Every modern vehicle draws some current from the battery while parked — the clock, the alarm, the body control module, the radio memory. This is normal and called 'parasitic draw' or 'key-off current.'
Normal parasitic draw is 20–80 milliamps. A healthy battery handles weeks of this without dying. Above 100 milliamps, the battery drains faster than the alternator can recharge during short trips, leading to chronic dead-battery situations.
Common excessive-draw culprits: trunk light or glove box light stuck on, aftermarket alarm or stereo wired incorrectly, faulty body control module, stuck relay in the fuse box, dome light circuit short.
If your battery dies every few days even after a full charge, you likely have excessive parasitic draw. A multimeter measurement at the battery (with the vehicle in deep sleep mode, often 30–60 minutes after locking) reveals the culprit.
Hybrid and EV 12V auxiliary batteries are especially sensitive — these batteries are small and any excess draw kills them fast.