iPhone 6s plus battery goes very, very fast to 1%

28 april I bought a new iPhone 6s plus from a marketing site where you can sell/buy items( https://www.marktplaats.nl/). It was a very new phone she sad. I began setting the phone up and everything was okay, until a couple of days later. I noticed that my phone was full charged at the beginning of the day and very low percentage after a couple of hours. Now, it goes from 100% down even till 24% in 30 min. I've tried calibrating, restarting, charging from 0% to100% but nothing works. I've searched for a battery replacement, maybe that works? Is there someone who can help me? Thanks