Totally dead battery

After a few weeks out of town, my battery was 100% flat. I have it on a good charger, and will replace it if it does not come up to full voltage and crank well. So, what am I in for in terms of programming and software issues?

for the 987 i think its pretty straight forward - program your radio stations, get the car to re-learn the window positions (i think its by holding the buttons down for 5-10 secs once you reach the end points) and turn the steering full lock both directions. I dont think there's anything else but it may also depend on the options that you have installed.

@eyeofthetiger is correct, the reprogramming things are minimal. I would be more concerned with the health of the battery. I would have the friendly folks at your local auto parts store give it a good testing, it may be damaged. And get yourself a CTek maintainer to use if you're not driving the car for more than a week or so. They work great and keep the battery healthy and ready to go.

If the battery was dead, don't bother with the test. Replace it. Lead acid batteries don't like to be brought down that far in charge level.

@N_Jay what method did you use to access the frunk with 100% dead battery? Or did it have enough left to use the sill switch?? After a few weeks with the doors locked, some of the systems shut down. I left my car in a storage lot for almost 3 weeks and when I went to retrieve it (in the dark), I tried the fob when I got close, but nothing happen. First thought was oh ****, dead battery. But I used the key and opened the door which triggered the anti-theft alarm and I shut it off with the fob. It started up with no problem. So I'm hoping yours wasn't 100% flat.

The battery was flat. The remote would not open the door a week ago (had to travel). No lights, not even a glow. Put the key in and it got locked in. Pulling the terminal in the fuse box, and used a jump box to open the hood. At first the battery charger didn't even want to start, so I put my jump box on and charged both in parallel for about 30 min. Then disconnected the jump box and restarted the battery charger. Ot came up to a resting voltage (charger off) of 12.6v. Not great, but atleast all the cells are working. Probably took a good bit off the life, but not enough to worry about replacing.

I like to have my battery tested at interstate battery, if you have one in your area they seem like they do a real test telling you battery health, its not like Autozone.

Very familiar with batteries, not as familiar with the Cayman 's (modern Porsche ) electronics.

You took the charger off when taking the voltage reading, which is great (to many forget to do that!). How long did you wait (with the charger off) before taking this voltage reading?

Took a reading after a drive, it was showing 12.3 with the engine cooling fan on. Was 12.3 in the AM, so will be getting a new battery. (Yes, I know that you need to wait 30 min or so after charging to dissipate any surface charge.)

Thanks for the info. Generally, I recommend waiting overnight (with the car off, of course) to check voltage. 30m is better than nothing, but voltage often drops beyond that - you need a stable reading without surface charge. Glad to hear you're replacing it. 12.3 isn't worth cheaping out on. Granted, I'm SUPER cheap, so I waited and my battery was down to 11.6 before I replaced it... so you can stretch it, but I don't recommend it. (My radar detector would show problems every time I started the car.) Yes, I'm just THAT cheap, haha. For those interested, most charts show that 11.6V is 20% SOC... yes, THAT low. JUST REPLACE IT unless you don't mind being stranded (I have 3 cars, so we can be fine with it, and I don't mind being stranded since I can pop-start with my manual).

At 11.6 you could have a bad cell, which can fail suddenly since it is being reverse charged during discharge. 30 min should be enough to get mostly past surface charge. At 12 hours you ate starting to see how it holds up to parasitic draw. Either way, I use 12.4 as my lower limit for usable, because today's batteries die very quickly at the end. One two, missed meeting or other disruption is worth tossing a battery that might have a few months left in it.

FWIW, the PO of my 987 had a battery issue (drained slowly and for a long time as the PO rarely drove the car) and the car's heated seats, homelink, headlight washer etc were turned off as the battery drained. Required me to do the infamous "handover procedure" to get those functions working again.

I don't disagree - one shouldn't wait as long as I did to replace it. I simply did because I could... I wanted to see if I ever got the weird errors others have reported, but I never did. That said, it's replaced now, so it's a non-issue. Many websites say you should wait 12-24 hours for a lead acid to fully stabilize. Eg: BU-903: How to Measure State-of-charge The most blatant error of the voltage-based SoC occurs when disturbing a battery with a charge or discharge. To get accurate readings, the battery needs… batteryuniversity.com Octopus Electrical Service – Power up your Dreams with us octopusasia.com etc That's why I recommend an overnight on a car. While this does show a drop due to parasitic draw, it's reasonably comparable to waiting for 12-24 hours in my experience. Besides which, a normal car should not be much (Optima says 100mA is signs of an electrical problem) so that draw shouldn't affect a battery much overnight, unless the battery is bad, of course... basically, it's implicitly testing the capacity of the battery... In any case, 12.4V is a good floor, and apparently others agree, eg: https://www.aa1car.com/library/2003/us20310.htm So, we're on the same page, regardless. Bottom line, a "12V" car battery is basically bad when it's below 12.4V (more importantly, it's WAY dead when it's actually at 12V!

I think the manual suggests, driving on a flat surface @ 6 mph, driving forward and in reverse several times, ( About 9 meters) so the ECU understands the car is tracking correctly. If I am crazy let me know, my wife will confirm.

Just got back from 3 days out of town. Hoping it starts in the airport garage.

12.37v after sitting 3 days. Probably wait it out a while and watch fir a good sale.

Agreed, 12.37 is relatively low (about 50% on a sealed battery, per Lead Acid Battery Voltage Chart ) I recommend Costco, if you have membership and access. Lowest price of batteries, in general. They don't go on sale that I've seen, so it makes the process easy. Walmart is my second go-to.

Yes, I usually use Costco, unless I find a really good deal (or coupon). Lots of times there are 20% off coupons on batteries. With a$200 battery it ends up less than Costco. Sometimes you stack on a deal, such as $20 off your "first" on-line order and really drop the price.

Our other cars are all on Costco / interstate batteries and really like their price / warranty. Our 987 is AAP / DieHard Plat (from the PO), However, I became a fan of AAP / DieHard as well. The PO had a 2+ year old Die Hard Platinum in the car and didn't hold charge / voltage, a few months after I bought the car. A long story short, AAP replaced the battery with a new one / under warranty.

My recollection is that lifting the door handle will "wake up" the car so that the fob will work.