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, DVD’s, pumps, fridges, winches, charge camera batteries and mobile phones etc, even microwaves, 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 useable energy, and over a longer period of time, and we need fast and safe ways to recharge them.
Many of the old ways used to charge these batteries are now antiquated, and they mostly never did the job well anyway, nor were they conducive to long battery life either.
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.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).Another way of doing it is to have the solenoid turn on as soon as you start the engine up, and turn off when you shut down the engine, this has been the cheap way of doing it for many years, but it’s far from ideal.
I believe the best way for most situations is to use a VSR (voltage sensitive relay) activated solenoid, and the one I like the best and use is one of the most inexpensive ones, it does the job very well, and as good as, and sometimes better than many of the very expensive ones too.
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 next thing the Redarc is looking for is 12.5 volts, in other words you have shut down your engine and started to draw power from the batteries, so very quickly the voltage across both batteries will fall to 12.5 volts, as soon as the Redarc sees this voltage it will open the solenoid contacts and isolate the batteries, leaving your starting battery for starting and your auxiliary battery for auxiliarying ;-)
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.The Wiring & Cable.
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.
Understand that when charging from vehicle alternator one of your biggest enemies is voltage drop, you need to have the highest possible voltage at the auxiliary battery, this will allow it to charge as quickly as possible, and as fully as possible.
Some standard set-ups I see regularly that have been fitted by the so called professionals is where they use twin 6mm auto cable (really 4.59mm˛), this is next to useless for fast deep charging.
A very basic explanation would be, that the larger the current draw, the larger the wire needs to be, and the further the distance the wire needs to run, the increasingly larger again the wire needs to be.
Water and power analogies work well here, so think about it, if you want to fill a bucket full of water fast, do you use a hose the size of a straw, or a ˝ inch garden hose, what one do you think will fill the bucket quicker? I always suggest a minimum of 6 B&S (13.5mm˛) cable here for linking batteries,
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.Just throwing them under a seat or in a cupboard is far from ideal and just down right dangerous, even worse some people place battery chargers and even inverters next to these batteries, just think about that for a moment, a hydrogen producing device next to a possible spark producing device, I guess some people just don't know or don’t think!
Wet cell batteries must be housed in a properly vented area, all but Gel and AGM batteries must be well vented externally away from any ignition source and from human habitat areas, hydrogen gas rises so sealed battery enclosures should be vented externally top and bottom.
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.Some tips………
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 battery charging, so here we use 50A Anderson plugs.
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 just two examples of how an auxiliary battery set-up may be done.
This is a basic layout for those
that might install an auxiliary battery in the back of their vehicle.
This is a two in one layout for those that might install an auxiliary battery under the
bonnet of their car
and/or one in a caravan or camper trailer.
On the right of this photo are two examples of the Anderson plug, a very well designed plug for connecting heavy cable together, incorporating no gender, both connecting plugs are identical, they can not possibly be hooked up wrongly so there is no risk of shorting out the batteries in the dark, they are self cleaning so a bit of mud won't worry them, the price for these is now only $10.00 each, and you need two to make a connection.
This is a Maxi fuse holder and a
100A Maxi fuse,
note the standard fuse on the left, well I did say MAXI
The Maxi fuse and holder come in at $19.90, and you need two of these.
The larger cable is the required 6
B&S (13.5mm˛) and matching lug, note the wire above it,
that's the 6mm (4.59mm˛) Auto cable used by many to do the job, it's just not up to it at
all.
The 6 B&S cable is $5.50 per Mtr, and you need enough red and black to run between
both batteries.
If you need more information please don't hesitate to contact me.
Val Rigoli
Fridge & Solar
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Copyright © FRIDGE & SOLAR 2002-2007. Last revised: November 21, 2007.