Battery? Alternator? Something else?

Hi guys. I have a 02 996 Turbo I am having some electrical troubles with. Last week we had a larger snowfall (20 cm) and I took the car out for a spin. For a short while, went through deeper snow like that before I had the paved road. Got stuck a few times, so involved a bit of going back and forth. After about five minutes, a ABS warning light goes off. But then it starts flickering. I shut the car down and tried to start it back up, but was unable to. I just heard a clicking sound as I tried to crank it over. Someone jumpstarted me which started the car, but on my three minute drive home things did not look well. The voltage gauge on the car was very low (10-11V), and I continued to get flickering of the lights on my lower instruments panel, and also my climate control and radio. Several other lights went off, I don't recall which. I made it home, and hooked up the car to a trickle charger for quite some time. I have a Optima Yellowtop battery with a Optima charger. When I started up the car the next day a error light went off. I hooked up my Durametric and i was P0562 - Porsche fault code 107 - Supply Voltage. With the car running, the Durametric read a supply voltage of 12.04V. I took the battery out and took it to the shop. The first shop I tested it said it was bad and should be replaced. So I took it to the place I originally bought it from, and after testing it they said it was good? So, I thought maybe my alternator was shot. I took the car to an car electronics repair shop today. Hooked up to the trickle charger for three days. Started up fine, but voltage gauge reads about 12V. Tends to jump a little bit above it to 12.5V, and then drops down to about 11.5V. They ran some tests and said that apparently my alternator is working fine, but that the voltage on my battery was low. I drove home, again no warning lights. Just fired it up recently and a warning light came on again "Battery Generator Warning". Voltage was down to about 11.5V as I started it up. After a few minutes of driving voltage went to just above 12V and the warning light turned off. The voltage never goes above 12.5V on the gauge. What am I missing here? Clearly something is not right.

Battery is KAPUT!

There are a couple of deciding factors you need to determine before spending any money. The tests below are pretty basic as load testing (preferable) alternators and batteries requires specialised equipment. Low voltages can be caused by a weak battery, faulty alternator or failing voltage regulator. Your fluctuating voltage gauge reading 12.5v - 11.5v points to a possible voltage regulator problem. 1) Disconnect the battery and connect to your charger for about 8 hours. Disconnect from the charger and leave for (say) 30 minutes - your voltage should read 12.6v for a fully charged battery. Less than 12.3v and your battery is compromised, less than 12v and you have lost a cell. 2) Reconnect the battery and start the car. Connect a voltmeter across the battery terminals (don't rely on the cars voltmeter) and check the voltage. The alternator should be putting out steady 13.5v - 14.5v @ 1250rpm engine speed. If lower than 13.5v with a fully charged battery, your alternator will need to come out and be tested. The voltage regulator is mounted on the rear of the alternator and is cheap & easy to replace.

"load test" your alternator

Sounds like alternator. The alternator, even when charging a fully depleated battery, should put out well over 12.5v. I'm thinking 13-14, even when trying to drive 60+ amps into the system. Have the alternator tested for load. It's possible that the battery is also bad, but most bad batteries can still show 14v when fully charged with the car running. Unfortunately with these cars, it's possible for the battery to show a status of bad because it has been discharged to the point where an old battery could now be bad because of the failed alternator.

Its an Optima battery. They follow different rules, as in the charging needs to be precise.

I'd start with a new voltage regulator in the alternator. the brushes wear down and causes these kind of issues. cheap part and a couple of hours labor.

Just an update. I let the battery charge overnight. I then unplugged it for half an hour and checked the voltage. Reading came back at 13V. I turned on the car and read the voltage across the terminals with the car on - 12V. I took out the alternator today (which was a pain in the ***) and replaced the voltage regulator with a new one. Then I put everything back together. The voltage in the battery was now 12.7V before turning the car on. When I turn the car on, the voltage across the terminals reads 12.1V. What am I missing? Could it be something else in the alternator, or something else? Is there any way I can test the alternator myself, or do I need to pull it out again and bring it somewhere? I don't want to pull it out again if its something else causing the problem. No warning lights showing up on the panel yet, but then again I haven't driven it long enough to deplete the battery. Thanks again for your help guys.

Or can the alternator be load tested without removing it? Because the last shop I took it to told me that they tested the alternator and that apparently it was fine? They just used the terminals in the back of the car to do the testing.

What is the voltage with: 1) engine off 2) upon cold start when engine is running 3) during a 20 min drive Anyone with a carbon load tester can test the alternator output at the back of the alternator (or the positive jump start terminal in the engine bay). If one of the diodes is bad in the alternator, it's output will stay low despite a good regulator. One can do a ripple test to verify that. ​​​ If the battery is 4yrs or older, you may as well replace it to eliminate it as the culprit.

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The voltage should have gone up to 13 after starting and running you car for only a couple of minutes. To be honest, yes, you can test the alternator while it's in the car. You already have done that. All that's ever needed 95% of the time is voltage. If you have voltage that's higher than the battery, it's working. If it's not, it won't charge the battery. Is there any problem with the main lead wire coming off the alternator? If you test voltage at the main lead and it's 14v, then the current is not getting to the battery. You also might want to check for proper ground. Does the warning light come on when you put your key into the ignition prior to starting you car?

Have you researched Optima battery issues? Unlike standard batteries, which degrade slowly, Optima's can (and do) fail and exhibit non-linear behaviors. I think that is what you are up against based on the disconnected battery voltage dropping far more than it should. Swap a battery from a different car and see if your problems don't go away. The alternator is supposed to supply 13.7V. With an unhealthy Optima it can't, it needs the right load.

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How does this work? I would have thought that any battery should still have 13.7V when the alternator is running unless one of the cells is actually shorted out. The potential voltage output of an alternator should be proportional to the current. As current decreases, the voltage should increase up to the limit that the regulator is set to, which is often 14V on many cars.

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This is so true! Had an Optima battery in the Mini. Worked fine one day, then the next day barely started the car. Backed car into driveway for a wash and tried to start the car to move it into the garage....nothing. Replaced with new Optima battery and all is well. Great battery, but correct that they work okay one day, then just completely give out the next. No signs of linear degradation like other batteries.

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When Optima cells fail they become enigmas, not shorts or opens, enigmas.

When Optima battery cells fail they become enigmas, not opens or shorts, enigmas. Then they confuse the alternator and the owner. Sorry about the redundant post thing.

Well OP - have you figured it out yet? Battery - right?

Just an update, it was the alternator that was shot. I tried replacing the voltage regulator first and that did not work. The battery was OK. After replacing the alternator everything is now working great again.

Glad it worked out. About six months ago, I had similar symptoms. After driving the car around for over an hour or so, I noticed the voltage dropping. I charged it with a trickle charger and then drove the Autozone where they tested the battery (it was fine) and the alternator (no good). Next day, I had the alternator replaced and everything was back to normal. I think the simple rule is that if the voltage drops when you're driving the car, it's highly likely to be an alternator issue and not a battery issue.