Why is replaced battery not lasting?

Hello All,

I recently replaced my 2011 macbook air battery after an issue with the original battery swelling while not in use for a few months. All seemed well after the replacement; everything registered properly. However, if the computer is removed from the charger, the battery only lasts about 40 minutes while in use (excel, safari). I got the "new" battery from iFix it, not the refurb - so unsure why this would be happening. It does get very warm (as usual with that model), would that affect it? Any guidance to resolve?

Thanks!