Chances are the battery is the symptom and the alternator/voltage regulator are the casue. I have been through multiple batteries in my 98 430, and mine only has 60k on it. For some reason the trigger voltage for the thing to start charging is off in these models. If you ever notice, if you park in your driveway at night with the lights on and engine running, the car will go through wild voltage swings (watch the headlights against the wall). I have an after market stereo in mine with a dash nav. Sometimes after driving on the highway, when I exit (turn signal on) as I get to the top of the ramp, the screen will pulse in time with the turn signal. The reason is that the vr has shut down the alternator (since it was making good voltage when at speed) and the system is drawing it down. Basically, the system switches from "on" to "off" way to slowly. This can easily cook a battery, as it will, from time to time, just keep trying to jam voltage into the battery.

Also, from my personal experience, germans make great cars, but the worst batteries ever. I have an old Porsche 911, that needs a new battery every (or every other) year. Even though all german cars use giant batteries, they all go bad real quick. Another case in point: my brand new BMW M3 needed a new battery when it was exactly 1 year old.