Those external USB batteries are great little doodads. A Jackery 6000 mAh single USB port battery is much smaller than my iPhone 4s and can recharge my iPhone, my small USB helmet lights and keep my video camera running for hours until the 32 GB card fills up, all on a single charge. Some colors run as low as $15. I've had one for a month, based on another cyclist's recommendations. Works great, no problems.

The 6000 mAh Jackery and similar batteries is the largest I'd consider for mounting on a helmet alongside a helmet mounted video camera. It'll add some weight, though. But the shape will nestle into the air vents on typical road helmets if absolutely necessary. Might compromise crash effectiveness, but so would the camera itself, lights, etc.

If you go for the larger capacity batteries be sure to get one with dual USB ports to run or recharge two devices. If I get another I'll use it to run my video camera continuously while having another port available for the phone to record much longer, or to recharge my lights.

If you don't mind running long USB cords you can run some video cameras continuously off external batteries. Put the battery in a jersey pocket, backpack, handlebar bag, etc. None of my rides are quite long enough to need that yet.