Why does a 12 inch Macbook refuse to turn on with a lot of battery remaining unless it is plugged in? This Early 2015 12 inch MacBook was showing 75% battery remaining, and when I plug in my iPhone to try to record the iPhone, the Macbook began to sleep and refused to turn on when I press the power button.  It showed the "Plug Me In" icon.

So I unplugged the iPhone and started charging it, and then it would turn on right away, and showed a 69% battery level.

So why did it refuse to be on at such a high level of battery remaining and how can it be fixed or prevented?

P.S. It is a Macbook 12 inch that was introduced in 2015... so the battery is just a little more than one year old.
Assuming it is a 12" MacBook (implying at most it's still a little shy of being a couple of years old) and not a 13" model (which could be a lot older), then the first thing I'd do is reset the System Management Controller.

For a 12" MacBook you reset the SMC as follows:


Shut down your computer 
Keep the power cable plugged in. 
Press at the same time shiftoptioncontrol 
(on the left side of the keyboard) and the power button 
Let go 
Turn your computer back on with the power button. 


Once you've done this report back on how it went.

If in fact it's not a 12" MacBook, then please update your question with the exact model so we can make sure we're providing accurate advice for your model.