Why is the new battery worse?

I recently replaced the screen on my wife’s watch and figured I should replace the battery since it was open. The screen replacement went great but the battery I bought from ifixit seems terrible. Are the batteries sold, used or new? I have a series 1 Apple Watch that has its original battery and its life is still way longer than the series 2 replacement battery. I unpaired and set up the Apple Watch as a new watch and updated the software. I also turned off background app refresh but we can’t even get a full day from it. Am I missing something obvious? Thanks in advance for any help?