2000 986 2.7 - No low beams/wipers/cabin fan unless plugged in home trickle charger

[JSM1373]

Hello all - six year long reader, first time poster hoping the board can please help… I’ve scanned many posts and done multiple searches but can’t find anyone mentioning a similar issue. Apologies in advance it this has already been covered somewhere and I missed it!

Six years ago I inherited a 2000 Boxster 2.7 manual, the last of roughly ten different Porsches my dad owned over his life. It had just 33K miles then and as of today it has 42K, only ever driven for fun (or a few miles around the block to keep it from sitting too long). Sure its had a couple of the expected/normal issues like a leaky coolant tank but overall been extremely reliable and equally fun! It isn’t the fastest or most valuable car either of us owned but easily one of the most *fun*. Even at low speeds it always puts a smile on my face.

He always kept it on a battery tender trickle charger so I have too. In 2019 when near freezing the alarm would randomly start honking overnight - a buddy with a 911 suggested a new battery and sure enough that did the trick.

A few months ago, at the same time - the low beams & foglights, windshield wipers, and climate control fan all stopped working - but ONLY when standalone on its own power, with engine on or off. When plugged into the home power via trickle - all of those things work perfectly. Strange that when unplugged each setting on the light switch/stalk work *but* the low beams and fog lights… Parking lights, signals, high beams, even the dim low beams with parking lights on - everything except the full low beams.

Last week I finally got a new OEM battery from the local dealer (in other cars I probably wouldn’t get an OEM battery but dad always did so I paid (overpaid?) retail from the Porsche dealer. Like six years ago, I swapped the batteries myself thinking surely that would fix it — and no dice… While the new battery clearly charges much quicker than the old one, same issue - everything works perfectly when plugged in - but no wipers, low beams/fogs, or fan when unplugged.

I thought maybe a bad alternator - but after an hour long drive the other day the battery was still green light full when replugged (which I’ve almost never seen) so seems the alternator is charging the new battery - and the three features work perfectly with engine off and trickle plugged in… While the stickers have changed, the numbers on both new and 6 year old batteries seem identical so I believe the dealer gave me the right model battery.

If those things also didn’t work when plugged in I’d assume wiring or a fuse and have to take it to the local independent shop or dealer - but can’t wrap my mind around everything working perfectly on home power just not standalone…

Thank you very much in advance if anyone has experienced or heard of something similar! And please ask if I’ve left out any relevant details…