The BattPlug is a great little device, and for me, it's almost flawless. I wish I didn't have to unscrew the battery box to access the batteries, but because it's not made to be a primary charger I normally have enough power in it.
At $15.50 ($22.50 for Edge version, and both include free shipping) this is another one of those inexpensive, yet important items for Visor Prism, Pro or Edge users.
Overall (not an average)
Charge your visor using standard AA batteries