Hi John,

This should be possible, but it might be that you have to have the ignition key in and turned to the ACC setting. If so, make sure everything else in the car is turned off, such as the climate control, lights and stereo, as it could damage the charger. Also, it's likely that the charge rate will be low through this method, as you could blow the socket's fuse if attempting to charge at a high rate. 

If in doubt, check with Hyundai directly.