yeah,maybe N8X0 is not a mobile phone,but external battery pack is used for phones,For my phone, I use a solar power charger which gets me about 10-15hrs per charge, Universal USB Charger Kit, Emergency Cell Phone Charger (4-AAA gets about 30 mins) (I also have a few other travel, car/plane/plug/battery, chargers that I have not yet tried that might work out)

here are the links ,maybe you can get to know.

http://www.solio.com/charger/
http://www.234buy.com/