2012 G37x sedan

Car sat for 2 weeks due to COVID, battery was dead, got a jump, and was fine
Car sat for 3 days, battery dead again, second jump, this time went and bought new battery
New battery installed, car starts right up.
Next day, battery dead AGAIN
Use a 12V charger, nothing I do gets the new battery to work (50V quick start, 10V long charge of 3+ hours)
Use the 12V charger to charge up the old battery, starts up just fine
Both new and old batteries show ~ 12.6V out of the car, and seem to hold charge outside the car
Old battery in car with engine running only shows ~ 13.2V, even when revving engine, I would have expected to see ~ 13.6v idle and 14v+ when revving
Ran the car for about an hour, then turned car off. Old battery was down from 12.6V to 12.25V, meaning close to 50% battery drain

Am I at alternator replacement or is there anything else I can try?

 

Do a parasitic draw test. A bad alternator will not kill a battery when the car sits.

 

Seems simple but make sure terminals to battery are clean and tight.

 

As soon as the weather is nicer tomorrow I'll be doing this.

 

Yep I re-tightened everything and also cleared off most of the corrosion to improve the contacts, so I think I am ok there.

 

Were you ever able to resolve this? I am having the same issues on my 2011x sedan. 13.2 volts with the engine running. Thought it should be higher. Could the ecm be constraining the voltage too low? Looking up alternator testing now to see if a replacement is in my future.