2019 Chrysler Pacifica battery smoking

The car was dying everynight so I put it on a charger and it worked fine. But then I got home and noticed sulfur smell and look under the hood and the aux battery was smoking! I didn't even see the aux battery when I was charging the regular one. Do I replace just the aux? Both batteries? Can I just remove the aux and run on the one battery? My battery charger said the alternator was good but I don't know. Seems to be a common problem with this car so I'm hoping someone knows what's up. Thanks.