The DC-DC converter (
On-board Alternator Charger) method is an option also. These things use/converter the 12V from the alternator/engine start battery side to 2 (or 3) 12V *isolated charging outputs for charging the trolling battery side of things. And, of course, switch on only after the engine has started AND the start batt is topped off.
I've not used one so I don't know a ton about them and of course like anything else there are limitations including the charge current is limited to 10A/Batt, that's pretty weak!! Perhaps there are higher rated output units out there? I haven't looked.......
The Trollbridge/Combine (relay?)system mentioned in #6 I believe would be more efficient at using the alt's available output current.
But note and again with the limitations, it will have to divide alt's out current (whatever it is) by 2 (or even 3 if the start batt is down also) given the trolling batt's will most likely by default both be equally dis-charged due to their wired in series when in use.
Also, not sure (wasn't obvious stated anyway) if the Trollbridge/Combine system has a built-in "auto inhibit" for the switch over if the start/engine batt is low?....Might not be a good idea to parallel two paralleled dis-discharged'ish with perhaps a already weak'ish start battery.....Hmmm, Maybe just use the manual remote switch feature and let the start batt top off before allowing it to switch over.
As mentioned many times here, the alternator's current values are usually all based on full'ish speed running, like 3/4/5K. Idling or near idling isn't going to get ya very much charge as the alt's don't produce much current at low speeds. Running for hours off shore or in big lakes to the fshing hole is more likely where these systems would work best.
*Isolated charge outputs mean you can leave the trolling battery's wired in their 24V series config and just clip on the charger's output directly the the battery terminals, no re-wiring, no switches or relays switching around series/parelled connections and grounds. Same diff and similar to the 120VAC on-boards chargers.
