Capacity of a cell, electrochemically speaking, is the amount of lithium ions that can be exchanged between the cathode and the anode between the upper cut-off voltage and the lower cut-off voltage. Theoretical values usually differ from the practicably achievable ones, since only a part of the lithium stored in the electrodes is available for the electrochemical reactions. In practice, the capacity is calculated by the integration of the current over time. Additional complexity arises from the fact that cathode and anode potentials change with temperature and hence influence the upper and lower cut-off voltage criteria. Thus, to define the capacity, both the current rate and temperature information are needed. Further, it must be specified if the capacity is measured during charge or during discharge.
Cell manufacturers usually provide a rated capacity value for their cells: Rated capacity means the capacity during discharge, usually measured in ampere-hours, of a cell as measured under predefined specifications. Usually, the cell manufacturer provides information to determine the rated capacity such as temperature, applied current and cut-off voltage. The influence of different parameters on the measurable capacity makes it challenging to compare tests from different parties with the same battery cell. Only if every detail is specified and agreed on, a direct comparison is possible. Therefore, there is not one true capacity, but just various ways to determine the available capacity under certain conditions.