Re: Battery charging question.

My 981S is now a little over 5 years old with a little over 50K miles. It still has the original battery. Normally, when I'm driving, the voltage shows about 13.5v. In Sport Mode, it normally shows about 14.3v. Yesterday, during a 250 mi. round trip, I notice that voltage starting to climb. Then it seemed to fluctuate up and down for a bit. Then, about the last 100 mi. of the drive, it climbed to 14.8v and stayed there. It didn't matter whether I had all electrical accessories on or off. I checked this morning (about 12 hours after turning the car off) with the engine off, The battery shows 12.5v. There have been no warning lights or messages.

Can anyone suggest what the problem might be - maybe voltage regulator?

Damage that this voltage could cause?

At 12.5V the battery is down to around 75%.

What you are seeing could be a voltage regulator but my (limited) experience is when one goes bad the voltage level drops rather than climbs.

The voltage regulator in my 996 Turbo started acting up and the voltage level (observed via the in dash gauge) would start out ok but as I drove it would fall. Once after some time (< 1 hour if I recall) the voltage was down around 12V. It was then I booked the car in for a Porsche re-manufactured alternator. (Came with the same warranty as a new one but for less money.)

With the climbing voltage I'm more inclined to suspect the battery. And it is playing the odds. My experience is far more batteries fail compared to voltage regulators and alternators. My Boxster required 3 or 4 batteries over 16 years but not one alternator. The Turbo required about the same number of batteries -- one failed prematurely after 7 months and was replaced under warranty -- but only one alternator.

5 years ain't that old for a battery but 5 years is getting up there.

Really, I'm more inclined to believe it is the battery.

Mark, I don't understand how a 12V battery showing a reading of 12.5v is down around 75%. That would mean that fully charged, it would have go give a reading of around 16.5v. I've never seen a reading that high. I'm not sure how that's possible on a 12V battery.

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Guenter in Ontario
Mark, I don't understand how a 12V battery showing a reading of 12.5v is down around 75%. That would mean that fully charged, it would have go give a reading of around 16.5v. I've never seen a reading that high. I'm not sure how that's possible on a 12V battery.

My info is a 12V battery is made up of 6 cells, wired in series, with each cell contributing 2.2V (fully charge) to the battery's total voltage. Fully charged that is 6 * 2.2V or 13.2V fully charged. After "resting" for 12 to 24 hours the voltage level will drop to above 2.1V per cell or 12.7V.

Forgot to add: So a battery that after 12 hours resting is down to 12.5V indicates a battery with one or more cells that can't hold a charge very well. A battery never gets any better from this point.



Edited 1 time(s). Last edit at 10/15/2018 03:29PM by MarcW. (view changes)

Your voltage readings don't look that much different than mine in various situations. The first time I noticed a high voltage was on some trip with long downhill braking situations, and that was early on in the life of the car. That said, here's an excerpt from a manual of sorts I was given when I got my 13 Boxster S:
(It speaks of the coasting function, which my stick shift car doesn't have, but I think this is mostly applicable in any case.)

"Vehicle electrical system recuperation

When the accelerator pedal is released quickly, the DME control unit detects that the driver does not want the coasting function. Trailing throttle cutoff is initiated in conjunction with the vehicle electrical system recuperation in these cases.

To this end, the generator power is increased in a targeted manner by the generator regulator during braking and the recuperated energy is supplied into the starter battery. The increased generator power acts with a higher braking torque on the crankshaft of the combustion engine via the belt drive. This leads to vehicle deceleration, boosting the conventional brake system. The voltage is lowered again during acceleration and the available energy can be fed into the vehicle electrical system in order to supply the electrical consumers. This reduces the load on the combustion engine, making the full drive power available for propulsion. An intelligent algorithm in the energy management system evaluates various input variables of. the components involved, thereby allowing active coordination of every recuperation operation based on the battery charge condition and driver request."

Pedro Bonilla
1998 Boxster 986 - 314,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

---

Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna

Thanks Laz and Pedro. There were no long downhills on this drive - at least not that last 100 miles. That was relatively level driving except for one 1 mile uphill drive. We had actually stopped in NOTL where we met and had dinner with MikenOH and his wife. So the car sat for about 3 hours. After I started the car up, the voltage climbed right up to 14.8 and stayed there for the whole 100 mile drive. Headlights were about the only electrical accessory I had on. It was the first time that I remember ever seeing a reading that high.

I also wondered if the reading might have gone higher but not shown up. I wondered if Porsche capped the reading at 14.8v for some reason, like they do the coolant temperature reading that's capped at 90 C - 194 F.

I will keep an eye on it and see if there's any change.

So, Pedro, there's no danger of damaging any electricals if the voltage is running too high?

... you should confirm the reading.
Using an accurate volt meter, read the voltage at the battery with the engine off and with the engine running and compare to the car’s readout.
Although 14.8 (if real) is a bit high, it won’t cause any harm.
Normal voltage when charging is 13.7 a 14.7 volts.
Happy Boxstering,
Pedro

Pedro Bonilla
1998 Boxster 986 - 314,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

---

Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna

I'll look into checking that out.

After various issues with other cars I have had, I don't keep a battery for more than 4 years.

I see well into 14 Volts regularly, and 14.8 sounds familiar. Again, I saw these readings within the first year or so of ownership, so I wouldn't suspect the battery. Also, whenever the car will be off for more than a week, I keep it on a battery maintainer. At my last dealer visit a few months ago, I expressed concern as to the battery's age, and they said it's ok.
By the way, the lowest I've seen, and this happens regularly when turning on (but before the motor is actually running) after a long off period is in the mid to high 11s.

Minus 40 degrees... Is that Fahrenheit or Celsius?



Edited 1 time(s). Last edit at 10/15/2018 11:01AM by Laz. (view changes)

Thanks Laz. If I recall correctly, you usually drive the car with Sports Mode engaged. In that mode, I normally see 14.3v. In this case, the car had been sitting for just over a week I wasn't driving it in Sports Mode and the voltage showed around the normal 13.5v for the first half of the drive. It's on the last part where the voltage went up to 14.8. I'd think that if the battery were low from sitting, that charging would show up as high voltage at the beginning of the drive, rather than the last part.

I don't have a voltmeter, but plan to get one, or go and have the battery tested with one, as Pedro suggested, just to make sure I'm getting an accurate reading with the car's voltmeter.

I've only started using it more recently, partly as a reaction to driving my friend's GT3. I also use the PASM Sport setting more often, also because of the GT3. I haven't taken notice of voltage differences, but am now curious and will see what occurs.

Minus 40 degrees... Is that Fahrenheit or Celsius?

Guenther,

You're fine.

Keep your battery on a tender during the winter sleep, and I'm sure that you'll enjoy at least a few more seasons with it.

-- not entirely familiar with the 981 platform, but if it's like the 986, the the battery is up front, the engine is behind the driver... I would expect that it doesn't get as much heat as convention layout cars with the battery and engine up front. Plus you have a silver vehicle, so the hood temps are lower.

It's heat and combination of age that kills the battery, cold makes it perform less CCA. The roads in the 50 mile radius around you are 100% boring, leading you to look at trip computer functions on your dash to pass the time. I would expect that most healthy batteries will come to rest around the 12.5V mark.

Final answer.

Thanks for the inputs, everyone. I haven't bought a voltmeter yet. Can anyone suggest a good one to get?

The last two times I've had the Boxster out for shorter drives (15 - 30 miles), at first, the Voltage did climb to 15 and after completely getting warmed up, the voltage dropped down around 13.5 and seemed to pretty much stay there.

One other interesting observation I made. While it was still showing 15v, I decided to try activating the start/stop function (which I normally have turned off), thinking if the ECU detected the battery too low, it wouldn't shut the engine down during a stop. And, it worked normally, stopping the engine. What I did notice was that the oil pressure didn't drop to 0, it dropped to 1 psi. So I guess the electric oil pump maintains some oil flow to keep the engine lubricated for the next engine start up.

I would look for a Fluke digital multimeter on eBay. That has more than a voltmeter. The resistance function can be helpful in electrical troubleshooting.

If you are sure there has been a change in the car's behavior, if it behaves differently than when new, I would replace the battery. I personally have had 3 instances of batteries which developed an intermittent bad cell.

I was advised to charge the battery, remove it from the car and let it stand at room temperature or thereabouts for 24 hours then test the voltage with a meter. Then use the following scale to determine if it is still good. It should read if a wet cell 12.6 volts or better after standing for twenty four hours. Note there is a difference between the meaning of the readings for fully charged for an AGM (Gel Cell) and a conventional battery (wet cell) on the meter scale.

12.6 or better 100%
12.4-12.5 75%-100%
12.1-12.4 50%-75%

If it fails that test, it is obviously bad. There is also a load test which is much better and can be done by any battery shop and the battery doesn't even have to be in the car (be sure you have the radio code if a 986, you'll need it after reinstalling a battery). However, suppose the battery turns out to be fine. They won't be able to test the charging circuitry without the car if that turns out to be necessary.

You can also buy a battery and alternator tester for around $30 (Actron CP7611) but it is just a sophisticated voltmeter.

Better battery load testers put a load on the battery and see how it responds. They are much more accurate than the simple voltage test outlined above.