I've posted many a message so far and thanks everyone for all your advice. But my dead battery problem will not go away.

The summary:
Starting about a year and a half ago, my car battery died and the saga began. Since then, I estimate to have replaced the battery at least 5 times, with countless chargings in between. Jumping the battery works sometimes but not always, often the battery would have to be taken out and over to Sears for a charging. Any time the car gets a fully charged battery dropped in, it works like a charm, but leave the battery for 5 days without running the car and it's a goner. I've taken it to one independent mechanic as well as to the Volvo dealer, neither of which could find anything wrong.

What we've ruled out:
faulty batteries
the alternator (new as of about a year ago and tests okay)
the charging system has been tested and is good.
attaching a voltometer to the battery doesn't return any draws

I had started disconnecting the battery whenever I parked the car overnight, and that was the only thing that kept the battery from going dead. Which seems like it points to a battery drain, but neither the mechanic nor the Volvo mechanics could find one.

HELP!! I'm at my wits end. My only option seems to be to sell the car just to be rid of this problem, but I can't really even do that until I know what the problem is, who's going to buy a car that doesn't start?