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.