I had the alarm come up aux battery fault, changed the battery. Could not fault the old battery, stored it for 2 years still had 12 volts charged it and fitted it, all ok, second battery can not fault, now in storage for the next time.This saga sounds to me like a fault with the auxiliary battery rather than the main battery. The auxiliary battery is used to power various electrical consumers when Eco mode ie start/stop is active and the engine has shut down, leaving the main battery to deal with starting the engine. Any problem with the auxiliary battery will result in Eco/Start Stop being disabled, and potentially some or all of a variety of unrelated systems that rely on the auxiliary battery eg Distronic Plus, Active Driving Assistance system and Hold function also being disabled. This was a (very) common issue with the W212 E class and was due to a fault with the auxiliary battery itself. The fix was to replace the auxiliary battery. So hopefully this was what has happened with you and that the car is running - or should I say not running - as it should.