ALternator putting out 13.02 volts

KEITH KOBLITZ

MEMBER
2005 VOLKSWAGEN PASSAT
2.0L
4 CYL
2WD
AUTOMATIC
280,000 MILES

We have a diesel Wagon, the battery had drained flat and we checked output and it was 12.4 volts running, battery was made in 2015 and seems strong. So we changed the alternator, and it is only putting out 13.02 volts at rpm, 12.94 at idle. The battery light does not come on except for initial startup. It seems to me the voltage is way to low, but here is the thing. After we changed the alternator we had to jump the car to start it, jumper cables are poor, so even being jumped it barely started. After starting we checked voltage at battery and that is when we found that the output appeared low, and yet after running for maybe fifteen minutes and driving around the block with all accessories on, I shut off engine and restarted and engine cranked fine and started right up. Did this several times and each time cranked fast and started quickly. The fact that the battery was low and barely started, and then after a relatively short time running, with what appears to be a weak charge from alternator, that the car starts and cranks so well does not seem to jive. Battery terminals were cleaned and made tight, even grounded alternator directly to battery, no change in numbers. Any ideas?

1 Reply

CARADIODOC

MECHANIC
33,871 POSTS

Check the voltage again in a day or two. You are right, the charging voltage is too low, but I suspect it will come up in time. We want to see between 13.75 and 14.75 volts. If the battery was badly run down, it will take time for the acid to become conductive again to the point the battery will start to charge. As it does, the voltage will rise. It can take a good fifteen minutes before it really starts to charge. You can prove this to yourself by watching the amp meter on a small portable battery charger. If you start with a totally drained battery, (it will measure close to 12.2 volts), on a 10-amp charge rate, you will see it starts charging at often less than five amps, but during the next ten to fifteen minutes, the charge rate will go up. It will slowly come back down in a couple of hours when the battery is fully-charged, and with the charger turned off, it will measure closer to 12.6 volts.