XSeries-forum

Back home today after a 3hr drive at 17:00 without any problem. 2 hours later, the car is totally dead bar from the nearside main beam weakly illuminating when I opened the drivers door with the key - could not then turn this off so had to disconnect the bulb. Any ideas anyone?

Bad/loose battery contacts?  Alternator output gone bad?  Battery cell failure?

What's the voltage showing when measured under the bonnet?

Richard

[quote="X5Sport"]
Bad/loose battery contacts?  Alternator output gone bad?  Battery cell failure?

What's the voltage showing when measured under the bonnet?

Richard
[/quote]

Thanks

Actually I was wrong on my timings - the car had stood ~ 5 hours (not 2) before I found it to be dead.

Voltage across the jump-start terminals was 4v at midnight and still 4v at 09:00 this morning. The battery was new just over a year ago. Anyway, I have now got the engine running via jump-starting from the wife's Clio - surprisingly, it started immediately (Clio only has small amp-hour battery and alternator amp output)

All the car systems seem to be working OK so I guess its a battery drain issue - if I can get the car to go to sleep with me inside, I'll do a voltage drop test across the fuses.
 

If the battery is below 10.6 volts then it's an internal failure.  It looks to me that 4 of the normal 6 cells have gone if you only have 4 volts. 

You should see 1.95V per cell at full discharge open circuit and 1.75V connected, hence my suggesting 4 cells are not connected, have shorted, or are faulty. A fully charged battery should show 12.2 - 12.4V (or 2.25V per cell) disconnected.

Time for a warranty claim on the battery.

Richard

Well can't find any battery drain and the car seems OK now - strange. Suspect that's not the end of the matter though!

Check alternator (no idea how) but also the terminals are tight to the battery. Mine came lose once and I couldn't get the car started.

But see if you can get battery changed under warranty to rule it out first. If it happens again you know it's not the battery. And go for the Bosch S5.

Odd.  Sounds like connections may need a bit of battery grease on them. 4V is a major drop and if it was a vehicle generated short I would have expected a battery burst.  Those are definitely a bad idea!!  Only seen it once and the mess was impressive and required large amounts of Sodium Bicarbonate to stop the steel body rotting away.

If the alternator is running correctly you should see 14.4V at the jump connections under the bonnet with the engine running at a fast idle of about 1800rpm.  Much lower indicates a poor battery state or no charging.  It should not fall below 13V when running and should rise with increased rpm as the regulator pack will then kick in.

Richard

Thanks for all the advice - noted

For what its worth I had a similar problem (overnight drain) with my last Mercedes, the dealer was stumped couldn't find a problem their only action was to replace the battery (3 in a month). Once started the car ran perfectly alternator output was spot on (it should have been on a 2 year old car) and they couldn't find a drain yet over 7/8 hrs it would kill the battery.
Eventually stuck at a hotel in Yorkshire called the AA their mechanic took a quick look and stated "Soft diode in the regulator pack" he said he had seen several others same model as mine!!!
I rang my Merc Dealer's a explained what had happened they said "bring it in and we will have a look" 2 days later they called to say car OK, when I picked it up the Service Manager (with a smile on his face) told me "we found it there was a problem with the regulator pack on the alternator. At least it was all under warranty shame we can't warranty the time and frustration.

[quote="Greydog"]
For what its worth I had a similar problem (overnight drain) with my last Mercedes, the dealer was stumped couldn't find a problem their only action was to replace the battery (3 in a month). Once started the car ran perfectly alternator output was spot on (it should have been on a 2 year old car) and they couldn't find a drain yet over 7/8 hrs it would kill the battery.
Eventually stuck at a hotel in Yorkshire called the AA their mechanic took a quick look and stated "Soft diode in the regulator pack" he said he had seen several others same model as mine!!!
I rang my Merc Dealer's a explained what had happened they said "bring it in and we will have a look" 2 days later they called to say car OK, when I picked it up the Service Manager (with a smile on his face) told me "we found it there was a problem with the regulator pack on the alternator. At least it was all under warranty shame we can't warranty the time and frustration.

Thanks - I have come to suspect a faulty diode in the regulator but wonder if this could cause an intermittent drain (the car is still OK after its little drama) - I would have thought a diode would only have an enduring good or bad state

Bin OK since  :-[