Everybody seems to have failed to notice you put in a second battery...
Put the battery on the charger overnight. In the morning you should find it at 13+v a 12v battery at 12v is low...
Run the car and see what the voltage is at the battery. I think you've got classic bad alternator symptoms.
A battery clamp that looks even slightly wonky should be replaced. It takes a lot of current to start your car, and even more when its cold out...

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Sadly Benz-less