I started looking for a battery-powered smart phone charger, and the search has turned out a bit harder than I thought. Regular (dumb?) phones don't demand as much power, and can be charged with a AA battery. I think smart phones need more. I found some candidates on Amazon, but nothing with rapturous reviews. In fact, the brand I used to use seems to have fallen down flat on its face. Any recommendations? Thanks!

Bingley

EDIT. Forgot to mention that this is meant to be an emergency charger for a common battery size (like AA or AAA). Small size and portability are important for this purpose.