2015 Macbook Pro Screen hoes black at 30% Battery

Try and reset the SMC of your Mac. This can be a bit technical, But I will guide you through this, and this could be the solution of battery problems on an intel based Mac

For a 2015 MacBook, this is an intel one and it doesn't feature the T2 security chip. So you will need to follow these instructions in order to reset the SMC of your Mac

1. Shut down your Mac.
2. On your built-in keyboard, press and hold all of these keys:

• • Shift  on the left side of your keyboard
  • Control  on the left side of your keyboard
  • Option (Alt)  on the left side of your keyboard

1. While holding all three keys, press and hold the power button as well.
2. Keep holding all four keys for 10 seconds.
3. Release all keys, then press the power button to turn on your Mac.

If you need help, contact Apple Support. 

Hope these instructions help, and please let me know if you're in need of any more guidance!

Sounds like the battery is failing if the laptop is powering off at 30% charge level. You may want to check the Battery Condition to see if it says "Service Recommended". You can also try running the Apple Diagnostics. Unfortunately many battery failures never show up with either test. If your battery is more than 3 - 5 years old, then it is most likely bad.