Originally Posted by
noglider
Charging while riding is much more complicated than you would imagine. There are a few gadgets you can get which will take the output of a generator and charge a battery, but the cheap ones are crappy and maybe dangerous. The good ones are surprisingly expensive. B&M makes a headlight that also provides USB charging output, but I believe you can only run the light or the charger, not both at once. And it's about $180, so it's justified only if you're going to use it a heck of a lot.
The battery on the light you bought is fairly big, and it needs a lot of charging current. To provide that much current with solar, you'd probably need panels covering the entire bike and your entire body.
So as I said before, plug it in every night and consider getting a spare.
I have small foldable solar panel that weighs 12 oz, measures 9"x6" and it puts out 10W in direct sunlight. It will charge an i-phone in 60 minutes. I have not used it on my bike because my Panasonic 3400mAH batteries provide plenty of power for an allnight ride and carrying a spare set is much easier and lighter than trying to charge on the fly.
This is the panel and battery.....Instapark® Mercury 10M Solar Panel Portable Solar Charger with Built-in Dual USB Ports for iPhone, iPad & all other USB Compatible Devices, 5200mAh Battery Pack