Macbook Pro Shuts Off When unplugged and Airplaying/Using Thunderbolt

I have a 15" MacBook Pro Retina, Mid-2014. I use it with an external monitor with my thunderbolt connector and sometimes for work I airplay my screen for presentations.

I've noticed recently that if I am Air-playing or mirroring to my external monitor, if I unplug the power adapter from my Macbook, it will shutdown until I plug it back in. I did a diagnostic and it looks ok, the battery is still in good order. I also tried an SMC reset. It still seems to be happening. Any advice?