Is the trolling battery wired in to the system IE: a switch? You could then charge somewhat with the motor, though most outboard charging capabilities are limited. Most don't do much unless running over 3000 RPM. So you would have to be making some pretty good runs between fishing spots for it to do you much good. A solar charge system shows varied results in my search, common response was you get what you pay for, and most boats don't have enough room for a panel big enough to do a real charge on a battery. Older outboards with an alternator were marginal at best, from what I'm finding they only put out a few amps at 3-4000 RPM and sapped the motor of some of it's power. Your best bet is to spend the money on the best battery you can get, and keep it fully charged when the boat is not in use. A radio is only going to draw a small amount on a deep cycle unless your running a CD player or something.
