Battery or Alternator??

I've searched and read regarding both, but would like some input please. I don't drive my C4S much...when I started her Friday (08/01), she turned over slowly - it had been almost 2 weeks since I'd driven her. The voltmeter is just under 12, no warning lights/message, so I make a 20 minute trip, stop to get gas, get back in she starts up just as slow as before - meter stays about the same. I make two more stops where she's off for about 1 or so hours each time. She starts each time (still no warning lights or message). I'm 30 miles from home, but close to a friends place so I stop and check the battery - almost totally dry in each cell!!! I top battery off with water and we hook up a slow steady charger with battery still in car (didn't disconnect) for about 20 minutes -I take the charger off get in, and she wont turn over - pull the battery and take it to Autozone, they put in on the tester and it says to charge battery then retest... I wait about 30 minutes and they say battery retested good - great, I put the battery back in and she cranks up no prob..still no warning lights, etc...voltmeter is between 12 &13 (IIRC) - I take off into the night and notice the one-touch function on my windows isn't working...after about 4 minutes driving, the battery light comes on and I get onboard message warning of battery/alternator probs.. Battery is original Porsche equip. - didn't look at the date, but, given the above scenario, would your best educated guess be battery or alternator? Thanks!!

Battery is KO'd I would add electolyte rather than water. If the alternator was bad, the warning light would come on.

Voltmeter should be reading higher. Like almost 14. Charged battery at autozone runs down later because alternator not keeping it charged. Search voltage regulator you may get away with just changing that.

Sounds like a battery to me.Lots of options other than a Porsche battery for a replacement.

Diehard.....not the movie. J

I just had the same signs on my TT. No warning lights gage reading around 12V and slow battery. It was the alternator on mine.

Thank you all for your varied responses... In theory, if it is the battery, would it show up as a bad battery if I take it out now & have it re-tested? The most puzzling part is although the battery was slow to crank the car on it's initial fire-up Friday (and all subsequent starts), I never got a warning light or message indicating battery/alternator problems...When I drove my car to my friends to check the battery, that was the last start and drive of the day I made without any warning light/message - afer inspecting the battery and seeing it was very low on water, I added water and charged the battery in the car for 20 minutes - it was then when I tried to crank her, it didn't have the amps to turn over. The ensuing trip to Autozone & the test/charge/re-test all occured immediately before I hooked up what I had been told was a good battery with a fresh charge, drove 5 minutes and got the light and message. So far the tally is 3 say battery 1 says voltage reg. 1 says alternator - anyone else?

What happened to testing? Test the at the alternator for 14v output.

I'd say the avg. life of a battery is 5 years. If the battery is 4 - 5 years old, there is nothing wrong with replacing it. After doing so, I would suggest having the charging system checked and see if there is an issue. When checking an Alt., you will check the volts, amps, and if there are any diode issues. One other suggestion, check the connections to the battery and related ground connections.

When I said alternator/Voltage regulator I meant that most bad alternators are done in by the voltage regulator that is attached to it.Either the brushes fail or the spings inside become fouled with the dust the brushes make. I have owned a lot of german cars and the bosch product does this. I would just change the voltge regulator if I could get one or even just change the brushes(harder to get seperatly) you still have to pull the alternator to do this. The volt meter should go to almost 14 after you start a car that has sat for a couple of days.The problem is that a sick alternator will kill a 3yr old batter and after a few complete discharges you have a very tired battery with little life left. Winter is comming!!

If the alternator was bad, the car would not run more than so many minutes. The amp draw from the computer, AC, and other items would drain a brand new battery quickly. The battery can be weak and the warning light NOT come on because the alternator is supporting system.

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+1 -With the car running,the battery shoud read ~14.4v on the multimeter. -With the car off and 'rested', the battery should read ~12.7-13.x volts. If the battery reads ~14.4v with the car running, then it would be the battery that is bad. Don't forget to clean the battery post and cables for a solid connection.

1. The voltage regulator is part of the generator/alternator. It is usually the part that fails but since it's hard to get a replacment and you have to remove the alternator anyway most people just replace the full unit (alternator). 2. The advice to pull the negative battery terminal to test the alternator is bad. This is an out dated technique that can damage modern cars electronics. It is specifically called out in the fsm. 3. The advice that the car would not run more than a few miles with a bad alternator is also bad. As long as the battery has a sufficient charge you can drive quite a while. It will eventually fail and leave you stranded but it will go for a while. In my experince "a while" is about 15 miles or so in stop and go traffic. I had a bad alternator and watched the battery drain each day on my drive to work while I waited for a replacement part. I charged the battery before each drive and could have continued doing that forever if I had the patience. 4. As other suggested you should check for the charging behavior of the alternator to confirm it has failed. See springgeyser's post for voltage readings. From what you've said it sounds like a good battery and a bad alternator. The low battery levels were just coincidence.

Before you say it will go a long way, what car are you speaking of?

Thx again for all responses - Springgeyser I'm a bit nervous to drive the car more than 10 minutes and there's no indy or P-Car dealer within that time frame...so, can i get my alternator load tested at a shop like (gasp) autozone? Cory , I thought the same thing - I'd been warned not to take that chance in this car..but thx Gotta love 107* heat and wrenching in the back of a P-car!!

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It's bad for electronics because the ground would be floating if you pull the negative terminal of the battery. The best way is to just take a simple volt meter and measure voltages like springgeyser mentioned.

Battery test - load test and ~12 volts static after a full charge. Alternator test - I myself would look for readings above 13.5 volts when car running.

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2001 C4. 15 mile commute over approximately 35-45 minutes in the winter. I could watch the charge on the battery drain on my gauge. I think I did this a total of four times over two days while waiting for the part.

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Well ok, summer would of been a different outcome, would we not agree?

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If a cell falls and "grounds" the battery out, you have a short.

So what was the outcome??

Hey folks - it's the alternator...I'm going to attempt to pull it myself tonight, I've found these instructions for Secondary Air Injection removal. http://p-car.com/996/diy/sai/mainpage.html To anyone who has ever removed their alternator , are all of these steps necessary to remove the alternator? Anything to suggest/add to these instructions? Thx again for all the help

Yes, I will add one part of the instruction that will make your life easier. When you loosen the bolt that goes through the idler pulley, back it off a couple of turns and then tap the bolt head with a hammer until the bolt is flush with the pulley. This will drive the bushing in the alternator back a little to allow clearance. In the instructions they say it will be a little tight, if you do this, it will come out very easy. Remember the wire are back there though.

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Thanks Cory, how much clearance should this yield (how much is too far?)

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If you back it off a couple of turns or so and tap the bolt head flush (back against the pulley), that is good enough. You are just trying to make a little clearance. When you retighten the bolt, the bushing will clamp back around the bracket it is affixed to and the gap will be closed.

Tippy's advice is dead on in terms of loosening things up. You shouldn't need a pry bar as described in those instructions if you follow his advice.

Thanks again - I'm not the most mechanically inclined, I'll update when I get it pulled (which is looking like Thursday) FWIW, I'm having my OEM rebuilt here in Dallas

Okay, got it pulled - thanks again Cory for that helpful hint - I remembered to tap the bolt head flush - but didn't remember to retighten it!!! D'oh! I studied the instructions I posted above from P-Car.com, and managed to skip a lot of steps. After pulling the airbox, I only removed a few vacuum hoses, a bracket and a total of 6 or so bolts/nuts...If I can do this, I would suggest anyone with the time and even a slight bit of wrench turning saavy, this is a DIY

Now that it's out just change the voltage regulator!

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I have never changed a 996 alternator, but isn't it an internal regulator?

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ahhh, thanks but it's at the shop getting whatever is needed to fix

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it's bolted onto the back in fact