Been done to death I know, but here's something unexpected. With my 2008 E320 CDI AvantGarde saloon I have been plagued for years with a constant current drain that proved elusive. MB main dealer and an independent specialist showed the resting drain was below 50mA and shrugged their shoulders - "no fault found." Yet both the original battery and its new replacement still went flat in 7 to 10 days. The car lived constantly on a mains charger, later changed to a solar panel resting on the dashboard. Carried a battery starter pack around in the boot as well, just in case. Lately, my son bought me a very handy gadget - a battery monitor, made by CTEK, that links by Bluetooth to a mobile phone. It records charge capacity data every 5 minutes which you can download to your phone. So, you can see on your phone a graph of battery charge status over time - for hours, days or weeks. Very handy as it turned out. At last I could see what was happening - 10% loss of charge per day. The leak always started 2 to 4 hours after the car went to sleep. I went round unplugging everything I could think of - all the usual culprits, watching what happened to the charge graph overnight. After a lot of unplugging and plenty of patience, it turned out to be the Navigation DVD drive in the boot that was quietly and steadily leaking current to earth, only starting several hours after system shut down. It's staying unplugged. For the first time in years, the battery now stays charged for weeks, without any external charger or solar panel connected. I'll investigate if it can be repaired or maybe I'll chance a second hand one. I don't know if a replacement would need coding in to the car or not. But regardless, the long term battery drain is gone!