This might come in handy instead of an extra battery.

"The 5-inch by 3-inch device known as Soldius1 converts solar energy into 6.58 volts at a maximum of 320 milliamperes of power after only about 3 hours in direct sunlight."

Here

I use two iSun solar collectors and an iSun battery pack that holds 10 rechargeables for backup power. I've mostly used it with XM radio, but it would also be fine for the phone. I've got larger panels, too, for my tablet and my Airstream. - sheureka

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man for that price that is not too bad. i could mount that to my sunroof. will it work thru semi tinted glass do you think?

I dont think it would work as well under tinted glass - but in your car a car charger would be cheapest.

On a related note, does anyone know it the SideWinder would connect to the 6700?

Look here