Ipod Touch 4th Gen Battery Case?

Hello everyone. I've had my 4th Gen ipod touch for about a year now. The battery life is really going downhill though. I was just curious if there is any type of battery pack case made for these. Like the mophie ones that just snap on the back and start charging when the device falls below a certain percentage. I've seen them for the Iphone 4 and 4s but I've heard conflicting reports on whether or not this would work on the ipod. Thanks everyone for reading.