MacBook Pro battery rapidly loses charge I have a MacBook Pro Mid 2010 that I changed the battery and logic board on, a few moths ago.

I have an issue with the battery that after a full charge and removing the charger plug the battery suddenly drops below 80%, but runs normal till about 50% and then the MacBook suddenly shuts down and doesn't turn on again until I plug the charger in again.

I've tried reseting SMC and calibrating the battery with no result.
Apple has better diagnostics than third party plug in, so I would seek a quote / evaluation from AppleCare online or a store if that’s more convenient. 

The shutdown at 50% should be logged well enough for them to diagnose if you need another battery since that’s exactly what happens when one cell is failing or behaving like the other cells and the predictive model of how much charge is left doesn’t accurately tell you how much run time is left. 

Over time, I would expect the battery status to go to check battery or replace battery if it’s the battery that’s failing. 

It could be the logic board, too but that’s far less likely. Only swapping one and not the other will let you test which item is causing the discontinuity.