You can test the true capacity of a lithium battery using a simple device: a frequency counter (or dometer). The details are as follows:
1. Select the current magnification range to an appropriate value, such as 0.5A or 1A.
2. Connect the lithium battery to the magnification via the wire and turn on the magnification. At this point the magnification will begin recording current and voltage data.
3. Allow the lithium battery to discharge to approximately 3.0 V, then turn off the magnification and record the battery power at this time.
4. Charge the battery to full power and record current consumption and duration.
5. Calculate the actual battery capacity based on the above data. Actual capacity = discharged capacity/charged capacity x nominal capacity.
It should be noted that this method only approximately tests the actual capacity of the batteryie lithium and that there may be certain errors. If you need more accurate test results, it is recommended to use professional battery testing equipment for testing.