I posted this on the Tech help page as well but based upon my review of the site, it appears this is a unique problem:

I have a 1999 ML 430 approx 98k miles. I'm on my 3rd alternator and 4th battery. I live in a region where it gets up to 100 degrees - 104 degrees in July, Aug, Sept time frame. It seems that whenever it get's hot, the car won't start. Actually, it will start in the morning, you drive 3 miles, stop, then it won't start. Obviously, the battery and alternator have been replaced multiple times but I have a feeling that the mechanics are treating the syptom and not the cause. I have checked the charge on each battery prior to replacement and they have been dead.

Thoughts anyone?

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.

Does the car crank? Or is EVERYTHING dead?. Have they ever done an amp draw on the vehicle?

DCF - what (if anything) did you do about it?

No crank - I attributed that as a characteristic of newer cars. I have had a number of different cars that did the same thing when the battery needs replacement. At one point, it has enough juice to start then it's done. Still power but not enough to crank vs old cars that just turn over but not enough for ignition