From Jeremy S Cook on Tindie blog:
Ted Yapo had a small problem. As an amateur atronomer and astrophotographer, he needed a way to mark his expensive equipment so that he wouldn’t trip over it in the dark. Glow-in-the-dark materials were out because of they only glow for a short time, and glow sticks were also less than ideal because of their single-use nature. Tritium light sources would be perfect, barring the small details that they’re radioactive, expensive, and in the US only a few uses are allowed, most are prohibited by law.
So Yapo instead came up with an LED light that can run for not 20 hours, or even 20 days, but 20 yearson a single CR2032 coin cell battery!