I have experienced something similarin a 200 car. Turned out that one of the "brushes" in the alternator had got stuck, barely making contact with the contact surface on the rotor in the alternhator. Good contact is necessary to deliver sufficient voltage/current to magnetize the rotor for current to be generated by the alternator. I think, although I am not sure, this problem should show up as a faintly lit charging lamp, but this can be difficult to see, especially in daylight. These things are a unit with the regulator which is placed in the rear side of the alternator. Fairly easy to replace, not expensive.

If your problem instead is due to current leak when ignition is off, you should be able to diagnose this by disconnecting the ground lead (-) from the battery and measuring current between the loose lead and the battery. If more than milliamps, then you have a leak which has to be located by disconnecting fuses or different parts of the electric system. Someone here should be able to help you further with that. Look also in 700/900 FAQs (uppermost right-hand drop-down menu), the 700/900 cars are not that different from a 200 in this respect.

Gustafkalle