ttodd,
No, the diode discussed above wouldn't solve the problem. Your battery drain sounds like a typical shorted diode in the alternator...other than the fact you said you tried another alternator and it did the same thing. Was it a known good alternator?
I'm not clear from your post how you have this thing wired, but it still shouldn't discharge the battery. A shorted cell can self discharge a battery so do keep that it mind. If you are sure you are disconnecting all the other loads on the battery other than the alternator, then it's either a faulty alternator, a high resistance short to somewhere between the alternator and battery, or the battery itself. This is how I see it.
It sounds like the alternator isn't wired in a conventional fashion, could this have something to do with the problem.
Stuart
