iPhone turning off on 10 - 20% and battery percentage not showing right
I agree with Tyson, these are classic "dead battery" symptoms. If you want to be certain before ordering a replacement battery or having it repaired, use a battery utility, such as coconutBattery (for Mac) or 3uTools (for Windows). It will tell you what the health of the battery is (Design Capacity). If your battery shows anything less than 70% of design capacity, it will require replacement.