I connected 3 LED's in series to a 12V car battery. The LED's are rated 3.5V 4V max. The question is when the car is running the alternator is sending 14.5 so I was wondering about burning the LED's. Should I install a resistor in the circuit? What value should it be?
For sure you would need a resistor, you can figure out the value from the LEDs current consumption assuming you want to drop 4 volts across it - if the LED's want 40ma then you need like 100 ohms.
I would not be inclined to do this myself though. I'm not sure how minor differences in the LEDs would affect the result. There are "12 volt" LEDS meant for automotive use that don't need any coddling - could you try one of those instead?