why iphone battery died fast help?

idk why my iphone battery dies so quickly like i would charge it at home and then all of a sudden when i'm at the store or out to dinner it's like at 50% and then when i get on SNAPCHAT it dies faster on snapchat for some reason .. ugh it's so. annoying anybody else have this problem? if so what do i do??? I get on low battery mode the yellow thing but still IT DIES😡i hate iphones