I had a battery inexplicably drain after a storm. I have also had an ignition coil fail, and an overvoltage relay fuse blow. Your problem sounds like one of the latter two.
Does the ABS light stay illuminated? If yes, it is the overvoltage relay fuse. If no, my vote is with G-Benz on the ignition coil.
The good news is that a faulty ignition coil gives you several weeks of warning, i.e. hard starts and random engine stalls -- often at embarrassing moments --- before failing entirely. If the coil is indeed the problem, you are on borrowed time and I would suggest ordering a new one now, at a very affordable cost ($99.00), rather than waiting for it to fail on the road and having to pay $250 + towing for the dealer to replace it.
This is where it is, and what it looks like: