Battery life starts going down super fast at 15%

Hello, I have an Acer Iconia Tab 7 A1-713 with Android 4.4.2 KitKat. I've been having the tablet for about 7 months and this just started happening. When battery reaches 15% and notifies me about it, the battery starts rushing down so fast that it goes from 15% to 3 or 2% in the timespan of 30 seconds, which is brutal.
I haven't installed or downloaded anything recently, nor rooted it. How do I fix this?

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