So... rather than replace the old, oddball battery(s), you think a different
switch is an appropriate fix? Gimme a break!

OK guys, I'm not going into huge detail here, I have done that before with this
problem. My Johnson outboard has an un-regulated charging system. There is
really nothing on my boat that uses any current to speak of. Once the starting
battery is fully charged (very quickly) the voltage go's up to 16+ and this is
not good. Its much cheaper to buy the switch to let the voltage go to the
trolling motor battery which is usually discharged enough to keep it "busy"
than it would be to try and put a regulator on it. To further complicate
matters the charging system is only 5 amps so when I used a combiner, both
batteries wound up low (I cant win!). Sooooo, I figure I can watch the guage
and just flip the switch when the voltage gets too high and let the trolling
battery have it. At 5 amps it would take it a looooooong time to recharge the
trolling battery. If anyone else has a better plan I am listening.