This is a little bit of followup to my previous post on dead batteries. After my car had drained a new battery, it could no longer hold a charge. How does that happen? Install a new battery, a few weeks later the battery would be dead. Take it in to get it tested and am told that the battery isn't holding a charge. This happened numerous times with different batteries, so it doesn't seem that any of the batteries were actually defective. Is there any malfunction in the car that could be ruining the batteries, rendering them un-chargeable within weeks?