I believe there is a defect in the charging software or hardware. I used several iPad 2 car chargers rated for at least 10 watts. (5.2Volts x 2Amps) They take 5x longer to charge than the iPad. Talk about proprietary hardware/software.
The OEM charger EP-TA10JBE is rated at 5.3Volts x 2Amps. The voltage loss in the 4Ft cable supplied must be at least .1 volts, so I do not understand that the voltage difference should cause the charging rate to slow to a trickle.