red dot on the stud, this is where you connect the cable coming from the MEGA fuse on the starting/cranking  battery, and the other post is for the link cable that is running to the auxiliary battery positive MEGA fuse.

The Enerdrive VSR has one small black wire coming from it, and is simply an earth that should be secured under one of the screws that you mount the VSR to the vehicle with, if that is metal that is grounded to the chassis, otherwise you will have to go to another nearby good earth point (bolt/nut or screw) with proper grounding.

Strong Tip,  for the very best VSR operation, the VSR needs to be mounted as close to the cranking battery as possible, and the cable size between both batteries also plays a huge part in optimal VSR operation, also battery charging speed and efficiency.

It really is simple, next the earth/negative cable simply gets connected to the negative terminal of both batteries, but again using the good sized 6 B&S cable..............

Now see how easy that was.............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 :-)

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.