Charging XB1 Controller

I have a few Nyko battery packs but I'm confused about which way to charge. Should I plug the USB into the slot on the battery or the slot that's on the controller itself? What is the difference between in the two? Thanks

Then what's the slot on top of the controller for? Just wanted to know the difference between the two

Both ways work. I imagine the slot on the battery is in case you only have one controller but multiple batteries? You could play while the battery charges

If you plug into the controller, surely that would be counter productive. The controller is meant to do one job no?

I'm not sure about the Nyko batteries, but the official Xbox rechargeable batteries can be charged with the USB cord that comes with it. You simply plug it in the controller while the batteries are in. I imagine this is the same thing with the Nyko version if it came with a usb cable.