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Larry

Not true. I have a spare depth sounder (a cheap Humminbird fish
finder) that goes berserk at anything over about 15 volts. Keeping the
input voltage to all those other, more expensive instruments
controlled at 12 volts lets them run at lower temps, which in
electronics, equals longer life and greater reliability.

It is impossible to put 15 VDC across a normally-charging, functional on
all cells, lead acid 6-cell battery.

If there is 15V on that battery you are WAY overcharging it or have
corroded battery terminals causing series resistance the charging current
is flowing through. This is not a problem bacause a NORMAL alternator with
a working regulator will simply cut the current back, as it folds over the
voltage at its 14.2VDC setting.

Your fishfinder is going berserk because the regulator has lost control of
your alternator's charging and is charging a battery with high internal
resistance caused by a dead cell, probably, to create pulsating DC across
the bad battery....that whining sound in your car stereo is caused by it.
Now with several volts peak of pulsating DC riding on top of the battery
voltage, the pulses are causing havoc in the sonar's digital circuits.
They expect it to be running on SMOOTH DC....