Measuring relative battery capacity with "bootloader life test"

Hello everybody,

currently I got the “feeling”, that my Fairphone 2 battery has lost some of its initial capacity. Before buying a new one, I want to know if this is actually true and, if so, whether it is actually significant. As an engineer in the measurement instruments industry, I do not like to act based on “feelings”, but like to perform (as far as possible) objective measurements.

For my previous smartphone, I “invented” a relatively simple test for measuring the relative capacity of a battery:

  • charge battery to 100 %
  • remember current (date and) time (=: START_TIME)
  • put phone in bootloader with the usual power button plus volume button combination specific to the phone model
  • wait until phone turns off
  • remember current time (=: END_TIME)

If you have done the above test when your battery was new, then you can repeat the test later and check how much your battery capacity has degraded. From my point of view, BOOTLOADER_LIFE should nearly scale linearly with the battery capacity.


  • For my tests with 3 phones (2 vendors, 3 models, ~5 batteries) BOOTLOADER_LIFE usually was between 3 and 5 hours
  • If phone firmware is updated, the power consumption of the device in bootloader can of course change. However, bootloader changes regarding the power consumption should not be very frequent.
  • (Slightly) different hardware (modules) in the phone of course may affect the measurements as well.
  • As with normal usage, the measurement method may show different results depending on operating conditions (typically only temperature).
  • When measuring battery of my Fairphone 2, I put the phone into Fastboot mode by pressing and holding volume down button and then pressing power button until Fairphone logo appeared and status LED turned blue
  • If one (like me) cannot (or does not want to) watch her phone the whole time, one can put it in front of a laptop camera or a webcam or similar and record a (long) video (in low quality and frame rate). It may be reasonable to put a clock next to the phone for not depending on timing issues with recorded video.
  • One may write an app for PC which helps automating the process when connecting the phone to the computer with a USB cable. If phone is detected in Fastboot mode, the app can easily measure the time from first connection until disconnect. However, I doubt that it is possible to turn off the USB power for the port for preventing the phone to charge over USB while still communicating with the device. To prevent this, one could modify a USB cable by cutting the VCC line off (only an idea, have not tested this, yet).

Now my question to the community:
Could anyone please perform the above mentioned test for the Fairphone 2 with a (nearly) new battery and tell me his or her results?

My result with a battery from 2017 was about 3:14 [hh:mm].

Of course, any other comments will be appreciated as well.

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