Battery problems

I have the I phone 6s. Ever since the Apple store gave me a new phone cause it wasn’t worth just fixing the screen, my battery dies within a few hours, it goes from 20% to 10% out of nowhere, sometimes it just shuts of anytime under 20% and when I plug it in cause it explains it dies using the plug in animation It turns back on with 18% battery ? I was told to check the serial number but it’s not wI think there time frame but it’s mimicing everything the web site says. Could I I’ve gotten an old battery when they gave me a different phone 9 months ago?