Battery drain problem!

I'd check for voltage drop in the charging circuit. With the engine running and headlights etc on, check the voltage with a decent DVM from the alternator output to battery positive. Should be no more than 0.1 volts or so. Do the same from alternator body to battery negative.

Then that is the sole cause of the problem. You are never replacing the charge used to start the car so it gets less and less everytime.

My sister in law had exactly the same problem. A 20 mile run once a week prevents it.

Yes its upping the voltage which it will do but its not replacing the current that was used. This is why using the voltage as a measurement doesn't work.

Yes, that's because every day you draw say 25% of the capacity but only replace say 15%. 10 days later and you'll have a completely flat battery.

Like I said, voltage cannot be used to do anything else except say that it is getting a charge but not how much it is getting.

What's the charging voltage when the cars idling , & at 2000 rpm with the headlamps on? Have you checked the alternators earthed?

Conductance, it's a tricky & expensive thing & you need to know the battery characteristics, or specific gravity of the electrolyte. If the alternatives a new car then try a replacement alternator, that's less than a weeks depreciation.

So how do you measure the poential energy within a battery?

The car in question usually makes a couple of 100 mile round trips every week! Plus as already said our car usage has not changed since these problems began in Feb from the previous 4½ years when we have had know problems!

You've already said that you checked for drain and there is nothing wrong. If the alternator is drawing current it will show on a multimeter. Unless you have some intermittant current drain, then the battery is faulty. You said at the start that you put the battery into the van and the voltage was higher. As I've said, this means nothing. Different vehicles will maintain different voltages. Did you try putting the van battery into the Merc and see how it goes. I once had a Fiat with a dodgy cigarette lighter that was contacting and drawing current. it would heat up and move slightly but not eject. When it cooled down it would contact again. Took me a long time to figure what was wrong, as any time I put the ammeter on, there was no drain. Unless you have some weird problem like this then I would say it's a dodgy battery.

I swap the batteries over evry Sunday, The van recharges the flat battery and the car flattens the charged battery.

The problem does seem to involve some intermittent drain, what I would like is an ammeter with a log, so as I could leave it attached all night and then view what has been happening.

My next move is to see if the battery is discharged with the alternator unplugged, unfortunately it is quite difficult to get to the back of the alternator.

It is an intriguing problem, I will let the group know when I get to the bottom of it, but don't hold your breath!

Thanks to all for the help and suggestions offered!

Do you leave the ignition key in by any chance? Some cars keep pumping up abs brakes and things if the key is left in, likewise courtesy lights and things.

mrcheerful