| 749S here and had various battery issues in the past couple years. Optimate after long stand is not good - but don't use it and the alarm runs the battery down in about 3 weeks.
You should check the earth strap if you ahve the clock resetting like that. It will also cause trouble starting because it can't get the current through the connection. The earth strap connect to the engine behind a frame tube just above the clutch slave. It's an allen screw which you can jsut about squeeze in past the frame. At least it is on my 749S.
As for changing the battery, ECU won't mind, but the alarm might if you ahve one fitted. The Datatool Series 3 needs to be put in service mode to do it, or it will go off on it's internal battery. Turn ignition on, then press the disarm button while you switch ignition off. It will beep periodically then, you've probably heard lots of them beeping away in the showrooms. | 
Hybrid Mode
