Battery ok, now to Alternator

[viking64]

Hi, only had my svx for 2 mths and the red dash lights came on, chanded with revs then left me stranded at servo and would not start.

Replaced battery not knowing its age/comdition and then checked car Volts
car off - 12 V
car running - 14.2V
car running lights - 13.8 V
car running A/C on - 13.8 V

three weeks later new battery showed signs of original problem, and checked readings again and got the same as before except that with both A/C & lights (only had one or other on before)on Volts were constantly dropping at .01 V per second when car is at idle, back to 14 V when revs >1500.

Does this mean alternator is faulty, car just drawing too much, or there is a volt reg problem? (electrics are definitly not my strong point)

bruce

[ItsPeteReally]

The alternator is just fine, if it can maintain 13.8 volts across the battery when the lights are on, and the A/C too, then this is still charging the battery, and pretty strongly too .

But just 12 volts across the battery, when the engine is off is bad. I'd like to see a fully charged battery reading 12.7 volts or so. A battery reading only 12 volts is fairly deeply discharged. I'm afraid that your new battery is probably already beyond hope.

Automotive batteries do not survive repeated deep discharges. A quick Google search will quickly confirm this.

http://www.scoop0901.net/AWAKE/FAQ/Battery/carfaq.htm is a pretty good guide to batteries and their maintenance, and shows how to troubleshoot the problem that you have.

[viking64]

thanks, that was a pretty good site as it just about answered all my questions in regard to volts, anps, load etc
- just checked again as I posted the fig's from memory when I came in, and battery is at 12.6 V (so almost 2.1 per cell) when off and when running with both A/C & lights it's draining the battery by .6V every minute while engine is at idle, but ok when reved or idle with lights on or A/C but not both - so stop go city driving is risky.

[ItsPeteReally]

Glad to help.

I don't think that there's much, if anything, wrong if you've got 12.6 volts.

I'd try the parastic load test that site mentions though, just for peace of mind.

[dannmarr]

I had the same problem. It was a faulty regulator in the alternator. The voltage would gradually drop and would regain only when the engine would rev. The regulator would think the battery is charged and would not kick in untill it was really low. This also kills the battery with time. Have it checked. Good luck!