Hi everyone. I set up the circuit recommended here, but it just flipped the 5v relay constantly at idle.
I simplified to a single regulator rectifier fed by the headlight circuit running into a y splitter that came with the battery pack. My battery also has it's own switch which aided a simple setup.
The voltage coming through either of the 12v regulator rectifiers flickers the light (still ac). If I turn on the battery the light is steady and my battery pack seems to charge above 2k rpm or so.
I need to test my setup more, but generally, I'm thinking that cheap /high quality batteries and LEDs have made all the struggles on this thread out of date.
For $40 you can just run a standalone LED and removable lithium battery that can be charged anywhere like a cell phone.