Mercedes Repair Questions? Ask a Mechanic for Answers ASAP
if the battery won't hold a charge then I'd check that the ECU is getting 12V on its fuse with the ignition on as if the battery voltage is below 11V then it will never start
and if it is low then it may be time to replace the battery
Also check that the alternator is actually charging the battery in the car by taking a battery voltage reading with the engine off then another with the engine running. The 2nd reading should be higher than the 1st and climbing steadily. And also check that the alternator is secure to the engine as this provides the ground for the charging circuit and make sure that the engine / gearbox earth strap is secure and the contacts are clean and bright.
if the alternator appears not to be working - take it to a electrical specialist and ask them to test it as sometimes you can replace just the diode pack and this will repair the alternator without requiring replacement
And one last thing is – how old is the battery? – As even good quality ones need replacing every 5 years and faulty battery just won’t hold a charge