To do this electronically would get messy since you would have to use either a DC to DC converter or build a pulse width modulator and run it at about 50% duty factor. Another thing that could work would be to use a series resistor. Problem being that you have to know how much current the motor draws while being used to make the calcualtion of the resistance. If you wanted to drop 7 v with a motor that is drawing 5A you would have to use a 1.4 ohm, 35 watt resistor (that is the smallest wattage you could use, better would be 50W). YOu can look around on the web on how to make a pulse width modulator with lm 555 chips, or regular PWM chips. All you would need would be the PWM chip, coupld of FET's and a variable resistor. but just using a smaller supply is the easiest.