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[4PLUS2]

Hi Guys
Now I hope I can explain this properly so please be understanding

A fully charged battery should I think read 12.6v and a charging battery should be 13.8v ish showing that the alternator is doing its job. Now if the battery over night drops to 11.8v can I assume that the battery is faulty. Now it does not matter if the car is standing overnight or a few days so I don't think I have a circuit draining the battery.
Now if various sencers are expecting 12+v and don't get it they show a fault and switch of ie TC, now after driving a few mins stop and switch engine off and restart all is ok. On checking the ECU no faults are stored but the code reader shows  low voltage.
Just one other thing if you start the car and leave it ticking over for five mins no faults whatsoever.
Is my thinking correct and the battery is on its way out or am I missing something
cheers guys  

[Kevin Wood]

I would say during charging you should see 13.8 - 14.4 volts. What you see is a little on the low side but not desperate.

What I would do is to charge the battery - using a mains charger, for example, so that you know it is fully charged. Let it rest for a few hours and then measure the terminal voltage, which should be over 12.5, and then switch on, say, the headlights and watch the voltage over a period of a few minutes. The voltage will drop a little but if it drops to 12 volts and below the battery has had it.

If the battery appears OK it's time to check for something draining it but, as you say, if it's dropping to 11.8v overnight it would be flat after another day standing.

The ABS is probably seeing low supply voltage as it does a self test when vehicle motion is first detected. Giving a little time for the battery voltage to get dragged up by the alternator is probably helping here.

Kevin

[Grumpy]

If the battery is dropping that much, and you're sure you don't
have a current drain, then I would say it was on it's last legs.

Funnily enough, I've just changed my old diesel Merc battery
today for the same reason. It wouldn't hold a charge for more
than 2 days and I was getting 14V with the engine running.

Mind you, I've had the battery for over 9 years now. So I think
I've had my moneys worth.