onboard charger

Hey guys. So I have searched for a couple hours and can't really find what I'm looking for charger related so yup here goes another thread.

I am running 3 batteries and a perko. I have a 2 group 27 interstate marine batteries for the stereo and a group 31 blue top for the starting. And I have a guest 12 amp 2 bank onboard charger laying around. Will the 12 amp work? Or do I need to upgrade to a 20 amp? My plan is leaving the perko on bank 2 with the 2 interstates for starting and cruising around as well as sitting with the boat off and jamming out. And if when I go to start bank 2 won't do it then I always have the bank 1 to get me home. This minimizes how often I have to swap the switch. I rarely sit and jam for longer than an hour or so. So I shouldn't have to ever swap the switch unless I drain bank 2. So will the 12 amp do it? Also thinking since I already have it to use it on bank 2 and just a trickle charger on bank 1 just to keep it topped off since it will rarely get used.

You wouldn't need two chargers, since you already have the 2 bank use it on the three batteries you have. a 20 amp would be better but the 12 will work.

Even though you have three batteries, you really only have Two BANKS of batteries since you have the two Group 27 batteries wired together

Correct, I just wasn't sure if the 12 would cut it. It only gets used on the weekends, so the boat would have all week long to charge back up. Just curious.

It won't be optimal but it will get the job done!

I have 2 group 27 batteries in parallel on my stereo bank. I have a guest charger that puts out 5 amps/channel. It always gets the batteries charged overnight.