I have a string of battery powered LED Christmas lights that use 4.5v (3xC-size batteries). I want to power them from my scooter's 12v battery. I have two different colors, one is red and one is green. I see a 20 ohm resistor in one and a 10 ohm resistor in the other. I'm not sure which one has which. There are 30 LEDs in each string. I know I have to add another resistor to the circuit to make up for the increase in voltage. But how much do I add?