Time to charge

Date:1 August 2011 Tags:

Q During the winter, I’ve always used a trickle charger to keep my motorhome’s battery charged. I discovered that, if I put it on a timer so it only ran an hour or two a day, I didn’t have to add water every month. Last summer I replaced the battery with a pretty expensive AGM one that doesn’t have ports for adding water. I also neglected to use the timer. Now the new battery won’t hold a charge or start the motor, and the guy who sold it says I have to buy an expensive charger to keep it alive over the winter.

A The problem isn’t your AGM battery; it’s your old-tech charger. It’s essentially a constant-voltage device, providing about 14 volts regardless of the battery’s state of charge. A really dead battery might pull the output down to a more appropriate 13,2 volts or so, but, ultimately, as the battery charges, the voltage rises and overcharges. With your old fashioned flooded cell battery, it wasn’t a big deal – as the distilled water was electrolysed to hydrogen and oxygen and evaporated, you simply topped it off every now and then. Putting the charger on a timer like you did is brilliant, because that strategy proportionally reduces overcharging and the rate of evaporation.

Modern maintenance free batteries have no filler caps – and thus no way to add water, so it’s vitally important to avoid overcharging. And AGM-class batteries are even more advanced. AGM stands for Absorbed Glass Mat: the sulphuric acid electrolyte is soaked into a porous glass fibre mat, and additional chemistry inside the battery recombines the hydrogen and oxygen back into water, making these truly maintenance free devices.

Until you overcharge one with a constant-voltage trickle charger, of course. Your first step is to give that old trickle charger to someone you don’t like – and get a modern battery maintainer. These devices generally have three or more charging phases and are designed to be used full time.

They start by assessing the battery’s charge state, then charge at a constant voltage (like your trickle charger) up to 80 per cent of the battery’s capacity and finally taper off to slightly above the nominal 12,6 volts to avoid overcharging. Periodically the smart charger checks the battery’s state of charge and briefly ramps up the voltage to replace any energy lost during the battery’s normal self-discharge. Some even include a desulphation mode to break up any lead sulphate crystals growing on the plates, another problem caused by normal use and exacerbated by chronic undercharging.

I have probably a dozen battery chargers and maintainers lying around the workshop, ranging in cost from a few bucks to several hundred bucks. Assuming your motorhome’s battery is fully charged by the engine alternator when you park it, the smallest ones (around 1 amp) should do the job. Some larger chargers which have a maintainer function and are rated at 6 or 10 amps will actually recharge a decentsized battery from dead overnight – without cooking the water out. They’re great for things such as trollingmotor batteries that need regular recharging. Maybe that’s what this fellow is trying to sell you; otherwise, look for a device in the R150 to R300 range that should be available from several suppliers.