12 volts nominal.
A 12 volt car lead-acid battery will measure about 12.2 at full charge and no load. Car charging systems usually keep the voltage around 14 when the car is running, though it might...
If it is an alternating current (ac) circuit you could use suitable a step-down transformer.
If the circuit uses direct current (dc) then you could use a drop-down resistor if you know the load's...