Led's are a great solution when replacing incandescent bulbs in many applications. One problem is that LED's don't like changes in voltage. For example, the voltage from a car battery can fluctuate quite a bit from 11 - 14 volts and this can be a big problem for an LED circuit. If you design the LED light for 11 volts then it will shortly become a DED (Dark Emitting Diode). If you design the light for 14 volts it will always be very dim when it is only gets 11 volts.
took that from some website.. theres alot of info on it on google, i probs should of searched there b4 coming on sau