Powertraveller's Powermonkey Extreme is a combination of a battery and solar panel specifically aimed at charging 5V (USB or others) devices. I tested the Powermonkey Extreme with a Zoom H4n audio recorder and a bunch of GoPros to see if and how one Powermonkey Extreme can get you through a whole day of recording.
I received a black to set up my test environment. My goal was to see if the solar panel would be able to charge a GoPro HD Hero2 with the new WiFi BacPac for a whole day recording, and how long the battery would be able to run a Zoom H4n audio recorder driving two 48V phantom powered microphones.
The Powermonkey Extreme is rated for 5V devices, including USB. It can charge iPads, iPods, iPhones and every other 5V device that draws up to 9000mAh. The whole system, from travel case to battery unit and foldable solar panel, oozes quality of build and well-designed features.
Charges anything via USB
My tests with the Zoom H4n powering two 48V phantom-powered studio microphones with the Powermonkey connected to its mains input, confirmed Powertraveller's claim of this device being able to charge an iPhone and a Garmin Edge 800 up to six times and standard mobiles up to 12 times.
For the GoPro test I used a slightly different approach. I imagined someone wanting to shoot continuously for a whole day. Doing that would be very difficult, even with GoPro's extra battery pack installed.
Here comes the sun
Now I had two different charging methods to my disposal: one would only use the solar panel. A day of bright sunlight allowed me to try out the solar panel sitting behind a window. I let it charge the GoPro HD Hero2 and that worked out brilliantly with the GoPro not taking more time to get its full charge than otherwise.
However, with a Powermonkey Extreme you can sort of-daisy chain your power devices. That was my second option. This was my setup: the solar panel plugged in to the battery pack's input port and the USB cable plugged in to the GoPro's mini-USB port.
This arrangement ensures that you have a stable power supply (the battery) while the solar panel maintains stamina. This was 100% certain to give me a whole day of GoPro shooting.
Two words of advice, though: while the Powermonkey Extreme system is waterproof, it's not when its ports are exposed. Wrapping the ports firmly in plastic so they stay dry is best. The GoPro enclosure is waterproof too, but the waterproof case hasn't got a hole to plug in a USB cable. I didn't have this problem, but I can imagine drilling a hole in the waterproof enclosure and isolating it with some silicon kit will work miracles.
From my testing, I don't even think you would need to plug in the battery unit in the solar panel if you want to run the GoPro unattended for 12 hours on end. Given that the power hungry H4n audio recorder kept working for half a day with enough power to spare for what I estimate would have been another four hours or so, I think the solar panel backup is needed for peace of mind more than anything else.
A Powermonkey Extreme will set you €141.60. Considering the test results, that's pretty cheap.