Dead battery- what could it be? - 986 Forum - The Community for Porsche Boxster & Cayman Owners

Got to my '99 base Boxster on Monday morning to get to the airport and got nuthin' when I turned the key. Not a click and zero dashboard lights The battery got replaced 4 years ago along with the alternator. I generally drive this every 3 or 4 days, though the two times I was away for two weeks this winter, it started up right away with no complaints. I had driven the car 4 days prior to Monday with no issues. I park in a garage 2 blocks away, and there's no known history of rodents. I didn't leave the lights on, no dome light, trunk wasn't left unlatched with the bulb burning- I couldn't find anything that I had done wrong. When I get back Saturday I can pull the battery and have it tested and charged I suppose, but I've never had a battery do this without warning. Any ideas what would make this happen? Just realized it's my Boxster birthday, bought 7 years ago today!

Batteries last about 4-5 years in these cars. Most likely a bad cell.

Start with the simplest thing first. That is.... have your battery load tested. That will tell you if the battery is still good or if it needs replaced. If the battery is still good then you need to move to a parasitic draw test.

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I had the same thing happen with my 986 soon after I got it. I hooked up my battery charger, which has a repair mode, but no dice. So I bought a new AC Delco, the largest one that would fit.

The common wisdom is that AGM (absorbent glass mat) batteries withstand periods of limited use better than traditional lead acid batteries. I don't drive my Boxster every day. Some weeks I might drive it 3 times and other weeks as much as 5 times. I installed an AGM battery in my Boxster two years ago and one in my Mercedes C230 in April 2018. So far, both have been reliable. The battery in my Mercedes is older, but I usually drive that car more often than the Boxster.

Yes, batteries can go dead in an instant! Had it happen to a 1999 Z3 in 2001. Drove to my Son's school to pick him up one afternoon...no issues. We walked out of the school, go in the car and nothing! Bad battery.

The Bosch battery that came with the car I purchased 8.17... still running strong. It's held up well to this SoCal heat. Further, it takes up ~3/4 o th battery tray... so not the long battery.

9 years old... I have the original battery in 2015 Audi Q5. The car was in CA for the first 3 years and since then in MI. Car has about 65k miles and no problems starting whatsoever. It is parked for 2 weeks quite often with no interactions. Alarm is not armed. So they may last longer... As for you, what that could be? Maybe a loose connection? Can battery die all of sudden? Yes, the cell will short when you are driving and it will discharge quickly once parked. If the battery was left completely flat for more than a week, it will be a toast. No charger will bring it back to life. Yes, you could rinse it with DI water and charge it, repeat 3 times and then flood with acid. I had done it years ago. Worked. But nowadays - no access to cells. Plus getting sulfuric acid is not easy. If you think you have a leak somewhere (current leak), you could use this gauge. I chased my daughter's car issues with it. https://a.co/d/6YPXpbk

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Thanks for that, good to know! But I heard you can't use a regular charger to bring an AGM battery back to life...

Okay, new information for anybody still following… I'm pretty sure I'm on the trail of what drained the battery so quickly. I used a friend's charger and topped off the battery, and I just put it in this afternoon. I immediately heard a clicking sound like a relay going on and off every two or three seconds. Sitting in the driver's seat, it sounded like it was coming from the passenger door. I go around to the passenger side and it sounds like it's coming from the space under the dash above the passenger footwell, but I couldn't get in there to see any more because of the way it was parked in the garage, and then I ran out of time. Are there some relays up there? Any idea what my next troubleshooting move should be? Thanks once again!

In the passenger wheel well behind the wheel well liner there is a switch and a valve that are linked to the EVAP system. The clicking sound could be related to those.

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Thanks! Will look into that. But can you think of a reason that it would have been clicking on and off while the car is sitting, no key in the ignition?

New development for anybody still following! Used my pal's charger to bring the battery back to life. When connected, I immediately heard a clicking from the passenger side in the car, and it sounded like a relay or a door lock constantly locking and unlocking, once every 1 to 4 seconds at irregular intervals, whether the key was in the ignition or not, whether the car was running or not. Today I had time to try again, and re-connected the battery, then started pulling fuses. The clicking stopped when I pulled out E4, which controls the passenger seatback switch. I put the fuse back in and then moved the seatback forward and back and the clicking stopped. Great! But what caused it? I don't know. And what would keep it from happening again at random? I don't know. I had the seats out 2 winters ago to put L-seat seat covers on and may have missed something when putting things back together, thought the seats have worked fine since. I guess I have to pull that seat and snoop around. Too bad electricals are a bigger weak spot for me than all my other weak spots. Anybody have an clues?

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Thanks for the tip, but it turned out to be something under the passenger seat. Pulled the fuse for that and it stopped. Put the fuse back in, it started again, but then I hit the seat adjusters and it stopped. I have to take the seat out and see if there's anything obvious I can see but I'm not optimistic, and I don't know if I could count on it not happening again and running down the battery at the worst time, like in an airport parkign lot. I'm outta my depth with electrical stuff so might have to throw myself upon the mercy of a shop.

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Thanks for that! I won't go as far as to get the gauge- would need more background learning before i could make use of it. btw FWIW it was a 5 year old Interstate battery and a night on the charger brought it back to life (for now). It was dead for between 6 and 11 days but I don't know when the short happened. Guess I should be glad it didn't burn down the car and the garage, but I guess that's one thing fuses do. It turned out to be something under the passenger seat. Pulled the fuse for that and it stopped. Put the fuse back in, it started clicking again, but then I hit the seat adjusters and it stopped. I have to take the seat out and see if there's anything obvious I can see but I'm not optimistic, and I don't know if I could count on it not happening again and running down the battery at the worst time. Hope to get to that soon.

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Did your alternator die at the same time that day? The only other battery problem I had was when I drove around Chicago, did errands for an hour, then 20 minutes back on the highway, then it died while parked doing errands locally and I had to bump start it. In the next two days I ended up replacing the battery and the alternator, and the shop that diagnosed it said they couldn't telll which took which out. Now I wonder if in my case the alternator died first, took the battery to the grave with it, and then we replaced the battery. If I remember correctly, I think the new battery died the next day, indicating the alternator pre-deceased it. The shop charged me like $350 for what might have been a faulty diagnosos, the new battery, and installation, so then I swapped the alternator myself which was barely doable due to tight working conditions in there. But I also heard the battery and alternator often take each other out and you can't always tell which went first. I dunno, not a strong area for me..

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Thanks, good to know. For now I have revived the old battery with a friend's charger and it seems good. But I do sometimes go 5-6 days between drives so I wonder if I should look into an AGM for my next battery. But I heard that that kind of battery can't be recharged with a standard charger and you would only somehow cook it. Know anything about that? TIA.

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Most modern chargers/maintainers have a specific profile for AGM batteries as they finish at a slightly higher voltage than a conventional flooded cell battery. Again, most modern chargers have "float" profiles built in to prevent overcharging, which was common with older, non-electronic chargers.

A month ago I parked my car at a friends house as I needed the space to work on a different vehicle and noticed that the battery struggled a bit to start the engine (2016CS with original battery..), the car was parked at her house for about two weeks but left the hood latch covered with a towel just in case. Afterwards, I installed a new AGM battery and it ran great (starts the engine with much more power), however yesterday I plugged the Durametric to reset the oil service message and I noticed that I had a P1559 code on the engine module.. Could the P1559 code caused by the weak battery, and just needed to be cleared? Thank you!

Weak batteries throw all kinds of ghost codes.