Fast drop of battery charge level should not happen in case of new battery.
But if your phone is intensively used and you charge the battery often, then it could happen that battery level drop faster from 20% or 10%.
Fast drop of battery charge level from 20% to 1% can also happen:
- If you often give short charges to your battery (short charge cycles)
- If you don't use the phone for a longer time
- If there is a cold weather (battery drains faster when it is freezing)
- If charge indicator is not working correctly