The 12v sockets in my Audi, unlike the one in my wife's Honda, remain live even when the ignition is switched off. Three weeks ago, when I put the Audi in the garage I forgot to unplug the dash camera so, hardly surprisingly, when I went to start it yesterday the battery was completely discharged showing only 2.5v across the terminals. Not a problem, I thought, my Cetec superwhizzo 'smart' charger will soon sort it out. How wrong I was.

To my surprise, after being connected for more than four hours the Cetec still hadn't started to charge and was stuck at the preliminary analytical stage. What I didn't know was that unless the battery you are trying to charge has a minimum residual voltage (typically 9.5 - 9.75v), a smart charger will not recognise it as a battery and accordingly as a safety precaution will not begin its charging cycle. Old-fashioned chargers don't have this problem but if you don't have one of those to hand, there is a way to get round it.

The trick is to connect a well-charged battery to the discharged one with a set of jump leads (+ to +, - to -) for a few minutes after which when uncoupled the discharged battery will have sufficient surface charge to be recognised by the charger which will then start its cycle as normal. You can leave the discharged battery in situ while you do this.

As my battery had been so badly discharged, I was concerned that the plates might be sulphated so I put it in 'recondition' mode overnight and to my relief by this morning everything was back to normal (12.98v) and the car started immediately.

I've heard it said that if you fully discharge a car battery you will need to replace it with a new one (£100 in my case). Others suggest they can be rejuvenated by pouring out the acid, flushing with Bicarbonate of soda, filtering the acid and returning it to the cells prior to recharging but a) this doesn't always work and b) messing about with acid somehow just doesn't appeal.

As most battery chargers sold today are of the 'smart' type this problem is bound to become more and more common and people may well be persuaded to throw away perfectly good batteries that could be restored using the above method. Worth a try?

Quote: Aaron Calder wrote in post #1

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The 12v sockets in my Audi, unlike the one in my wife's Honda, remain live even when the ignition is switched off.

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Thanks, Pete. I'll look into that.

I'll keep an eye on the voltage, Ray, and if necessary will replace the battery as I already have a newish spare that a friend gave me. The problem is, the discharged battery is a newish Varta silver 85ah and as a tight-fisted Yorkshireman I'm reluctant to ditch something that might be serviceable. Fingers crossed.