What is happening with my iPhone 4S?

I have a used iPhone 4S that has no damage whatsoever and it use to be my dad's phone and he never had any problems with it but I've noticed that for me, the battery percent is unreliable because it dies at all types of different percentages like sometimes it won't die until completely no battery and sometimes it will die at percentages as high as (so far at least) 87% or anywhere in between so I can never trust it. And the percentages quite a bit will jump around because not even using it for long the battery will suddenly drain 20% or when I put it on the charger the battery will just around to different percents. What's happening and what can I try doing to fix it? It's with AT&T.

I reckon you might have a faulty battery or hard rive but there's nothing you can really do to help it yourself you need to take it to repairs or get a specialist to look at it for you.
(Have you tried rebooting the phone sometimes it clears out a lot of apps and storage off your phone as it could use up a lot of your battery and make it very unreliable)
Trust me there's nothing much you can do yourself just get it repaired if rebooting doesn't help.

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