A lot of cell phone chargers are 5 volt from what I have been told. You might take a voltmeter to one and see what you have.
To reduce the voltage on a DC system, you use a choke. For many years most cars used 9 volts for the instruments in the dash and the voltage was choked down with an IVR (instrument voltage regulator). When they would fail, most of the gauges would be pegged on the high side. I have changed quite a few.
