I recall getting same message almost every time I started the car no matter how long it sat. My main battery checked out OK at several places I took it to incl. a Mercedes Dealer that used a Mercedes tester.

The battery was fairly old (8 years) so I got a new one. Message went away and I have not had any other issues in 2 years.

I think batteries may loose their ability to hold a charge over time.

__________________
-- Chris

'95 E300, 216k miles, Silver Surfer
'05 E320 CDI, 138k miles
'07 S550 4matic, 69k miles

Gone but not forgotten:

'76 300D, 350k miles?, SOLD in 1995
'75 240D, 300k miles, SOLD in 1991