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Old 11-23-2023, 02:30 PM   #1
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Join Date: Aug 2023
Location: PNW
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98 Boxster is constantly killing battery cells

Hello 986 Forum!

I have been having trouble with my 986 completely killing new batteries after a heavy rainfall that started it all. The two front drains were clogged and some water built up underneath the battery and the terminals were wet. I cleared the clogs and dried the terminals out, but my battery was completely dead.

Now I do not mean that my battery discharges and goes to 9v, I mean that after a day of a brand new battery getting put into my 986, the battery can only get up to 6v. I bought a new battery from Costco last Friday, and didn't try to drive my 986 until Monday. When I tried to start it, the battery was dead. I tried hooking up my trickle charger to it and it said it had 0.1v. I was not able to use my Jump Starter box to even get the car started. I went back and replaced the battery with another brand new battery, and replaced it in the Boxster. It started right up and worked for the rest of the day as I was driving around town. I went to check the car later that night and the battery was dead again. Again the same problem happened to the battery the next day. I was able to charge it and get it started in the morning, but when I checked it at midnight, the battery was completely dead again, and this time only charging to 6v. I also took out the Engine Compartment Fan fuse to see if that was causing the drain when I jump started it in the morning, but It does not seem to be that.

Do any of you have an idea of what could be killing the battery so bad that its killing the battery cells of every brand new battery I put in there? This issue is driving me crazy! I also don't think Costco is just going to continue to replace my battery every week.

Some Notes:
I have had an issue for a while with the engine compartment fan always turning on to 100% when I turn the key to the on position, without the car having run prior that day. I also have a very dimly lit CEL that is not throwing any codes when I hook up my Foxwell scanner to it. I just changed the Coolant tank and the Air Temperature Sensor for the Engine Compartment fan, and noticed that one of the Air Temperature Sensor's wires looked bent and like it had a possibly wire break in the sheath. I also replaced the Climate Control LCDs and I took the radio out in preparation to install a 3rd party one. I also get the spoiler light when the car starts up, but then it goes away when I start driving. However on the last drive, the Spoiler Light turned on when I hit 70mph. I pulled off and checked the spoiler with the little switch in the fuse panel under the driver side, and this time the spoiler did not move up and down like it did previously, with the light turning on at start and turning off when starting to drive. I also still have the trunk liner/trunk light removed. I also replaced the starter when I did the coolant tank.

Last edited by EmergencyMexican; 11-23-2023 at 02:34 PM.
EmergencyMexican is offline   Reply With Quote
Old 11-24-2023, 12:20 PM   #2
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Join Date: Aug 2018
Location: Woodland Wa
Posts: 1,287
Start by going to youtube and researching "parasitic voltage draw"...
Learn how to test out your electrical system
That will help you diagnose your electrical issues.
That will at least be a starting point.
blue62 is offline   Reply With Quote
Old 11-25-2023, 02:45 PM   #3
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Join Date: Jan 2023
Location: Beaumont Texas
Posts: 63
I had a bad rectifier in my alternator running down my battery overnight. Not totally dead but run down. I put in a new alternator and problems are gone. It was also overcharging when running. It should charge at 13 to 14 volts.
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Old 11-27-2023, 01:31 PM   #4
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Location: Sanford NC
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New fully charged battery what is the voltage at the battery after 10 minutes of running? Tested the alternator output recently?
Prior '70 914, '99 986 Boxster, '01 Boxster S
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Old 01-13-2024, 11:41 PM   #5
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Location: PNW
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Hey everyone, thank you for your suggestions! The problem that was killing the battery was the voltage regulator on the alternator. I found this by hooking up my multimeter to the battery terminals when the car was running. It was reading something like 14.699 volts. I replaced the voltage regulator and the battery has not died since.

However now I have a new problem. The coolant temperature gauge light is flashing slowly, which makes sense because the engine compartment purge fan runs at 100% from the moment I turn the key, until a few minutes after the key is taken out. Also the CEL is still half lit. I bought an ebay engine compartment fan and swapped it out, but the same two issues appear.

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