detrjo

Hi OK, so i'm a complete novice at this electronics stuff and need a little help with something here. Here's what i want to do: I have a 2-way radio that i wish to power off a motorcycle battery [for use whilst riding so i can talk to other bikers], instead of using the Nicd battery that came with the radio. But - i have conflicting info on how to do this. The facts: Motorcycle battery = 12V [or as close as you get with a vehicle battery] 2-way radio requires max 4.5V 500mA [Nicd battery supplied = 3.6V] The solution: Run wire off 12V battery terminals to 2-way radio terminals and reduce voltage using resistor. The problem: OK, so this is the part i have a problem with. I am given conflicting info from two people on how to work this out. opinion 1 I must first subtract the voltage required by the radio from the source, and use the remainder to find the resistor value required - ie: 12V - 4.5 = 7.5 / .5 = 15ohm ... and power rating: 7.5 * .5 = 3.75W requiring 15ohm 3.75W resistor to power 2-way radio from motorcycle battery opinion 2 No need to subtract the voltage required by the radio and the resistor required as follows: 12V / .5 = 24ohm ... and power rating: 12 * .5 = 6W requiring 24ohm 6W resistor to power 2-way radio from motorcycle battery -------- Can some kind person please shed some light on this? Thanks detrjo