Nothing too exciting, but still sorta neat. I soldered a Magicshine battery connector to a DC/DC regulator with USB outputs. So, I can now charge usb devices (tablet, phone, etc) from my Magicshine/Gemini/etc batteries. Great for camping.
usb charger from bike light battery pack by bbaker22, on Flickr
Here is a link to the cheap DC/DC regulator ($9.99):
Waterproof DC DC Converter Regulator 12V Step Down to 5V 3A MicroUSB Female USB | eBay
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Thread: USB Charger from light battery