I'm currently helping a friend who is renovating a VW camper to fit some LED lighting as she has no electrical experience. She has a 10 metre set of LED fairy lights that are powered via USB (bigclive reviewed something similar, or indeed the same here). She has until now been powering them by plugging them into one of these mounted in the van but would like a more permanent solution. As a result I bought a 12V to 5V DC-DC Converter and a small switch which I wired up to draw power from the vans leisure battery.
I cut off the USB plug a few inches down the cable and connected the two wires directly to the output side of the converter. As soon as they were turned on though only about the first quarter of the LEDs lit up and they trailed off in brightness which would suggest an obvious drop in voltage. The DC-DC converter was also getting warm so I turned the whole lot off before any of the LEDs burnt out.
Assuming that the LEDs were drawing far too much current I dissected the USB plug looking for a resistor but couldn't find one. I'm hoping I haven't done permanent damage as they would be a pain to replace! These lights do have a bundle of 3 twisted wires so going from what bigclive says it would appear they do not require a resistor.