Q: Battery of iPhone drops way too fast

Over the past few months my battery performance has been sub par at the least however today was the last straw when I watched my battery go from 43 down to 28 and then down to 18 in a matter of seconds. the battery usage section in my setting says that snapchat takes up most of my battery even though I close out of it every time and have background app refresh turned off. Additionally, I'm on low power mode 100% of the time. This should not be happening. Any ideas for a fix? Or is my battery just defective?