USB charger? Answered

There exist inexpensive DC-to-DC converters made to charge USB devices from input power from a car's cigarette lighter receptacle. You've probably seen these before, e.g.

http://www.google.com/search?hl=en&tbm=isch&q=usb+...

The output, 16 VDC, from your laptop brick should be comfortably in the input range acceptable to one of these converters. Essentially this adapter is a switching style DC-to-DC converter with a regulated output of approximately 5.0 volts DC. There is a pretty wide range of DC input voltage over which the converter will work.

Actually, I've got a picture of one of these things in my image library, um.... here,

https://www.instructables.com/files/orig/FTM/8LPC/G...

and the label printed on it says it has input voltage range of 12 to 24 VDC, and I think that's typical for a gizmo like this.

Using a linear regulator, like the LM317, is usually a bad idea when the input and output voltage are very different, since this voltage difference, multiplied by the current draw, must necessarily be dissipated as heat. E.g. for 16V in, 5V out, the efficiency is 5/16 = 0.3125, and 11/16=0.6875 of the input power is wasted as heat.

The 317 isn't capable of handling 2.5A. If I were you, I'd find a cheap 16v-5V buck switcher.