Battery totally dead without warning

Got a boost start and was ok for a day, but next morning-zilch. Read that was how they work- always full power until sudden death. At least it doesn't bugger about kidding you that it just needs a charge. DaveK.

You didn't leave an interior light on did you? That would flatten it in a few days - I did it to a colleague's car at a conference just before Christmas after map reading for him on the way there. Oops!

Tony

I'd have wondered that myself, but nope, it hasn't worked in years!

David

Had a similar fault some years ago when I was running my Scimitar. I pored over the wiring diagram & decided that the only bit connected to the battery when everything was switched off was the alternator ( this of course was built in the days before electronics, alarms, and central locking ) To this day I don't know what caused the problem but I changed the alternator, * just in case* and it never happened again.

The trick is to insert an ammeter twixt battery and alternator with the engine off. If the alternator rectifier has a faulty diode causing it to draw current all the time you'll see it, as it will be several amps.

Your description is consistent with a battery at the end of its life. An absolutely normal mode of failure involves the accumulation of shed plate material in the space below the plates. When the level of this stuff becomes high enough to touch the plates, the battery slowly leaks charge through it (as if a light was left on). This gives you a flat battery after standing for a long time, but one that's still able to take a charge and appear normal if used frequently.

If this is the cause, it will fail completely soon. The deep cycling accelerates the shedding of material from the plates.

John

Its surprising how even a courtesy light - or boot light which you might not see - can drain a battery over that length of time. Faulty switch somewhere?

Not really - say you have a 10 watt courtesy light. That will draw just under an amp (about 833mA). Say the battery capacity is 50Ah. That will mean that it can deliiver 833mA for 60 hours, or 2.5 days so the courtesy light will have completely drained it in less than three days. And that is of course in ideal conditions with a battery in tip-top condition and fully charged.

Tony