Battery/Generator light

I have a 2005 987 Boxster . We were on our way to a BBQ yesterday. Stopped for an hour to see my daughter, then 10 minutes at the supermarket. Steady moderate rain with ocasional heavy bursts. Started it after the supermarket and the Battery /Generator light came on. Decided to return home (about 6 miles). Had the fan, lights and wipers on but got home safely to swap cars. Put the battery on trickle charge with my Smart charger. No rain today so put my Durametric diagnostic. No obvious fault codes that I had't had before but when I started it, the light came on again almost immediately. Battery was showing 13.1v and dropped to 12.6v after I started it and ran for a couple of minutes. Battery is less than a year old and showing the green light. Does anyone have any ideas? Could the rain have something to do with it? I have read that sometimes the wiring shorts out in wet weather. Will it cure it's self when it dries out or does it sound like an alternator problem? It's kept outside but is rarely driven in rain.

To check the battery , you must let it sit idle (off the trickle charger) for a few hours and then take the voltage measurement. That will very roughly show you the SOC (state of charge). However, even this is not an ideal test - an actual load test is better. In the USA, auto parts stores will often perform this test for you, at no charge. Has the battery ever been dead? If so, that can damage it to the point of needing a replacement. Recharging and even "reconditioning" won't fix that. It could also be the alternator . Note that the ECU controls the charge voltage dynamically, so it's hard to see if it's working properly or not with just a single reading. Generally, you should see over 14v when the engine is running at some point, at least. I usually see this level soon after starting the car, but it can come down to lower values after that.

Thanks for the reply. No. The battery is only 6 months old and has always been around 3 13v. I will do the checks you suggest. Maybe disconnect the battery while the engine is running to see what happens. A replacement/reconditioned alternator is not too expensive so don't have a problem. It will go on the list of things to do this year; brake disks, leaky radiator, leaking windscreen washer container, bent bonnet after the catch released on the road, alloy refurb. Actually quite a list 🤣

I think it must be the alternator . Reading 12. 1v with the engine running. Cut out completely when I disconnected the battery . Time to go shopping I think!

Alternator , Voltage Regulator , or wiring. Not the battery .

Cutting out by disconnecting the battery is not typically a reliable test on a modern car. For one thing, the battery is a large capacitor and can cause excess noise and voltage into the electronics. But, more importantly, most modern cars will not run with a battery disconnected like that. It was fine in the old days but it's bad practice on modern cars. https://www.cartalk.com/content/do-quick-check-alternator-i-was Again, there are devices that can be used to test the car (battery and alternator)... shops have them, and maybe even parts stores (again, here in the USA, at least).

Thanks. I'll look into this. A big help

I'll double up on @schwinn 's warning - don't do that. That was fine in the days of relays and filament lights and vacuum tube radios. It's not a good thing to do with the thousands of semiconductors that are in the 987 series cars. Your alternator is dead. Kaput. Nailed to the perch. It's an ex-alternator. You need a new one. If you can find a rebuilt Bosch that would be a suitable replacement. Most other sources (can't say 100% for the UK) of "rebuilt" alternators are simply alternators out of wrecks, cleaned, sometimes painted pretty with almost no testing down to them, and if you're lucky - just the brushes replaced (many times they just paint them.) Rather than go that route I'd rather buy a used one directly from a wrecking yard (in the US almost all of them have warranties on the parts they sell.) The Bosch rebuilds in the US often look like brand new alternators. Either they're fabulous in their rebuilds, or they're simply selling new ones as used to get around exclusivity deals they made with the auto manufacturer. I've never seen a Bosch rebuilt that didn't look new. Good luck - when you have a chance, please visit: New Member Introductions and tell us all about you and your 987. Photos are always welcome!

Good "remans" should look new.

So what you're saying is that it's an ex alternator , ceased to be. It's f%#@ing snuffed it? I will certainly visit the intro page to introduce myself soon. I was waiting for a time when it was looking the way I want. Could be some time 🤣 Thanks, Simon

As I said "Nailed to the perch!" - looking forward to the resolution of your problem AND an intro message when you have a chance and it's all shined up nicely! Welcome to Planet-9, the Friendly Porsche Forum!