Over the past few months on a couple of occasions I've come to restart the engine after a short run of 3-4 miles and the starter has struggled to crank the engine and in some cases failed completely - leading me to a quick jump start.

Seems to happen more often when it's hot - but never from a cold start (60F in San Diego).

The battery is about 6 years old and is probably failing - so I made some measurements to hopefully make sure.....

Idle no load 13.45V - at 2000rpm 13.49V
Idle with lights on 13.35V - at 2000rpm 13.35V
Idle with lights + heated rear window 13.2V at 2000 rpm 13.3V

at 0rpm 12V with lights on 11.6V with lights + heated rear window.

I'm thinking the alternator is charging and the battery is just weak.

But I would expect the charge voltage at the battery to be higher ?

Any comments thoughts / suggestions for a replacement battery ?

Thanks Tim

 

95% probably the battery. Many have had them go after that time period. Alternator has only been a problem with one guy that I can remember in the 2 1/2 years I have been on this board.

 

I had the same problem for 2 months, absolutely no problem with cold start, but struggles with warm start. Didnt help after changing the battery. Untill last week, when she died suddenly on the road. It turns out to be the fuel pump! Had it replaced and no problem now.
Kevin said that this is very common for 996TT.
Have it checked before it fails in the middle of the highway, which can be both dangerous and embarrassing.
My 2c.

 

Thanks Derm.
Pretty sure it's not the fuel pump by the behavoir - just a slow crank or sometimes stalling the starter motor. Plus I have the fuel pump fail during the 1st year of ownership - it was a pipe coming off in the tank as I recall.
Tending toward a new battery - any suggestions as to brand etc ?
Would like to keep it stock.

EDIT - just fitted a new battery from AutoZone - what a difference to cranking speed, starter motor didn't know what hit it.

Time will tell if all is ok long term - mean while I think I'll keep my jump start battery in the trunk, just in case.

 

Very likely that your alternator is dying. I had the same problem and thought it was dead battery at first, replaced a new one but the new battery died after a year. So I called up AAA and the guy came and ran some test. Right away he knew that the alternator is bad, beacause its not charging the battery. So I replaced a new alternator and the car fire up like when its new. Even with the dead battery your vottage shouldn't fall below 13 when your engine its runing.

 

I agree with the comment below which would indicate a failing alternator

However even at idle rpm with the lights and heated rear window on to create a significant electrical load the alternator in my case was able to keep the battery at above 13V ie 13.3V.

I was just expecting the voltage to be a little higher when I revv'ed the engine - maybe the regulator works better than what I'm used to.

None the less I'll keep a close check on the battery.

Just a shame Porsche doesn't provide a charging current ammeter as seen on 'classic' cars.

 

Same issue, suspected the alternator, even looked at voltages, etc... it was the battery.

I believe the battery, when one or more cells begin to fail, will hold the system voltage down- it will mimic a bad voltage regulator,.

6 years on the battery and you'd hesitate for a microsecond to not replace it? Hmm

A

 

suspected it was the battery all the time, but wanted to make sure the alternator was on the way out as well.

Just been out to measure the voltages under various load conditions.

12.6V with ignition off, dropping to 10.75V during cranking and the battery holds steady at 13.85V or there abouts load or no load, idle or 2000rpm.

So pretty sure the alternator / regulator is doing its thing !