http://batteryscore.com operates by running a stress load on the phone and then measuring the battery degradation over that period that the test runs.
The app analyzes the battery life of the iPhone by measuring the time taken in seconds for the phone to move through two API battery level thresholds and uses that to compute an overall score for the battery.
The result is a score, which you can easily compare to other phones to see if your battery is still in good condition. The higher the value the better the battery.
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