Why does my iPhone 4S die when my battery is full?

I recently got a used iPhone 4S and the battery is terrible. My battery can be at 100% and ill take a picture and it'll die. I can't use my phone for more than 30 minutes at a time. I've even had to buy a portable charger and it will still die. It's dies randomly all of the time even if my battery is full. What should I do? What's the problem?

My iPhone has never done that. It might be the battery. Go to the apple store.

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