It for sure would charge batteries. Would also work with a ego passthrough or passthroughs with batteries since they are not really passthroughs when you vape with one plugged in your still
vaping off the batteries the USB cord just keeps them charged. If you have a straight threw passthrough or a true passthrough i doubt it would work very well it more then likely has a output of 5 volts 500ma to maybe 1000ma at a max. Which is not enough to power a atomizer. With a 1000ma you might get a enough vapor to piss you off is about it. So it can work depending on what you plan to use it for. Be sure to find you a 20% coupon for HF is you decide to get it.
Just saw the 3.9 volts thats got to be a typo for a usb charger it should be 5 volts for sure.