I converted a lightbulb to run from a USB powerbank.
I opened up the lamp and found that the LEDs were 6V ones (actually 2 in series in each package), in a series string to make the voltage quite high. I put link wires so that they were in parallel, with half in reverse. Sadly, I could then only run half of the LEDs like that.
I used a boost circuit, modified to limit the current (See
this post and the ones after it) to drive the LEDs and the supply for that came from the power bank.
It was simple, in that I didn't have to design the boost circuit to run from anything other than 5 V, and the charging and low-voltage cut-off was taken care of. It wasn't too inefficient, as there was no transformer needed, no boosting to higher voltages. However, it wasn't very efficient as it had two boost circuits (the one in the power bank and the constant current one, while a more optimised design could have boosted from the lithium battery directly to the LEDs.