when I bought the car it had to be jump started prior to drive away.
drove 200 miles, parked it, next day battery dead. installed new wmart battery, she started, at idle battery charge light remained on, a little throttle and light was out. ran her at idle for 30mins, next day, battery dead. Not sure what is pulling the battery down... Anyone have an ideal what is causing this... Alternator? voltage regulator? bad ground? What say anyone. Thanks. Seadog

 

If you live in the US go to a Harbor Freight Store and get a Digital Volt Ohm Meter. They have some for around $5 or less when on sale.
When you get it that will tell you what your actual charging voltage is.

You need to get your Battery Recharged as soon as possible.
To see if it is the Alternator at night you could disconnect the Alternator Electrical Connector (However, do not start without the Connector insatlled as it can ruin your Alternator). If your Battery does not discharge than you know the Alternator is to blame.

Another item that sometimes causes a Battery Drain is the Glow Plug Relay. You will do the same disconnect thest by removing the 6 sided connector and also either unscrewing the screw that holds a Flat Copper Wire Down or removing the Strip Fuse.
Be careful as the Strip Fuse can be brittle and only NPAP or the Mercedes Dealer is likely to have a new one.

If yo have an Automotive Amp Meter (Ammeter) you could leave everthing connected and remove a Battery Cable and connect the Amp Meter between the removed Cable and the Battery. If there is a short you should see it drawing amps on the Meter.

Watch the Meter and disconnect the Alternator Electrical Connector, do the same as I suggested above with the Glow Plug Relay. If the current flow drops when you disconnect something that is item is causing an issue.
If neither the Alternator or the Glow Plug Relay is the cause you open the Fuse Box and watch the Amp Meter as you disconnect one Fuse at a time. When the current Flow Drops when you remove a Fuse consult the Fuse chart that is in the Lid of the Fush Box for what items that Fuse protects. You will have to trouble shoot the wireing to those items.

If the Alternator is charging and you need to drive the car every time you park you should disconnect the Cable from the Negative Terminal of the Battery so it will not drain.

 

I want to repeat this back to ensure I understand and do things right.

- Start engine and disconnect the the ALT? ALT is "BAD" if battery does "NOT" discharge?

I think I have everything else... I let everyone know how things worked out.

Thanks for the tech feed.

 

Never run the Engine with the Alternator disconnected it is supposed to be bad for the Alternator.
Car sitting and Engine not running Disconnect the Alternator Electrical Connector down at the Alternator. If your Battery does not discharge while sitting overnight you have a problem with The Alternator.