I have a set of led headlights for my truck that won’t power up when plugged in or directly wired to car/automotive battery, yet when I use a little 23a 12v battery they do power up. I’m confused!! Why does this happen? And why won’t they work in my truck?

 

(1) Did you wire them to the battery backwards, (2) are you sure the "direct" method had 12V? Double check it with your meter. (3) Some cars with electronic headlight controls won't work properly with LED's unless you add a load resistor to it. The LED's don't draw enough current and the headlight control thinks the bulb is not on and turns the power off, though this usually results in blinking.