I use Barnacle Wi-Fi on a regular basis. At a mall, cafe, in my car or in areas of my building that don't get wifi, I use my X10 Mini Pro to tether my laptop, iPod Touch and iPad with Internet. This does take its toll on the battery life of my little Android.
Although my battery can last me a couple of days with regular use, tethering is a huge battery drain. Lugging around a charger is of no use if I can't find a power outlet to plug into, and not very pocketable.
Enter the Momax Portable battery. There are a lot of other battery solutions out there. And this review is more of portable battery packs in general. Pick the portable solution that is suited to the device(s) you have.
What I likes about the Momax battery pack was that it had a micro USB plug, which can charge both my phone, or Bluetooth headset. This also means that the chargers of my X10MP, Jabra Stone, and Momax are all interchangeable as well. Big convenience for field/travel use.
It provides approx 1200mAh of power, which is a little over a full charge for my phone effectively doubling my battery life. This gives me 8 full hours of tethered surfing or about 4 days of power with regular use. Why not just carry a spare battery? Portable battery packs allow uninterrupted use. And having a standard charging tip, allows use on more than one device.
iPhone users may like the battery jackets instead that wrap around the phone. These may make the phone a lot bigger but offer more usable power for the larger iPhones, and are more convenient to use since they act as a case for your iPhone.