I finally took one apart. It has a 200 milliamp/hour NiCad conventional (with button on + end) AAA battery inside, plus a little circuit board. Trimming excess plastic away from the solar cell, plus abundant use of self-fusing silicone electrical tape for assembling and mounting to the handlebar, and I now have an automatic cyclocomputer light. It looks more like something from thereifixedit.com, though.