GregD

I noticed my car's alternator seems to be putting out 14.5 - 14.8V of change into the battery (per the gauge, while running), drops to 13.8 or so after 45 minutes...
Seemed excessive to me, suggesting a low battery, so I checked the battery itself once I got home and it said 12.35 or something.... That's 50% discharged !!!

The car sat for a bit on the dealer lot and also was not driven much (12K miles in 4+ years) before my ownership. Starts well though, no issues... I'm putting it on the charger tonight, BUT if the battery drops again, I'm Ok replacing it - 4yrs seems the norm nowadays with factory batteries...

Q is - and I searched - I did this on my CAyman S without issues, but the 991 is a different beast maybe... Do I really need to get the dealer to teach the ECU about my battery or is it BS ? Should I really invest into another power source to maintain power while I do the swap, or just do it, let the car sort if out after a few miles ? Ah - modern cars....

ieatfishburritos

I'm new to the 991 and hope you can help me get up to speed. Initially looking at your post I do not understand the concern.

- What is the normal alt charge value for a 991 and acceptable deviation? Can others chime in?
- can you help explain the 50% discharged comment?

Thank you!

GregD

Well as a 2 weeks owner, I don't know the normal charging values of a 991 but 14.8 seems damn high to me and an indication of a discharged battery that the alternator is trying *hard* to charge... That is a fingernail from the red zone on the dash display... I'd expect a normal charged battery to receive 13.5-14V from an alternator.... I could be wrong on that account, except for the fact that when I checked, my battery was indeed low at 12.35 after 1 hour of getting 14.8V (commute home), which corroborates this.

My understanding - recently confirmed by a casual read of Rob Siegel's electrical book - is that a 12V battery is made of 6x 2.1V cells so a perfect battery should show 12.6V. 12V is supposedly dead, as counterintuitive as it seems. Anything in between is 25/50/75% charged, I am guesstimating here because I'm too lazy to google it ;-) I also recall my 996 and Cayman Batteries died around 4-5 year mark so if this does not take a charge and keep it (so far so good but it's been taking 14.5V in the 'nads for 3 hours and it's still not finished per the charger) I might want to preventatively change it...

If so I was asking if I need to take precautions such as keeping a power source active, to not lose any computer settings or cause glitches... I don't want to pay the dealer $200 of labor for this, seems excessive ! By the same token if something needs a dealer reset after I mess up, I might as well not bother and let them.... Just wondering...

PCA1983

AGM batteries run at higher village than others. 14.5V is perfect. 14.8V probably indicates the battery needs proper recharging and desulphation. Be sure to get va maintainer/charger that can be set for this higher voltage, like the CTEK 4300. Otherwise, it cannot properly do the job.

GregD

I did drag my lazy butt to google, here's a value table:

100% 12.7V
75% 12.4V
50% 12.2V
25% 12.0V
Discharged 11.9V

GregD

Thanks for that - so 14.5 is normal in traffic, with no extra draw ? (no lights, seat heaters, just car and radio)
I cannot find a CteK4300 - did you mean 3300 ? I havea schumacher ones I used on my old cars but if I need a newer one, fine by me...

Either way battery at 12.35V at rest was discharged (closer to 75% than 50 but I got that more or less right). I'm gonna cross fingers it was because it sat for a couple months at the dealership... Still charging now... Gives me time to look at options and figure out if I need some sort of power source while doing a swap...or not... (to not lose settings - PCM is so counter intuitive I'd rather keep my settings, plus PSM and whatnot)

Porsche_nuts

My volt readings are more or less that same as yours GregD and my battery is 2 yrs old. I think readings are normal.

GregD

Thanks ! Might be... But my battery is NOT retaining anymore than 12.34 V and that is a dying battery... It goes as high as 12.9 after a good charge, but 30 minutes later is back down to 12.34, like a swiss clock... Long term I think this is putting a strain on the alternator and I'd rather take care of it given modern Batteries' life span and avoid having to get a jump one day (in the rain, when I'm late or course)!

Can anyone tell me if replacing the battery "conventionally" (cutting power to the car) will cause a dealer visit, or if I absolutely must keep the car powered while exchanging it ? If so is there a device you recommend ? I ordered a CTEK charger and an accessory cigarette lighter plug, but I'm not sure if those charging devices deliver a constant voltage when plugged into a cigarette lighter and no battery is on the circuit - this electricity thing is a bit of magic to me... Will that work to keep it powered or do I need a battery like device instead to maintain power ? Thx !

Dyim

I have seen 15.1 V when it is below freezing outside.

To me, it seems to run high when it is cold outside or when sport mode is turned on.

Car is almost 4 years old, 30,000miles. Parked and plugged in during winter.

pfan

No need for concern about showing 14.5v while driving. When Sport mode is selected, the charging rate automatically goes to ~14.5, so in itself it doesn't indicate low battery charge.

GSIRM3

That's what my 2015 GTS does. In normal mode voltage is 12.4-12.5. Upon braking, it will increase to around 13.5, then usually drops back to 12.5. As soon as I switch to Sport mode, voltage increases to around 14.5.

jmh981

Yep, noticed this with my Cayman and watched the gauge like a hawk, soon realized that the reading goes up as the temp goes down, often as high as 15.1.

I e-mailed the dealer, sent pics of the gauge and argued till I was blue in the face that this was not normal but always got the same response - alternator, charging is determined by many circumstances and behavior was normal. Finally got the car in and they checked everything - normal, normal, normal. That was a year ans a half ago so I guess they were right. Deleted the voltmeter from the display and worry less.....lol.

PCA1983

AGM batteries need high charging voltages, but drop to near 12V overnite b4 starting. That is normal. Relax! It's OK.

GregD

I'm afraid you are right !
Here's the deal post battery change: With the new battery in, the car reported 12.5 before starting, then the alternator did not even bother charging for a minute or so. Then it charged at 13.5V - I'd never seen 13 before with my old battery, hi 14s only.. During my commute, the charging was alternating between none (12.6 and some: 13.6, with a couple seconds into 14.5 but very briefly). Again compared to before, my car was always in the high 14 range....

So it looks like the battery is newer and fuller and needs less charging. OR not having the dealer reset the ECU, maybe it's charging wrong now ? Not sure I buy the dealer thing... Overall I think this battery is better...

Odd thing: the old battery was reading 12.19V in the car. It read 12.51 outside the car... (then again It was on the charger all day yesterday and never stopped charging - never got a full charge) So I guess once you pop the frunk to read voltage across the terminals, some electronic modules come on and drain it a little, and therefore you cannot get a good reading until disconnected...

So you could very well be right, I possibly overthought this a bit... Those batteries do drop close to 12, maybe I jumped the gun... Then again new tires, new battery (both 4 y old and suspect - especially the rock hard factory tires) and now I have a spare battery on my welder cart for when I need to run a timing light or boost a car... compared to what I spent on tires, no biggie for peace of mind... I think I'll turn that Voltage display off the gauges now ;-)

myamymy

Mine always used show around 14.2-14.3 but recently started going up and now it is around 14.8-14.9 ( always on sport mode) it is 2014 991 base. Is there a thing to be concerned? Thanks