Shuts off if not connected to power

I have successfully swapped out the logic board on my MBP with a solid part from the iFixit store. It seems to be working great except for when I unplug from my Thunderbolt Display and work without power adapters. It works for about 5 or ten minutes then it will lock up. I’m planning to open it back up and triple check (because I already double checked) the power to the board. But I wanted to see if there was anything else I should be aware of before doing that. SMC, PRAM, Apple Diagnostics all came back fine.