split charge idea to try.
Lookin at the previous threads regarding split charging i had an idea years ago for a cheap reliable way of doing it. I havent tried it on an outboard so it may not work on some engines.
we had narrow boats with a conventional split charge system with a relay connected to the alternator light, so the second battery was connected in parralel with the main battery once the alternator is charging, this was fine for a single boat, but one of the boats was the tug for a "butty" boat which has no engine, it had its own battery for lighting and was charged of the main boats alternator along with the main boats lighting battery.
the sytem i used here was TWO blocking diodes, one for each battery,
so now both batterias are charging at 0.4 volts below what they should, the trick then was to insert another small diode of similar Vf in the regulator sense wire of the alternator, this lifts the voltage of the alternator by the same voltage and both batteries recieve the correct voltage. we added a resistance to limit max current too, but its not really needed.
it may work for some outboards if the alternator has a regulator, if it hasnt then the extra diode wouldnt be needed anyway.