Cayman 987 3.4s parasitic battery drain (Alternator ?)

[Rethw]

I am having a bit of bother with this. My battery is draining down overnight, dead as a dodo. My usual Indy (not auto electrician) connected it up to his new bit of equipment to test battery etc. and it showed a slightly lower alternator performance. I then took it to an auto electrician to diagnose and he said it looks like the alternator and described a diode potentially open all the time that leads to the battery draw? He doesn't want to fix it and asked me to take the car away, which cast a bit of doubt over his diagnosis. My question is, Is it possible for an alternator, if faulty, to drain a fully charged battery overnight ?

[DH986S]

I had this issue on my Boxster and it would drain the battery in 24 hours. I used an infra red thermometer as a quick check after leaving the car overnight. The alternator was noticeably warmer than the surrounding engine components. I fitted a new alternator due to the original being over 20 years old but you can buy the diode pack on it’s own

[mikef]

Do you have Porsche Vehicle Tracking System? That can run the battery down if it’s not used for a few days. I now keep my Cayman on a CTEK charger in winter

[981Boxess]

Rethw said:

My question is, Is it possible for an alternator, if faulty, to drain a fully charged battery overnight ?

In a word, yes.

If you are in a position to do this, disconnect the alternator (leave it in place) and see if it solves your problem, if you still have the problem you have saved the money and effort.

[Rethw]

Thanks for the replies. New Bosch alternator fitted and tested seems to have done the trick !