Why is my Mobile Phone's battery always hundred precent?

Hey guys,
Last week I got a Samsung GALAXY J5 (2015 edition) as a gift from my cousin.
I has been working smoothly but yesterday it got shut down at 4 as I was surfing through the net. I connected the charger and as usual, it reached 100. But since then the percentage remains staic. I tried restarting and also tried force shutdown.
After the force shutdown, it seemed normal. The battery shows 98. But now it won't go down. When I looked at the Battery Status app, it says "Battery Usage data not available"
PS: The warranty card has not been filled (no bill either). Bought from a foreign country.
Is my battery faulty? Any tips to correct it?

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