Suddenly losing electrical power, but after jump start, test the battery and alternator, they seem fine

Question

99 corolla seca.

OK so first time it happened i was driving and i suddenly lost power (electrics / everything else) it just kind of putters a little and then that's it.

I called roadside assist and we jumped her and tested everything. and the numbers looked good. figured it was a buildup under the battery terminal possibly causing the problem, we gave that a good clean.

a couple of days later i was paranoid so i got the multimeter out just to check again while it was in the driveway, 12.8v without the engine turning over and 14.1V with the engine on. seemed nominal.

well today it just happened again, roadside assist looked at it and everything seemed fine again. perhaps it's something to do with the heat? (summer here atm) both times i had the AC running with the fans on. I'm a little stumped, perhaps the alternator is failing but only after warming up...

so i thought it was probably the battery so i replaced the battery, and with a new battery in the car while i was driving initially i read 13.9 volts at the lighter with a multimeter

but shortly after for a while it droped to 12.3 which prompted me to go home, in the driveway though it went back to 13.9

does anyone know if perhaps the charging system is realising the battery is full and just disengaging the alternator for that time? or does this indicate i have a definite issue with the alternator?

Answer 1

According to Wikipedia, open-circuit voltage at full discharge is 11.7V for a six-cell lead-acid battery. Thus, at 12.3V, it should have enough juice to crank the engine. You did not say anything about the alternator charging light, but you should probably have seen it if you are having alternator problems. Have you measured the battery voltage when the problem that you're unable to start the car occurred?

Are you certain that this is a battery issue? I.e. tried to crank many times without jump start and failed to start, and then tried to crank with jump start and it started successfully? I replaced once a battery on an old car before realizing the problem was the engine control unit relay starting to fail.

What happens if you remove the alternator fuse? Does the alternator warning light come on? In theory, there might be an issue with the warning light itself, so this can be used to test whether the warning light is working. Does the warning light come on before you start the engine?

The charging system should never allow the voltage to go as low as 12.3 volts. There is no mechanism to decide if the battery is full other than a sane charging voltage which does not overcharge the battery. So, the symptoms you're seeing indicate alternator problems, but I'm somewhat surprised that you didn't say anything about the alternator warning light. On more modern cars, the charging system might allow the battery to be somewhat discharged normally and then charge it quickly with a high voltage when engine braking. However, even then 12.3V sounds too low.

Answer 2

The symptoms seem consistent with the alternator belt slipping under medium-high loads.

You can test this in a couple of ways:

• turn the A/C on (to simulate higher load), pop the hood and see if the belt is slipping

• measure battery voltage with the A/C on vs off. If the battery voltage drops with the A/C on, this would confirm that the belt is slipping

I had a similar issue a while back. Hopefully your resolution is somewhat straightforward.