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battery isolator and how it
**July 2022 Update, warning, generally you should not use a VSR as described in this article below,
if you have a vehicle fitted with a smart alternator, the risk is that your auxiliary battery may not be charged correctly or efficiently!
Also when charging lithium LifePo4 batteries a VSR should not be used, the risk is grossly over or under charging.
In both cases it is best to use a modern DC- to - DC battery charger, with a charging profile suitable for your battery type!
Auxiliary Batteries are used to operate appliances while you are away from mains 240v power,
and also so you don’t run the risk of compromising you vehicles main starting battery
(I'm sure that you agree that jump or push starting your vehicle is not much fun and best avoided.)
People have been using and charging
auxiliary batteries in their cars and caravans with varying degrees of
success for over a century.
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
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.
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 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
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).
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 and does have a few pitfalls.
I believe the best way for most situations is to use a VSR (Voltage Sensitive Relay) activated solenoid, and I explain a little further on how they work and why I think they are so good.
years I had used the Redarc Smart Start VSR with great success, and I've sold hundreds of them, they
really are a good unit.
However a while ago after doing more research and testing I came across another VSR that I found I liked even better, with many great features, and I've listed some of them below in my explanation of how it works.
This one I like the best
and now use is
also one of the least expensive ones, and it does the job very
well, truly ultra reliable (has been used in the boating/marine
industry for many years), and I believe these are as good as, and often
better than many of the much more expensive VSR's.
So my choice now is the Enerdrive VSR battery isolator, I explain
how it works
and other info >>please
The Redarc Smart
VSR (Voltage Sensitive
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 12.7 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 12.7 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 ;-)
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.
However if you don't already have a Redarc VSR, and would like to learn about an alternative that is now my favorite, and less expensive, then please >>click here for my full thoughts on the Enerdrive VSR and Dual Battery Systems<<
Connecting up the Redarc Smart Start
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 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 4.59mm²), 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 (cable) needs to be, and the further the distance the wire needs to run, the increasingly larger again the wire needs to be.
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 ½ inch garden hose, what one do you think will
fill the bucket quicker?
Fusing & Cable Protection!
fuses in is necessary because of the potential of a short circuit, now
seeing as we are installing good heavy cable so that we can transfer
large amounts of current quickly, there is a possible downside
having a few
failures sometime back using the 100A Maxi fuses (they were melting, not
blowing, melting because of high
created heat with the high currents),
so after doing a lot of
testing I found that the MEGA fuse performed best in this situation,
the MEGA fuse has been used for many years when connecting Inverters to
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, and 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 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.
to wire the Redarc up.
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 to always (and it really is best) to 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.
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
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 you can solder them if you have a gas torch, they do take a lot of heat to solder on properly.
isolator has two main cable posts on the top of it, these are for
connecting the main positive
battery link cables to, so here the one with the Redarc sensor box
hanging off it is where you connect the cable coming from the MEGA fuse
on the starting battery, and the other post is for the link cable that
is running to the auxiliary
that simple, next
cable simply gets connected
to the negative terminal of both batteries, but again using the good
sized 6 B&S cable..............
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.
© Val Rigoli
Copyright © FRIDGE & SOLAR. Since 2002