Join Date: Aug 2006
Location: Arlington, VA
Thanked 13 Times in 11 Posts
Rep Power: 12
There appear to be two things going on. Your first question was why does an apparently disconnected battery drain? I believe you answered that yourself with the "common" wire to the alternator.
I completely agree with the other posts re: checking the condition of your batteries. Batteries lose capacity as they age, particularly if they aren't well cared for. This is not the same as "ability to hold a charge".
It's perfectly possible for a battery to appear to take a charge, and to hold at around 12.6VDC for quite a while with no load on it. However, the capacity of the same battery could be seriously compromised, so that it will only deliver a small portion of it's rated capacity before the voltage drops to unusable level.
The only way to know the condition of the battery's capacity is to give it a proper load test. The quickie load tests used by garages, and the battery condition devices which measure internal resistance (like the Micronics series) are indicative and reasonably accurate, but the only way to know for sure is to put a "20-hour" load on them and see how much energy they deliver over time.
My guess is that the batteries are shot, and you'd be well advised to:
1. be sure you've sorted out any problems in the wiring of your DC circuits;
2. replace the batteries with new ones; and
3. take very good care of them, 'cuz they ain't cheap!