Defective battery and warranty question

My MBP is out of the 1 year warranty, but I had my battery replaced less than a year ago because it was defective. Right now, my battery is losing health abnormally fast - according to iStat, it was at 98% health a week ago, and is at 75% health now - and falling after 321 cycles. I know I should have bought the 3 year warranty while I had the chance, but my question: since my computer is more than a year old, but a defective battery was replaced less than a year ago, is this battery covered under any warranty? Or is this somehow normal battery behavior, which seems unlikely.