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I haven't checked any, but I'd think most won't provide a constant current.
I did see a project in Popular Mechanics (IIRC) that used a solar panel to charge a 12v battery, with a 12v cigarette lighter socket connected so you could charge your phone from there.

Yeah. And the problem with an inconsistent current is that the damn screen turns on every time you unplug it so you'd probably keep losing what you just charged back up if it's really intermittent.
And that's an interesting project, but assuming you're referring to a 12V lead-acid battery, that's hardly portable. What a workout to keep that thing in your bag!![]()
I remember that I had to build one for a Solar Energy Final a few years back. Although it looked rickety, it somehow was able to charge the battery. Not amazingly but from 15%->40%. Solar Chargers are a great concept, but I feel they dont have enough funding yet =(, wish it did cause then I can take advantage of this sunny weather......and not complain about how the A/C isn't working.
{at least then ill be hot and have a charged phone =-p}
the Battery Extender takes your device from zero to 45% charged in just 20 minutes!