If you have a mod like Vamo don't bother with it, otherwise it's pretty handy. It's micro usb.
Before I got Vamo and couple of batteries to go with it, I used to have ego twist and ego pass through. When twist was charging I would use the passthrough and not worry about the battery. After I've got the Vamo never had any reason to use them again.
I've had a couple of passthroughs and the only thing I will say is that the supplied cable isn't all that. I used it at a computer and in the work van via a cigarette lighter adapter doodad and after a while they stop working, presumably due to a break somewhere in a wire.
I always use a ego passthrough Battery.... With a passthrough bat you will never go down low on volt/power as long as it's plugged into the usb of a laptop or car charger ...Why I love them! brilliant little things!