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 ?

[Rethw]

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