@David Johnston posted:Maybe 4 diodes in series.
. . . mmm, or a few more, since a diode will drop the voltage about 0.7 volts each. So, to drop about 5 volts, as the OP desires, that'd be about seven diodes in series.
Or, you could just drop in a small value resistor, or even easier, just drop in a buck converter to produce a regulated 12 volts. Even better, set the buck converter to drop the voltage to 9 volts, for smoother operation.