The voltage drop across the reducer will change according to current flow. It's just a high-wattage wire-wound resistor in most cases. For instance, if a circuit that draws 10 amps gets reduced to 6 volts applied to a heater motor, etc., you would need a 0.6 ohm resistor with a 60-Watt minimum capacity. The resistor would consume 6 volts, and allow the other 6 volts to be applied to the heater motor. If the amperage changes, the resistance has to vary accordingly. Once the charging system kicks in and the available voltage goes up to 14.5 or so, all the calculations change. I like to use big wire-wound resistors with a sliding center tap so I can fine-tune the voltage for actual operating conditions, not some "one size fits all" voltage reducer.
Jerry

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