[litteng]

i have seen you posts regarding the problem over at pelican too. to say the least it is baffling...
so lets start with the basics.

cables on battery post stif and hard or flexible, and signs of corrosion? if so replace ends and clean cable and coat with protective slime (nonconductive grease etc)

test battery voltage disconnected should be around 13.xx+ volts, let it sit for a day and recheck; still 13.xxv? if not the battery is junk get it replaced on warranty. swap it into some other vehicle and see if it goes flat there, this would indicate the battery is bad, and lastly you can take it back to the supplier and have them do a load test to confirm the battery.

charging system check with alternator running, what does porsche say? id expect 14+ v at say 3500 rpm

if as you say you cannot see any milliamp or amp draw between the ground cable and the battery or the positive cable and the battery

what recent modifications have you made to the car or what service work has been done. i remember you claim no mods, but even service work if not done correctly can screw something up... so double check this and ensure you haven't got something a foul

the biggest draw item on a car is the starter, so now with the advanced age of your "B" its time to check the cable from the alternator to the battery and the starter. check the place were they go thru the "firewall" to the front of the car, is the rubber grommet still there? any chaffing on the plastic sleeve?

thats all i can think off

i had the battery in my r1100 die over winter, if the bike sat for anymore than 1 day, the battery didn't have enough juice to start the bike, and had the same with a 2 yr old battery in a 2 yr old car, sometime you get a lemon, i know it s hard to believe but it can happen.

best of hunting and hope this is of some help


ps

ground straps, can come loose and the battery may have the juice to light the bulbs, but not to spin the starter, ive also seen the grounds straps totally corroded away, and with no ground strap no ground path no go, so this would mean checking the ground strap at the battery and down under the engine and also by the starter if there is one...