I ride to work in the morning, well some mornings, every week just about sunrise and bike home after dusk. Lights are crucial factors in staying safe, more than most of us think. Many articles talk about many studies and accident statistics to the point where I'm sold. For example

I had some clip on lights but I typically had at least one ride a week without them as the batteries would die at some inconvenient time and I was too lazy or eager to ride to bother replacing them until I got home. I also felt bad about the battery waste.

My Solar Powered Helmet (v2.0b)

This helmet has a solar panel, a solar charging circuit and a LiPo battery that it uses to power a strip of ElectroLuminescent tape and an LED panel. It charges up during my morning commute and lights up for my nighttime ride home.
For riding on cloudy days, the charger circuit has a USB port that you can use to charge from a USB cable or power supply.
All of the electronics are held in place with velcro so that you can change them or remove them in the rain. Water on these components wouldn't be good for you or for them.

The Parts

The end product

Some tips

Make sure that the battery is mounted under an overhang, the direct sun does bad things to LiPos.
My experience has been that my 45 minute ride in sunlight and some amount of ambient light that the helmet picks up indoors at work gives me more than an hour of lighttime, enough for my entire return commute with a safety margin.
You could use a higher capacity battery for more runtime and/or a larger capacity solar panel for quicker charging time.

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