[jords]

You have a different model to mine as mine doesn't say 'going to charge mode', but it must do, as I have never manually charged it, and it has been left for a few days without being used.

I think the unit's must go into a kind of sleep mode, because when it automatically powers back up when the doors are opened, it already locked onto the satellites.

It sounds like all is ok if it comes on when the door is unlocked/opened and goes into charge mode (without the screen being on) after the door is locked/closed. Best thing is to lock your car and go and have a cuppa. The go and check and it should have powered down.