I have a 2004 ML 350, 40k miles full service history with Mercedes.

Have had no issues with the car until two days ago when I attempted to start the car, there was a click but it did not turn over.

Jump started the car, noticed the radio was requesting a code, assumed the battery was flat. Drove the car for 30 minutes to charge the battery. turned engine off, would not re-start. No power to open windows or turn heater fans on. attempted to jump start the car again but get a Start Error message, when i remove the donor battery I get no message just a click noise.

I removed the battery to allow it to be slow charged but also give me access to all the connection cables. The battery was charged an a slow charge for nearly 12 hours, the voltage recorded after the charge period was 13.16 volts, as expected it dropping to 12.86 volts one hour later and by 12 hours the voltage was 12.6 volts, 24 hours later the voltage was still 12.6 volts. I have checked the connecting cables at various points without any shorts or connection problems.

I put the battery back into the car. When I attempt to start it there is a click but nothing, I am not sure if the click is from the relay (in the fuse box) or the solenoid on the starter motor. The interesting thing I did not get a Start Error displayed but if I hold the key in the start position, after the click, the side lights come on until I release the key.

- suggestions as to the fault and possible resolution

 

Remove the battery, take it to autozone or Napa and have them to load test it.
What do you mean by slow charge. Your battery should be 100AH if it is still the original, to fully charge it, it will take 10A and about 10 Hr. to charge it (or untill warm on touch and all six cells boiling evenly.
With 12.6V, it seems like all your cells in the battery are OK, just capacity of the battery is down, because it was probably never fully recharged since the day of manufacture.
These are not maintenance free batteries even if they are build to higher standards then any other car battery out there.
You can try any other battery that you know is good and see if the starting problem goes away. It could be also faulty starter solenoid that needs more amperage to get activated and moving but at your milage, I would reather put the blaim on battery age.

 

Thanks for all the help, had the battery tested - it was a faulty battery!