Why is my Tablet shutting itself off when it has plenty battery life left?

Hey guys. So I have a Prontotec 7 inch 1024×600 tablet, DDR 512MB, ROM 8GB, dual core Android 4.2.2. Recently its been shutting itself off at around 90℅ battery. When I try to switch it back on, it gets to the Android screen then switches off again. It only turns back on when I put the charger in.
I've discovered that at 100% it switches off if I have FULL BRIGHTNESS. Otherwise, it lasts twenty minutes tops. However, if I have the brightness at it's lowest and the WiFi off the tablet is fine as long as I'm not doing anything too atxing, like watching a movie. I've noticed that when it shuts itself off,the screen briefly flickers then dies. It started soon after the micro USB port broke due to the... Not great build quality. Its a cheap tablet but I would like to sort it out, only being able to use it plugged into a charger is IRRITATING. Is it due to battery damage or has it just run its course? I've had it around a year now.
Thanks guys!

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