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In article ,
says...

Briefly, I have an old (1967) 45 HP Chrysler engine. It has the old
"points & condensor" type ignition and it runs just fine.

I installed a Volt meter in the dash, and I run the boat off a deep
cycle battery, only because I carry a trolling motor in the event of
engine failure, not for fishing.

Problem : After running for a while, the Voltage across the battery
rises to about 17 Volts, damaging the battery and possibly damaging
any other equipment I might run off the system (lights, fish finder,
GPS etc.) I intend to install a toggle switch to isolate the (Purple)
Positive charging wire, and flip it on as required.

An open circuit is bad, as the spike voltage is what will kill the
rectifier bridge.

No matter what anyone says about volts in combination with amps, all
battery literature for wet batteries allows a maximum of 14.8 volts
under any circumstance. It would be obvious at some point your charging
system put out enogh current to reach 17vdc, even if the actual current
at 17volts is somewhat lower, the damage was done in the process of
getting there. Someone suggested to me to run with the lights on, and
this is what I do. I like the one posters suggestion of inserting a lamp
across your 'charge' switch. with the switch closed you get full charging
output, and with the switch open and the lamp bridging the switch
terminals the lamp will glow, and the current through the lamp will
trickle charge the battery. Try an 1156, that should work, and you can
find a socket easily enough.

Best luck. Mine now peaks at 15vdc, it was once like yours.