Once again before anything else test for the glow plugs intermittently remaining on. The drain they present when is so great they can cause a problem like you describe.

This is after a proper check of the battery with a load tester after a proper total recharge. Many small chargers take a very long time to bring a battery back up.

Any voltage read across the battery terminals after a proper charge that reads less than 12.6-12.75 volts. Either the battery is not properly charged yet or the battery is defective. A load test of the battery then indicates if the battery cells are able to deliver adequate current to meet the needs. This testing is available for free in many automotive parts stores.

Yes you can of course still have a bad battery. Far too many times people have not really checked the battery out properly and replaced it. Only to find out it was not the problem. Testing for the glow plug circuit intermittently coming on if the battery checks good is almost mandatory. Or would be in any 123d I own if the battery checks out.

Constant periodic draining of many batteries today drastically reduces their lifespan. This is also one heavy drain you do not want to find with the average amp meter portion of a multi meter. The maximum current they can deal with is only usually 10 amps. That is without an external shunt. Either an internal fuse in the meter would blow in my opinion or the meter could be damaged.

So just pull the battery and let an auto parts place check it out first. I was also wondering if it where really bad if there might even still be some warranty remaining.

The wiring system is in general pretty robust in these older Mercedes models as we do not seem to have as many electrical type problems as the average thirty year old cars would have in general.

Owning and learning how to use a multi meter is one of the better investments of time and little money. They can last a lifetime and are usable for many things other than cars. Saving you a fortune over the long haul.

The one thing that to me is pretty certain. Heavily draining a car battery very low many times can for all practical purposes kill them early in their life all too often as well.

We are all lucky that Germany did not copy the English lucas electrical systems. They have really earnt their title of being the prince of darkness years ago.