I was wondering what if any difference there is with the chargers (ac vs. usb). I only have the usb charger and it charges in about 3+ hours. I'm getting about 8 solid hours of use before it goes dead. I've heard that plug in chargers charge batteries better than the usb type, but that could just be hear say. I thought for sure it would last me all day, but it ran out on me on my commute home from work Friday. I think the first charge really lasted about 9.5 hours, and I may have been vaping heavily. There is no noticeable drop off in vapor production like the typical 510 batts, so there's really zero warning before it goes.