Do I need?

Hi. I bought a Ring Pro, took off old doorbell and hard wired it to where it was. Now, I want to put a Ring on my back door for safety. Does it have to be hard wired too? Or is the rechargable batteries alone needed? I have no doorbell on the back door, that is why I am asking if the second Ring needs one so it can be hardwired? Does it need a base to work? Or is the Ring Pro from my front door enough? thanks