iPhone charger question

[w8ing4intelmacs]

I'm curious. I have a car accessory that has a USB port, but it won't charge my iPhone even if I use the USB cord that came with my iPhone. Why? Don't all USB ports output the same?

 

[-Ryan-]

Wirelessly posted (iPhone: Mozilla/5.0 (iPhone; U; CPU iPhone OS 3_0 like Mac OS X; en-us) AppleWebKit/528.18 (KHTML, like Gecko) Version/4.0 Mobile/7A341 Safari/528.16)

It depends what USB standard it is, and whether the accessory allows charging over USB which this one probably doesn't.