My Ford Focus' battery warning light keeps coming on - what's the fault?

I have a problem with the battery warning light on the dash of my 2006 Ford Focus. I drove from Devon to Scotland with the damn thing on. I called out the breakdown services when I first noticed the problem - they tested the battery (12.5v) and there was 13.5 amps going to it - but the damned battery warning light was still on. The car went to garage for a day where they updated some software, tested the battery and could not find a fault. When I drove about 50 miles the warning light came on again and the breakdown services said they couldn't find the cause of the problem. I don't want to spend a fortune at a garage for them to find no fault again. Any suggestions?

Asked on 1 September 2010 by agfor1

Answered by Honest John
It's a problem with the alternator diodes.
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