[Q] Htc desire 816 Battery case topic

Hey guys. I have done some looking and cant seem to find an extended battery case (as in plugs into the micro usb port) Has anyone been able to find one? On stock Rom with powe saving mode on I lose about 10 percent in 35-40 mins of screen on time at low brightness, which is pretty bad. A extra battery would be Extremely welcome! please link below if you are able to find one! ay alternatives?
Thanks in advance,
The_Victorian