I installed a similar 5 socket cigarette strip at the nav station. After reading your post I found a 15 watt USB adapter that gets good reviews, has overcharge protection etc. I'll probably order a couple of them.
http://www.amazon.com/Dual-USB-Charger-3-1Amp-15-5W/dp/B00CD0JBZK/ref=sr_1_13?ie=UTF8&qid=1389535634&sr=8-13&keywords=Cigarette+plug+usb+adapter