Elau, if its the original battery, I concur with the aforementioned posters.

However, if the battery has been replaced, I fail to see how you can condemn the battery without further testing.

The vehicle may have a slight electrical discharge that will weaken the battery if it sits for several days.

The first thing to do, IMHO, is to fully charge the battery and take it to a shop and have them put it under a load test. This will show the available CCA for the battery and will tell you, immediately, if the battery is the culprit.

I'm not at all familiar with the issues on the C class vehicles, however, a very good battery for the diesels can be obtained at AutoZone for $70.00. I'd hate to shell out over $200. for a dealer to do it.