Why does my iPhone 5s battery drain so quickly?

I would charge my phone till 100% and stop charging for at least 6 hours. It's on airplane mode and the Wi-Fi isn't on, but after 6 hours without using, the battery becomes 7%. My battery skips the %, from 100%, within 10 minutes of usage, it drops till 25%. Do I need to replace a new battery? Any tips to prevent this from happening after I change the battery?

Hi because it has a faulty battery. Yes some do go faulty so why not go visit a phone shop.

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