Mac Book Pro keeps shutting down after log in. Only logs in if charging.

For the past few months my Mac Book Pro has been shutting down at 98%-ish or after I log into my user. However if my laptop is connected to a charger it works perfectly besides the quick black flash of where it would have shut down if the charger was not in. Can someone please help me soon. I am finding it hard to deal with. I have tried to run the disk through first aid ( what ever that is). Nothing has seemed to work.