Date registered: Feb 2008
Vehicle: 2004 W220 (S430) ; 2001 R129 (SL500) Sport
Location: Alabama Gulf Coast--formerly Charlottesville, VA
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Okay, I'll play.
If your battery is draining, there are three possibilities: the battery is bad and won't hold a charge, the alternator is bad and is not charging the system or providing enough power to run the accessories which drains the battery, or you have a closed switch somewhere that is keeping an electrical system active.
The car won't turn over most likely because the battery does not carry enough charge even with a jump to allow it to activate the starter. I'd say your first effort at repair is to have the battery load tested to see if it's good. If you have the original battery in the car, it's time it wears out. If that isn't the problem, check the alternator to see if it is charging properly. It may be charging enough to keep from throwing a code, but not enough to power up everything and charge the battery as well. If it is neither of those, you get to pay someone to do a scavenger hunt and find the closed switch.
So far as I know, there is no safety system that would keep the car from starting if there is sufficient charge in the battery.
I hope this helps.
"Don't pick a fight with an old man. If he is too old to fight, he'll just kill you."--John Steinbeck