I've never been happy with any bike light I've used. Some aren't bright enough, some need frequent battery replacement (why don't they use C or D cells instead of AA or, ridiculously, AAA?), some aren't sufficiently waterproof. My bright, expensive helmet-mounted light isn't bad, but since I don't park my bike near an outlet I can't mount the battery permanently to the bike because I need to recharge it, so I have to carry it with me; I don't like that because it's fairly heavy and because sometimes I don't have it with me when I need it.

Here's my attempt at a solution: a solar-powered bike light, modified from a solar flashlight. (I did find one product that is advertised as a "solar bike light", but it's not good: it uses an incandescent lamp rather than LEDs, and it's a one-piece unit so the whole big thing has to mount on the handlebars). With my home-made version, the solar cell and batteries mount on the rear rack, as you see here.

I consider this a prototype, not the final answer.

Fully charged, it gives over an hour of bright light; I only need about 20 minutes.

Will it get enough sun to recharge during a gray week in winter? Doubtful, but maybe.

Here's the front end, the six LEDs that I removed from the flashlight and mounted in a tiny plastic bottle. I'll have to work on this mounting system, the electrical tape will stretch eventually.

The flashlight came with a conventional-looking metallicized reflector, like you would use for a standard light bulb. You see that a lot, and it's stupid: an LED (at least, this kind of LED) has a very narrow projection angle anyway, so the reflector doesn't come into play at all. I guess they put them on just because people expect a flashlight to have a reflector.

Not shown is the switch, which is underneath the top tube, just behind the head tube.

Possible system weak points: (1) switch not sufficiently waterproof, (2) LED mounting system not robust enough, (3) battery-pack mounting system not robust enough, (4) vibration may break a connection somewhere (e.g. solder at the back of the LEDs).