Jay, My SeaPro uses 3 ea 12 v batteries tied to a Perko switch. Two
are primarily for the trolling motor and one is for starting etc. The
primary starting battery is tied to position 1 on the switch while one
battery for the trolling motor is tied to position 2. Should you
encounter problems with the primary battery you can switch to position
2 and use one of the trolling motor batteries (you can also charge
this battery under way in position 2). This will allow you to use two
batteries for a 24v system as well as have a back up. I also had a 3
bank charger installed so I can hook to 110v and charge all three
batteries (power cord runs to a plug on the side of the console). All
I have to do is plug an extension cord into the plug on the console
and all three batteries are charged simultaneously.

This makes perfect sense to me now. I didn't know that we can use one
of the 12V battery in a 24V system for starting, and I didn't know that
we can charge it without charging the other 12V battery in the mix.
Now, I understand what Lloyd meaned to say.

This means if the existing battery was new, I should get an onboard
charger that can handle 12V/12V/12V (three 12V batteries).
Unfortunately, the existing battery is not exactly new. This means I
may not want to pair it with a new 12V battery to make a 24V system. I
may still be better off getting one new 24V battery or two new 12V
battery to make a 24V system. Or I can get rid of the existing 12V
deep cycle battery and replace it with two 12V deep cycle batteries and
arrange them in the way that you have suggested. I am still thinking
of which way I should go. I probably will go for keeping the existing
batteries and use two new 12V batteries to make up a 24V system just to
avoid messing with the existing connections on the batteries.

Thanks for the info.

Jay Chan