Is the "more than 15 v " with the battery connected to the generator or when
disconnected and open circuit?
The simplest solution is probably a series resistor to limit the charge to
about 1 amp or less which should not upset a reasonably sized battery.

Try a 21W car bulb and measure the current.

Alec

wrote in message
ups.com...
I have an older Honda 10 hp outboard that has a charging circuit. The
small boat that it is on has a bilge pump and small AGM battery. The
outboard's charging circuit puts out over 15 volts. Can I put a diode
in the circuit to reduce the voltage? If so what kind of diode?

thanks