I have a 16 ft arima Sea Chaser, I put a second battery next to the original, I used optimas, and an isolator between them, just a switch. One can jump the other that way. One (orig) runs big engine, other is charged by the kicker, and runs the electronics and the down riggers/pot puller. Alternators are still available for the later Johnsons, and its pretty easy to set up with wires and switches and a battery tray from westmarine. My yamaha with a 6 amp alt keeps up easily with the radio and etc. while trolling.