I can'r say for certain in your case but I do know in other situations
removing the connection to the battery does cause an overvoltage that
sometimes will damage the diodes. It sounds like you know it all depends on
the reverse voltage rating of that bridge. I would look into a regulator
solution. A low budget solution might be to find something that will use
some of that power and wire it into your switch. Or wire something into
series with the charging line to the battery that could be controled by a
switch.

I added a "low" charge function to an unregulated battery charger by adding
a switched car headlight bulb in series. One of those newer small bulbs.
For normal charging the switch bypasses the bulb. For charging smaller
batteries like motorcycles and lawn mowers the switch adds the bulb in
series with the output line. It glows a bit when engaged. I'd guess the
total output current is not very high in your case too and it might not take
much to bring the voltage down.

"Derek" wrote in message
...
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.

Is there a possibility that this will cook the diodes? The charging
system has no regulator, just a bridge rectifier. Is there a safer
place to "break" the circuit? The ignition system takes it's power
from the battery, no magneto.

I'd rather this "wasted" energy of boiling the battery be removed 'cos
this old motor needs all the help she can get.

Note : The 17 Volts is across the battery, not the open charging
system. Also, do deep cycle batteries have a lower internal resistance
that's causing this high voltage? All connections and battery are in
excellent condition.

Thanks,
Derek.