comments:
Kahlas posted on r/dieseltechs1w
Just buy an adapter for your preferred 12-20v cordless tool battery. I don't even bother with a box anymore to test trailers. I just have a ground and live connector from a battery adapter and I just manually switch which terminal it's plugged into. Add a magnet to the non battery end of the battery adapter and stick it to non greasy edge of the 5th wheel plate. Automotive lighting can handle up to 36v without issue.