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The battery warning light illuminating while driving indicates the alternator is not charging the battery. The vehicle will continue to run on battery power until the battery is depleted — typically 20-40 minutes for a modern car with many electrical loads. Diagnosis: check alternator output voltage with a multimeter (should be 13.8-14.4V at idle with lights on). Check the alternator drive belt for slipping. A failed diode pack inside the alternator is the most common internal failure.
Based on 518 driver confirmations
Light came on intermittently. Took it to halfords for a free battery check, they found the negative terminal was slightly loose — vibration was causing momentary voltage drop. Tightened it up, never came back. Worth checking the obvious things first.
Parts replaced: Nothing
Tested charging voltage — only 11.8v with engine running, should be 14v+. Alternator had failed internally, brushes worn out. Recon unit from motor factor was £95, garage charged another £60 labour. Back to 14.3v charging. Battery was fine, didn't need replacing.
Parts replaced: Remanufactured alternator
Warning light came on, then within 5 minutes the power steering went heavy and the temp gauge started rising — all signs the belt had gone. Pulled over in time. Belt had snapped completely. New belt and tensioner, £65 fitted. Check your belts people, they don't last forever.
Parts replaced: Alternator/auxiliary drive belt, tensioner
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