While it's not a Mercedes, the method should be the same on all cars. It's an '04 VW Touareg V10 TD that has had what I thought is a battery drain since I bought it over a year ago. If it sits more than 48 hours the main battery discharges to the point it won't start the engine. Luckily it has a dual battery system so I can start it on the auxilliary battery all the time. I did put in a new main battery when I got the truck thinking after 5 years it's probably time but that didn't seem to solve the problem. I used a clamp-on ampmeter on the negative cable and it was showing a drain of from .3 to .7 amps.

Anyway I tested this now 1 year old battery and it shows to be bad, only 500CCA when it's rated for 825. I'm not sure if the constant draining and charging of the main battery killed it or it was bad off the shelf but I got a new one under warranty. I put it in the truck but before connecting the negative terminal I thought I'd try testing for current drain directly by hooking up an ampmeter between the cable and the battery. It shows barely a trickle, like .010 amps. The manual says .040 amps is the maximum allowed. Then I check with my clamp-on meter around one of the leads of the other meter and it shows .334 amps. Am I reading it wrong? Is the clamp-on meter bad? And yes I zeroed it first.

Thems some pretty fancy equipments you have thar. Your Fluke 87 will work better for you if you push the yellow button to switch it to DC amps...

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

OPPS! That may make a difference.