I used about 30 to 40% of the battery (according to my yn300 led 2)
To recharge the batteries it took me about 15 hours with 220V cord.
So needless to say that the usb charging would take several days. I prefer not even imagine the load time by cigarette lighter.
Nevertheless batteries hold charge very very good (2h00 of use has consumed 30 to 40% of the battery)
I RECOMMENDED: a try with an official sony to reduce the loading time.