As long as stuff is connected using Bluetooth and/or wifi it will keep using battery. This is the case with anything. But since the Bluetooth 4 protocol is very battery friendly, it won't 'eat' your battery as fast as in the past with connected devices tho.
Thank you for your quick reply. It was my understanding that the new keyboard does not use BT or WiFi, but the three contacts on the side of the iPad. Is that correct? Does that make a difference?
I'm not familiar with iPads at all, so I just replied regarding wifi and Bluetooth keyboards.
I just googled this for you and my understanding is the new keyboards using the Smart Connector on iPad Pro gets its power from it.
This way you won't need batteries anymore in keyboards connected through the smart connector.
My understanding is the newest keyboards are prone to consume more data than selfpowered keyboards.
I have a 9.7" iPad Pro that is attached to the Apple Smart Keyboard. If that's what you're referring to then know that the Smart Keyboard does NOT HAVE any negative effects on the iPad's battery life, at least not in my experience thus far.
Just like @phreddyl and @Just_Me_D I use my iPad exclusively with the Smart Keyboard, never had any noticeable battery impact. It must be said that I also never really noticed any drain with my iPad Air with the Apple bluetooth keyboard.