I found that this 12 volt device also works great, providing up to 2 amps total and 4 ports!
4 port 12 volt $4This 120 volt charger also has 4 ports, to charge multiple phones in one location.
4 port 120 volt charger $5This is a portable battery powered USB charger. It will hold 4 batteries, 3.7V each and up to 9800 mA hour rating per battery, so it will be a 19 AH battery pack.
$12 battery pack USB charger You will also need to order batteries and a charger to go with that battery pack.
If you buy a battery pack, you might also want to buy a LED flashlight that uses the same battery, it will come with the charger that you will need anyway, and then order an extra 4 batteries. The batter pack will recharge any cell phone (with USB charger input) several times between recharging the 18650 batteries. 18650 battery is availible in several capacities, from 2000 mA to 4,000 to 9,800 that I bought a few days ago.
Good luck,
Fred.