Mercedes-Benz W204 battery drains when parked
Why does the W204's battery drain when parked?
The cause is usually a circuit or relay that stays on, so the car does not go to sleep mode. Examples are a relay feeding the engine that stayed on after rain, a radio that does not go to sleep and a short circuit in the towbar wiring. A poor earth connection or a worn battery can also go flat.
How is the parasitic drain located?
1. Connect a multimeter in series between the battery positive terminal and the cable. 2. Remove the fuses one at a time and watch at which point the current drops. 3. Wait about 5 minutes after each removal, because some devices go to sleep slowly. A thermal camera can find a warm relay in the fuse box.
Is a battery that has been drained still usable?
Draining weakens the battery, but fully drained batteries have been charged back to working condition. The resting voltage tells the charge: 12.75 V is full, 12.4 V about 70%, 12.1 V about 50%, 11.7 V about 25% and 11.5 V empty.