Most of us fit additional electrical equipment at some point but unless you are careful you could find that you'll flatten the battery and be left stranded or your main battery and standard alternator will just not be able to keep up with the drain you are demanding.
So far, so simple. The problem comes when the batteries are of different types, or capacities and at different levels of charge. If the starter battery is low on juice for some reason connecting the relay will allow the starter motor to try and draw power from the auxillary battery, even if it wasn't designed to provide the massive current draw that this would trigger. If the auxillary battery is fused it will blow, if it isn't fused it'll just heat and melt the wiring unless you've fitted it with starter cable (not everyone does) Conversely if the starter battery is fully charged but the leisure battery is flat connecting them together will allow the flat batterry to draw power from the starter. In most cases it won't draw much in the split second it takes you to turn the key but if, for some reason, you have turn the ignition on without starting (like waiting for your glow plugs to warm up) the flat battery could conceivably draw enough juice from the full starter to make it struggle when trying to start the engine. The answer is to fit a 'smart' system that knows all this and that monitors your individual batteries and distributes the charge and power drain accordingly.
The Dualmate gives a very handy, and precise, reading of each battery's voltage. Much better than a line of green LEDs, the voltage can be measured down to one hundredth of a volt. When the drop of a whole volt can be crucial to the performance of a battery a basic green LED light is simply not a good enough indicator of what is going on. While driving it also monitors the state of both batteries and optimises the charge to suit. So if you have a high electrical draw, like running your air con, windscreen wipers, lights and radio it will disconnect the auxillary battery and concentrate solely on keeping the starter battery fully charged. The Dualmate also has the ability, when necessary, to deliberately override the battery isolation so that both batteries can be used to start the engine. For our expedition vehicle we considered this unit to be just about as perfect as it could be to ensure our battery performance can be monitored and optimised whilst on the road. I know a lot of people don't like to rely on electrickery and microprocessors in vehicles that are used out in the wilds but Dualmate have thought of that. A manual override switch is also provided allowing you to disconnect the Dualmate altogether in the rare event of failure or other problem. In that case the standard split-charge relay will have to suffice but hopefully you will now have a good idea of its limitations and can take precautions accordingly. Fitting the Dualmate Fitting the Dualmate couldn't be simpler. First you must have a standard split-charge relay in place. (many vehicles like Land Cruisers already have these as standard but if you have a Land Rover you will need to fit one.) Then it is just a matter of fitting the relevant wires to the existing connections. (full and clear instructions are supplied) The control interface (a box with a manual switch on it) needs to be mounted somewhere easy to reach but out of the way. In the engine bay or battery box is ideal. The Control unit can then be mounted somewhere visible if you want to be able to see, at a glance, what your batteries are up to. 25 minutes should be sufficient to see the job done. We give this unit a double thumbs-up!
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 Dualmate
 info@oecinternational.com
 http://www.acecom.com.au/dualmate.htm
 Price: £235
