Can I use MacBook in clamshell on battery (not connected to power)?

I mainly work at my desk with my current 2014 MacBook in clamshell, attached to an external monitor. When I first started doing this, I discovered that it would only work if the MacBook was connected to power. Take out the power lead and the monitor goes dark (presumably because the MacBook has gone to sleep)

Now I am awaiting my new MacBook Air which is supposed to have a long battery life and I want to take advantage of this by working away from my desk. But if I need a big screen, I’ll go back to my desk and plug in the monitor.

So let’s say I’ve been working away from my desk and have 50% battery left. I don’t really want to plug in the power, when I plug in the monitor because it will recharge the battery and if I do that regularly, I’ll soon degrade the battery’s capacity.

I know I could have Mac open but I really don’t want the distraction of a second screen. So have things moved on since my original investigation such that I can run as a clamshell on battery alone?

Gary