Can you still jump start the car using your 1200cca jump box (connected at the battery terminals)? If so, you need a new battery, the voltage doesn't mean you have enough amperage.

I have a battery dated 1/2012 sitting at home, it won't even get up to 12 volts anymore, it was in my 300sd when I bought it a few months ago. The car needed a voltage regulator and had been boiling that battery away (at over 15 volts) ever since it was installed. I found a guy on craigslist with a 300sd parts car and swapped the entire alternator for a newer looking one. The newer voltage regulators look different, kinda flush mounted except for a round part in the middle. My old voltage regulator was a blocky black or brown thing that reminded me of other 1980's solid state electronics.

After verifying that the charging voltages were more healthy, I went and bought that huge autozone battery with the 150 minute reserve capacity. No cranking issues anymore, makes me very happy.

EDIT: Anybody have a washer fluid pump they want to send me? All that battery acid dissolved my old one. I really need to get out to the pick and pull.