To add to the good advice the other people have stated you can try to better calibrate the percentage of your device by letting it run completely down to the point where your phone shuts off. While its off plug it in. Keep it off while its charging. Once its green turn the phone back on, still keeping it plugged in. The indicator will go green to red for a short time. Once your at 100% then you can unplug and use your phone. By running it down to zero and leaving it off and fully charging it you essentially are calibrating the device.
Currently its off, and if it drops to 90% or so within minutes theres a good chance it could do the opposite when it gets so 5% or less. It might sit there for an hour or so because the calibration is off.
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