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I was about to suggest the same thing. However, you can actually BUY something much better. This is an emergency charger stick powered by 2 AA batteries, and has a USB slot to plug your charger cord into. I paid about $5 for this, and it works great.
I ordered it from a place called American Science and Surplus. They have some very cool stuff, but it's hit or miss as far as availability. I ordered one for everyone in my family. It's a great thing to have in an emergency.
As long as the voltage in the charger exceeds the charge in the phone's battery, then yes, it will keep charging.