Battery is dropping from 50% to 7% - possible troubleshooting?

Hi all,

I replaced my MacBook Pro's battery with an ifixit battery and it worked well for a while, but recently an issue has developed where the battery will go from 100%-50% okay but from there it will instantly drop to 7%. From then on the battery seems to last the appropriate amount of time from 50% to dead, but it’s always a bit of a gamble! Is there anything I can do to recalibrate the battery? I've tried a couple of options but to no avail.

Does anyone have a ballpark figure of approximately how long the battery should last from full charge for a late 2011 MacBook Pro? The new ifixit battery seems to be doing the job but I have my suspicions that it has lost some capacity over a short amount of time, in conjunction with the battery indicator issue.

Thank you for reading and if you need any more information from me do let me know.