apple gave me a refurbished battery, doesn't last as long anymore
Okay, so over the summer, my macbook started shutting down/restarting by itself. I took it to the apple store and they fixed the logic board and something else in the hardware. As a bonus, they replaced my battery, but I believe it was refurbished (the receipt is not with me at this moment). Since then, my battery that used to last like 16-18 hours, started lasting only about 11-13 hours, which was still okay, because it would last me the whole school day.
But ever since I upgraded to El Capitan a few months ago, the most the battery lasts is about 6 hours.
Would I need a copy of receipt or would Apple still have this on file, considering this was done 8 months ago.
MacBook Air