They are no use with a fully discharged battery. Lead acid batteries don't like deep cycling or fast charging rates and battery lifetime will decline sharply if the battery is subjected to either.

It's cheaper just to run the engine for 10 minutes every week-and once the engine is warm, well, it would be a shame not to take it out for a spin :)

If you have a power supply nearby the best solution is a smart charger, simply leave it semi permanently connected.

CTEK are a well known and respected maker, and Lidl's sell their own version at about a third of the price (recently on offer might be some left), i have one of each and they both do the job well...other makes and types are available.

Edited by gordonbennet on 05/11/2012 at 14:09

Thanks for the replies. I already have a battery charger - one from Halfords - but I wasn't too sure about leaving it on all the time. It would be no problem as the car is garaged, but I was concerned about the safety etc issues. I see that the CTEK ones specifically say they can be left on all the time.