The battery falls after getting below 20%

[alene]

I have a mi9 phone with 12.5.1 Rom, after a year of use, I noticed today a weird problem with the battery, when it gets below 20%, it falls in 1 second to 1%, and then the phone shut down, and when I plug the charger the battery level shows 6% instantly.
I tried a factory reset, but the problem still exists.

 

[Schwarzzu]

Old battery? Does it last normally when above 20%?

 

[alene]

Old battery? Does it last normally when above 20%?

The phone work normally, but I feel the battery is stuck at some levels or drains slowly, I tried to turn on the flashlight and open all apps but the battery decreased by 1% after 5min, which is not normal.

 

[Chupacabra83]

The phone work normally, but I feel the battery is stuck at some levels or drains slowly, I tried to turn on the flashlight and open all apps but the battery decreased by 1% after 5min, which is not normal.

try gsam battery pro

 

[alene]

I noticed something, the phone works normally from 100% to 20%, but when the battery level drops below 20% and asks me to activate the battery saver mode, the battery level falls to 10% and then 1% in 2 seconds.