I've pondered how to deal with the trolling battery while on a fishing trip. Sometimes, the boat may be on the water a few days, so charging or switching the battery out is a chore. In addition to the solar panel that charges the trolling battery anytime there's sunlight, an electrical circuit to bridge from the engine battery to the trolling battery during times when we're driving from one spot to another seemed to be a workable idea. The trolling battery/motor is 12 volts, so this was an easy task.
Initially, a few tests were made with jumper cables and an ammeter, and the idea seemed to work OK without causing a serious load to the engine battery or alternator. Usually, since the engine battery isn't much above that of the weakened trolling battery, the charge current is around 10 amps, tapering off to much less as the trolling battery charge level increases.
Both battery negative (-) terminals were connected together with some heavy black wire from a set of scrap battery jumper cables. A 12 vdc relay was installed with the normally open contacts connected to the trolling battery via 20A fuse, and the common (moving) contacts connected to the engine battery. I put the fuse in the battery circuit in case something went wrong with the trolling battery or wiring, and to limit the current in the event we totally killed the battery and didn't want to load the alternator / engine battery too hard.
Up front, a switch was installed to control the relay. 12 volts was picked up from the ignition switch, using a switched source that is only live when the ignition switch was on. I didn't want this accidentally left on during times when the engine was not running. Aerator and Bilge Pump switches get bumped "on" all the time by accident, so a covered switch was installed for the Trolling Charge to require an intentional movement to activate the switch.
This circuit works pretty good. Care must be taken not to leave the switch "on" when starting the engine (the starter would attempt to pull from both batteries and probably blow the 20A current limiting fuse). I plan to take the automation of this a bit deeper over the winter months. The next phase is to sample the alternator output, and energize the bridge relay after a delay of maybe 1 minute (if the master bridge switch up front is "on"). Likewise, if the engine is "off" (no alternator output), the bridge relay would be disabled.
