Hey Becca, I haven't used a Priv but in checking it out the USB is recommended to be used for updates and not for charging. This is fairly normal for dual battery mods without DNA chips as they don't balance the charge. That being said, the batteries would best be charged externally.
This is where I buy my batteries and chargers, the X-Tar chargers are my go to...
Chargers - Batteries, Chargers, and PowerPax Carriers
Hope that helps, welcome to the forum!