Battery voltage issue

On my JK I have a Genesis dual battery set up running AGM platinum batteries from Advance auto. At the end of October, I experienced a bad cell on my "accessory" battery and swapped it out under warranty after a year under my hood.

Driving last night, the g-screen for the genesis started flashing red periodically signalling that one of my batteries was under 12 volts. I checked and the new "accessory" battery was only showing 11.9 volts. I was able to get under the hood this afternoon and verified with a multi meter that it indeed won't charge past 11.9 volts.

I can turn on all my accessories and watch the voltage drop on the battery and then watch it charge back up to 11.9 and then stop. I did check all the connections on the genesis setup and all connections are tight and in good condition.

I'm thinking it's just a bad battery, but not being too electrical savvy, can I put it on a charger and see if it'll charge past 12 volts? I'm just not certain on how to proceed. Thanks in advance.

Bueller???

Yes you can charge it but since it's a gel cell don't charge at more than 2 amps. You can cook the gel inside of it and fry it. Make sure it's on 2amps and let it charge for a long time

I'm not real familiar with dual setups, however, I would check your charging voltage with engine running and all Accessories on. Just to see. Should be at least 13.5, I like to see around 13.8 to 14.3.

Thanks guys, gives me a place to start.