2002 Mercedes Benz ML500 Electrical Problems

Something seems to be draining the battery while it is parked overnight.

First thing (to eliminate charging system concerns), take your car to AutoZone, Advance Auto Parts or Oreilly's to have the charging system checked. They will do it for FREE. They'll test the battery alternator/regulator, and it give you an idea of the condition of the wiring and wiring connections.

Check for things such as the trunk light staying on when the trunk is closed, glove compartment light, etc.

Or you can try this method (takes a while): You will need a Digital Multimeter.

Switch your Digital Multimeter to DC AMPs. Amps is usually indicated by an "A" on the Multimeter Switch. AC is usually shown as a " " symbol and DC shown as a "-" symbol. Set the DC, AMPS scale to read mA ("Milliamps).
To test for battery drain: Switch everything in your car OFF and close all doors, trunk and glove compartment (If your car has a hood light, unplug it, because the hood needs to remain open). Disconnect just one battery lead. For example disconnect the Positive Battery Lead. Connect the Positive Multimeter Lead to the Battery Positive terminal. Make sure the Positive Lead you removed from the battery does not touch anything grounded, like the frame etc... Connect the Negative Lead from the Multimeter to the Positive Lead you removed from the Battery. You should now see current drain measured in Amps. A rule of thumb is.35A or less is acceptable. However car manufacturers vary and some will accept higher current draw than the.35A for others.35A is too much. (You might be able to find out what is acceptable for your vehicle from the dealer of service manual).
Now, allow the vehicle to sit, undisturbed for about 2-3 hours. This will allow all of the vehicle's computers to enter "sleep" mode and therefore "stop" drawing power. Note the meter reading for future reference. You now have to remove the fuses to see if you can locate the problem. HERE’S HOW: Remove a fuse from the fuse box, look at the meter. If the reading drops significantly, then something on that circuit is drawing too much amperage with the car off. Check the owner’s manual to see what’s on that circuit. Re-install the fuse. Now unplug one of the components associated with that circuit. Once you unplug the component which is at fault, the amperage reading will drop back down to the same reading (or very close to it) as it did when you pulled the fuse, then you’ve located the faulty component. Replace the component and retest.

NOTE: If the fuse box is inside of your vehicle, you will have to "create" a way to depress the door jamb switch to keep the interior light off during the test.