I'm looking to get another deep cycle trolling battery for my pro 102 boat. The one I currently have is good but I bought it used and don't really know how old it is and it doesn't seem to last as well as it did last year. I plan to use the old/current battery to power my live well and sonar unit. I have a one bank on board battery charger and I'm wondering how/if it's possible to charge both batteries with this charger and can I have both batteries hooked up together? Thanks in advance.

You can run then in parallel. Well keep it 12 volts and allow them to charge as one battery. I am not sure if one of the batteries is going bass, how that will effect it. They will then be running anything that is connected to then.You won't be able to just run your sonar and livewell only.

If it were me, I'd use the old one like you plan. Get a new one with lots of reserve capacity. Use the new one on the trolling motor then rig up some cables that you can use to hook them parallel (+ to + and - to -) when you are charging them. Your 1 bsnk charger will do the job but it will take a lot longer since it will be doing 2 batteries.