Here’s something I learned today fromRic Ford at Macintouch

The same thing occurs with monitors connected via Mini DisplayPort (which is also the Thunderbolt connector) to a 2011, 2012 or 2015 MacBook Pro [in clamshell mode]. No power adapter, no external display image.

Source: 2016 MacBook Pro et al

Seems that if you want to use a battery powered MacBook Pro to connect an external display in clamshell mode you’re out of luck