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The battery my 928 S4, 1987 model year, went flat over the weekend. It had never done it before over a six month time period. While I need to get the battery checked to see if there is a fault in it it’s not that old and previously had not been an issue. When I swapped the battery out and checked the volts at idle I only got 13.7 at the battery. That’s about 1 volt over a fully charged battery, so the alternator is doing something, but I’m wondering if it should be closer to 13.9 or 14.
Nominal charge voltage for a 6 cell lead-acid is 2.30-2.45 volts per cell. So, 13.7 is right at the lower end of the scale, and will not boil off the electrolyte. Try leaving the acc off, including interior lights, rev the engine to 2400 and check the voltage again. It should go up to around 14.5 or a bit more.
I think you have parasitic drain that has finally caught up with your battery charge-discharge cycle such that it will no longer hold a charge well to start the car again. Common with 928s, and a lot of threads on parasitic load problems.
To measure your parasitic drain, you will configure your multimeter for amps, highest range to start. Remove the negative battery terminal. Connect the plus(red) meter lead to the battery terminal with a clip. Connect the neg(black) meter lead to the end of the heavy cable lug with clip. This will put the meter in series with the current flow to the battery. You will need to close all doors, and hatch and wait for about 5 minutes for the timed acc circuits to end. Note down the reading. Then start pulling fuses, and keep testing the same way. Note any delta when fuses are pulled, and what they serve. Most likely culprits are int lights, radio, alarm.
To measure your parasitic drain, you will configure your multimeter for amps, highest range to start. Remove the negative battery terminal. Connect the plus(red) meter lead to the battery terminal with a clip. Connect the neg(black) meter lead to the end of the heavy cable lug with clip. This will put the meter in series with the current flow to the battery. You will need to close all doors, and hatch and wait for about 5 minutes for the timed acc circuits to end. Note down the reading. Then start pulling fuses, and keep testing the same way. Note any delta when fuses are pulled, and what they serve. Most likely culprits are int lights, radio, alarm.
Depending on how your meter is fused (if at all), it’s safer to put a thick lead in parallel to shunt meter, and disconnect it to start reading drain on meter. Otherwise there’s quite a spike through multimeter which may blow fuse at best, blow the whole meter at worst.
Just tellin ya what me and Forgotten On experienced last Sunday...hasnt moved or otherwise since getting my new/better parking ramp spot at home..and has since become a storage locker of sorts.
The factory specification on current draw, on an S4 model , is 32 milliamps.
Do the math.
Tom, if your car makes 13.7 volts at idle with the engine up to temperature, you've got an exceptional electrical system.
I think I would pass out if a stock alternator would make 14.5 volts..at any rpm...when the engine is hot. I've certainly never seen one, out of the thousands I've checked. Truthfully, I'd send my Fluke in for calibration, if I saw 14.2.
Iirc the cold voltage regulator rating is 14V or less. Less originally. Get one of the HF meters that Hacker shows and fit it with test leads. It’s a 30-amp capable meter. Put a 20 amp fuse in it and you can safely use it to test drains at the battery end. Put a voltmeter across the actual battery posts to see what the battery sees, and then in parallel with ground strap to see voltage drop there under charge.
Thanks gentlemen- something odd is going on. I had a battery charger on it last night and it showed over 80% charged last night and this morning it was flashing red indicating severely discharged.
Prior to this it was a “good one” that would happily sit for weeks on end.
05-14-2018, 12:38 PM
13.7 volts at battery at idle too low?
