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In order to trigger the charging indicator (on the TF101), you need 11 VDC. It should be the same for the Prime.
Other products (including an iPad charger) will charge, but slowly with the screen off. iPad charger yields about 10% per hour on my TF101 with the screen off.
For a car charger, I took a USB power plug, ripped out the components on the circuit board and direct wired the 12 VDC straight through with a 1.5 Amp fuse.
Also the wall wart is rated for 15 VDC and 1.2 Amps.