Galaxy S5

The battery is normal, because the phone didn´t shut down out of nowhere or things like that.

A battery will get worse from time to time and lose performance. Batteries in electronic devices, including smartphones, do experience a decline in capacity over time due to factors like charge-discharge cycles and chemical changes within the battery cells. This decline in capacity can result in a shorter battery life and reduced performance.

Lets say your battery is 5000 mah big, after 2 years, it can be only 4000 mah big, which is completly normal.

Replacing the battery will get the lost capacity back.

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I don´t know how the battery life on the S21U will be with One UI 6, so… even if you replace the battery, it will get better for sure, but One UI 6 could drain it fast.