Most times Shypes the voltage of the batt (1.2V) is too low to power most LED’s (Assuming the_bit is talking led’s here). I *think* the lowest voltage is about 1.6V (for a red LED) the highest I’ve seen is 3.6V for some ultrabright blue LED’s.
The most popular is to use a 3V lithium battery and wire the leds to that. Although ideally you should also wire in a 220Ohm resistor to limit current to the LED’s.
I’ve been building DCDC converters that take the 1.2V from the bar’s batt and raise it up to 5V, very handy for driving those LED’s. And it runs off the car batt so no replacing the lithium battery.

If there’s enough interest I might bring out my own headlight kit based on these dcdc converters….
ph2t.