W211 280CDi doesn't start

[wombatwillis]

I have a W211 280CDi estate (March 2006). I unlock the car with remote key function as per usual. Central locking performs as expected. I put the key in ignition and the safety interlock unlocks (small familiar sound associated with this, I can then knock the Auto box out of Park with foot on foot brake). However in Park position as per usual when starting, when I turn the key to start the engine there is no response whatsoever. The starter motor does not turn. Nothing happens. I get the full dashboard lights as usual with key in start postion. I get no warning notices on dash. I have now removed the main rear battery which is 8 years old and measure 12.45V (no load). I didn't check the voltage before disconnecting, maybe I should do? I am currently charging it. It is taking charge from a reasonnable battery charger. Prior to removing the battery I did try a disconnect and reconnect, waiting about 2 mins before reconnecting. The was no change when attempting to start.
The car has been garaged with no use for several months with the rear battery disconnected at negative terminal. Yesterday I reconnected the battery and successfully started the car. I did 3 short journeys each of a few miles for which the car started ok. The car failed to start after a 4th journey of about 3 miles.
Could anyone offer any advise as to what the problem is likely to be?

 

[E270 Owner]

It could be the 3 short trips were not enough to fully recharge the low battery condition.

Re charge and take for a longer run it should be ok.

 

[wombatwillis]

Charged for about 4 hours now and charger showing fully charged. So I assume battery is relatively healthy as it is taking charge. 5 mins after disconnecting the charger the (open circuit) voltage is showing 13.44V but this will slowly drop and stabilise. I will check the voltage in the morning before putting back in the car.

 

[wombatwillis]

Battery stabilises at 12.7V. Battery charger when reconnected this morning immediately shows fully charged so no loss of charge overnight.
I am assuming battery is good. So I installed battery back in car. Everything works as before, lights, ignition unlocks, power tailgate works, obviously central locking, Command Sytem, Fan, Electric Windows. BUT still cannot start! The igntion unlocks and so I can knock the autobox out of Park position with foot on footbrake.
I assume some kind of relay problem? Anybody have any ideas or met similar issue with no repsonse from car when turning key to start position?

 

[mersum1es]

Starter motor fail? Try knocking the starter motor by long shaft while trying to start, sometimes it helps... Very ofter starter motors fails immediately without warnings. Try to check that all wires all still connected to starter.

 

[SimonsMerc]

12.7v stable means your battery is still good (after 8 years it's pretty amazing actually). Does sound like it's either the starter motor or the electrics getting to it.