All single wire alternators have a parasitic drain. With the engine and key off disconnect the wire from the alt. output to the starter battery terminal. Use a meter to measure DC ma (milliamps) between the alternator and the wire you disconnected. This is the parasitic drain, often 30 milliamps or more (0.030 amps). Multiply that by hours and you will find how many ampere-hours you are draining over a period of time. Example:: Measured: 43 ma; Time: 1 week- 0.043 X 168 hours = 7.2 amp-hours. If your battery was rated 20 amp-hours, that would be a bit over 1/3 down in a week.
Three possible fixes: 1) Install & use a battery disconnect in the battery ground lead. 2) Use a correct 3 wire alternator hookup. 3) Drive the car more often!