Battery Charge Drops As Revs Rise

[TheDocmeister]

I had a break down last night & had to call out the RAC. The van just cut out, but strangely I only got a couple of turns of the ignition/ starter before it lost enough power to turn the starter motor.
It started immediately from a jump pack (after a 'fun' 2hrs sitting at the side of a 'live lane' in the dark with only hazards on!) and drove 8miles home.
No alternator red light on the dash before it cut out, but it's staying on for a wee bit until you blip the throttle at start up. More unusually, the 13.7v measured across the battery terminals at idle slowly drops as you rev the engine - down to 12.5v at one point soon after it started back up.
It was dark & very wet (of course!) last night so I really only checked the obvious connections I could see by mobile phone light! I left the battery on charge overnight. Weather's looking better today, so if I can get home this afternoon what should I be checking? Any advice much appreciated.
I should also point out that the hazzards kept going for the 2hrs despite not enough power to turn the starter and the interior light didn't seem to dim excessively when I was attempting to crank the engine.

[bigbadbob76]

Sounds like the diodes in your alternator have short circuited.
If this happens your battery discharges through the alternator.
I'd disconnect the alternator from the battery ASAP although it may have already killed your battery.
I'd be replacing the alternator and probably the battery.

[colinthefox]

It's worth trying a new voltage regulator/brush pack. They often pack up when there's a heavy load on the alternator. It's a simple DIY job without removing the alternator. Usually just two screws to remove and replace. A new unit is only about a tenner. If it cures your problem, that's a lot cheaper than a new alternator. If it doesn't you have a spare regulator to keep in your van for next time.

Brickwerks have them.

[TheDocmeister]

Well, I packed a spare battery and went for a drive last night to see if I could provoke the same symptoms - and after an hour's kicking about, nothing!
Battery appears to be charging from idle - no red light on the dash. Across the battery at idle it was reading 13.8v, when revved this went up to 14.1v.
Switched on, switched off, ran with full lights, wipers and stereo on - no problem whatsoever...
Hoping this has just been a one of glitch, but will obviously now have something else to be keeping an eye on whilst out and about!
I think I'll order up the regulator/brushes just to have at hand.
Thanks for the advice

[colinthefox]

If you get a new regulator/brush pack, then it's a very good idea to fit it straight away. You can do it at home, not in a cold dark wet layby. Once the old one has failed (and it's almost always the brushes that wear out) the brushes sparking under load will erode the slip rings and make them rough, wearing out the new brushes quicker than they need to.

You will still have the old one to carry as a spare that way but you can be confident you won't need it for a very long time!