Dumb question (?) - Should battery charge INCREASE as you drive?

Trying to chase down and figure out an electrical issue. Short version question: Should i expect my battery charge to increase as i drive, because the alternator is running the battery? It doesn't seem to. In fact, it seems to drain a bit. Does this indicate a potentially bad alternator? Longer version: i have a 2001 996 cab, and have noticed a couple of issues i think reflect a weakening battery or bad alternator. On occasion, the convertible top button will "click" but not actuate the motor. Simultaneously the rear view camera will flicker or cut out entirely. I brought the car to Napa autoparts and had a battery load tester hooked up to it, and it seems to be OK.... though I did the test myself, and am not 100% i did it right. Car was NOT running, which i think is right. I have been watching the battery charge level and today when i fired the car up for the first time in a week, the battery was nearly @ 14. After driving around for a while, it was closer to 13. Shouldn't it stay @ 14 or go up as the alternator charges the battery?

"After driving around for a while, it was closer to 13. Shouldn't it stay @ 14 or go up as the alternator charges the battery?" You have answered your own question. The voltmeter should read 13.5v-14v whenever the engine is running and with some load such as a/c and low beam ON. If it drops below 13.5v, it won't be able to charge the battery properly. Your problem most likely is either the alternator (or the voltage regulator) or a bad y-cable. To pinpoint which one, one will have to check the output voltage at the back of the alternator directly.

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if the battery is in a low charge state - the voltage reading on the dash would be closer to 14 at 2k+ rpm . When battery is charged this could drop to closer to 13.5v - so that is normal. If if you have a volt meter - measure battery voltage at the battery - after car has been sitting overnight. Then measure voltage at idle. The drive for 30 mins and measure idle and off voltage again - post results here. also measure voltage at idle but with high beams , seat heaters , rear heater screen and fan on max ....(i.e. Electrical load) I had alternator diode problem that was causing battery to drain overnight - but voltage guage reading with car running was normal . your camera and roof issue are more likely to be bad ground connection somewhere ...

The voltage regulator is supposed to keep the voltage at 13.5ish volts to charge the battery, take care of electrically-powered accessories, and keep the ignition process going. The alternator should be capable of producing more electrical energy than is required to keep the coil packs doing their thing, powering headlights, fans, dash electrics, the stereo system, AND charging the battery to ensure it can start the car the next time it is expected to do so. If the car is siphoning power from the battery while running, the alternator or voltage regulator are not functioning properly or the cabling is preventing current from going where it's supposed to go.

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Based on my experience with my Turbo over the years -- since 2009 -- what you see is normal. The meter/gage shows battery voltage. After engine start the battery voltage will be high, nearly 14V. That is without any extra electrical load. Turn on the head lights, turn on the A/C, and the battery voltage level drops some as some voltage is directed away from the battery to the car's electrical system to satisfy the increased electrical load. If you continue to drive with no extra electrical load after a while -- 15 minutes or so -- the battery voltage level will begin to drop some as the battery gets topped up. Normal. Can't help you with the top other than you have to be sure the parking brake is set. Have no idea about the rear view camera. Wait? Rear view camera? That's not a factory option is it? If it is not then someone's been at the car's electrical system and often that ain't good.

first I would go to another auto part store and get a test that says, Battery is good, alt is good, theres a definite answer from testers, not seems

If the battery is charging normally - i.e. you have been driving the car for weeks and not had to use a maintainer/charger - and operating the top causes other electrical devices to brown out, it sounds like the top actuator is drawing too much current.

The symptoms sound more like a current supply voltage drop than a charging issues. One weak spot as noted prev is the charge line from the alt to the starter. This is a weak link in the system and may cause many different failure modes as it won't provide good current supply under high load conditions. Bench testing the alt is the best way to go. And a hydrometer test of the batt condition will provide the most info on the state of the battery. Do those first, and then I recommend changing that heavy cable from alt to starter and see what changes you find.

Excellent comments and thoughts. Thank you all. I brought the car to the auto parts store and had the battery tested. It's fine. And it also seems to be charging as I tested that with the same meter by firing the car up. I also ordered a battery meter that plugs into my lighter so I can have a clear digital read out and record when voltage seems to go down. And see if there is any correlation to temperature (which i think there may be). I will share the results of that here and continue to chase this down!

Well, here's the deal. The lighter socket is one of the many current picks after a fuse and a relay(like your top motor). So, you aren't actually getting the direct battery voltage. The lighter socket is also downstream of that big charge line I mentioned. If you don't want to do the tests recommended, or replace the parts that we KNOW cause issues, then why bring your questions here?

@docmirror - I definitely appreciate the advice. Only i don't know how to bench test the alternator. So i figured i will gather as much data as possible so that when i bring it into the Porsche mechanic in my town, he can spend his time on the most likely cause. I think it'll be telling if the voltage reading from the lighter socket varies from the voltage reading on the dash. I'm not sure if the voltage reading on the dash goes through the same path as the voltage to the cigarette lighter though.... So maybe i'm not learning anything. But for $7, i figured a voltage meter @ the cigarette lighter MAY give me more info before i start spending real money to chase this down w a professional.

I hope you are getting the innova battery tester plugged into the cig lighter. The voltage displayed will be very similar to the voltage you measure from the battery in most cases. Of course if you have a loose battery connection or some wiring issues between the battery and the cluster, the voltage will be different but those are less likely cases. The most likely cases are the alternator and the y-cable (if you confirm the battery is good and charged).

As an update, the car is in the shop, and they've discovered the battery in the car is actually too small. Not sure how much smaller than the car requires, but it's not the one that should be in there. They didn't find any voltage drop between the battery and alternator, so hopefully swapping the battery fixes it!

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I know a few 996/997 owners with a smaller battery installed (usually size H6/48) and they don't have problems because of the battery size. Make sure the shop installs the correct size (H7/94R). So unless there is some very strange size battery currently installed, I'd be skeptical that the shop diagnosed your problem correctly. "Improper Battery Size" doesn't sound like a thorough diagnosis to me. Let us know.

Lol I love parts changers. Looking forward to part two of this saga.

So I don't think it's the battery. I swapped out the old Boxster size battery for a new one of the appropriate size. Curious what people think of the following (a) current assessment, and (b) proposed solution: 1. Assessment Voltage still drops after the car gets warm, though not as much as with the older battery. My mechanic said he drove the car 10 miles or so then checked voltage drop, and didn't see any, but that's because he didn't get it warm enough. Having read a bunch of message boards it seems when others have had this problem (and others have said this heat-related voltage drop is a not-uncommon problem in 996s, even 997s...) it is a result of a bad alternator cable (from alternator to starter) or bad voltage regulator. And even a bad "ground strap". 2. Proposed solution I'm going to swap out the alternator cable and the voltage regulator. If i can figure out what a "ground strap" is, i'll do that too! Will post what happens http://www.renntech.org/forums/topic...e-low-voltage/

As you have already considered: bad cable, bad regulator, bad ground strap MAY be culprits too. Any plans to load test the ALTERNATOR NOW/FIRST before swapping out above? Almost all parts stores do this for free. P.S. My 2 cents (more for future reference): battery and alternator could (should?) have been tested at the same time as THE first course of action.

I agree, most battery testers have a alternator tester with a carbon pile to load the alternator and check the amp output. I believe the 996 output is 120 amps. should be capable of 110 amps at 1200 rpm's.

Look in front of your right rear wheel. There is a large fat wire going from the engine block to the chassis. That is your ground strap.

Yes, alternator and battery were tested.