I had the same thing happening to my street rod. I found that my alternator was drawing 1 amp with engine/key off. That is way more than it should. From my electronic background I knew I had a 'leaking' diode. My local repair shop couldn't replace the diode pack as the alternator is one of the new very small type that mounts very low on the engine (in front of the valve cover instead of above it). I did as Jerry suggest and installed a cut off switch on the ground side of the battery. The switch is out of a battery powered golf cart. Sliver plated high current contacts. I did run a #16 wire from the negative post into the engine compartment to bypass the switch so I can hook up my battery tender.
