What are the fundamentals of testing cycling battery cells?

The cycle life of a battery is defined as the number of charge-discharge cycles a battery can perform before its performance has degraded beyond its useful level. This is a somewhat imprecise definition, dependent on the amount of degradation that is deemed acceptable.

The cycle life can be more accurately represented as a plot with cycles on the x-axis and the percentage of battery capacity remaining on the y-axis. Further information may be given by plotting multiple lines to represent the cycle life dependent on the depth of discharge of the cycles. Battery cycle tests are used to obtain this data.

During the initial cycles of a cell’s life, electrolyte wetting causes capacity to increase. The battery then enters a phase of moderate capacity loss, as successive cycles cause a lithium compound layer to form on the anode, which consumes cyclable lithium ions. Later in the life of a cell, significant capacity loss occurs due to mechanical damage to electrode materials. Mechanisms include internal short circuits, thermal decomposition of the electrolyte, and oxidation of the cathode.

A battery’s C-rate represents how quickly it can be charged, represented as the number of times it could be fully charged in one hour; a rate of 2C is equivalent to a 30-minute charge time. The c-rate may also refer to the current required to discharge a battery at this rate. The cycle life of a battery depends to varying degrees on factors such as the depth of charge-discharge cycles, the c-rates used during charging and discharging, and the temperature reached by the battery during this cycling.

Cycle tests for cells typically ignore this complexity and are carried out with deep charge-discharge cycles, and relatively low c-rates, which in turn ensure relatively low temperatures. As a lithium-ion battery cell is charged from a fully discharged state, the charge current is initially constant while the cell voltage increases to its peak value. Once the peak voltage has been reached, the saturation charge phase begins, with the voltage remaining constant while the current decreases.

Typically, charging is terminated when the current reaches 3% of the rated current, corresponding to approximately 85% SOC. The peak voltage reached during charging is generally higher than 4V, but following charging, the open circuit voltage will drop to between 3.7 and 3.9V.

During discharge, the voltage decreases almost linearly with capacity until a voltage of approximately 3.4V is reached, at which point the voltage begins to drop more rapidly. Discharging from 4.2 to 3V releases about 95% of the energy stored. Discharging below 2.5V causes significant battery degradation and negligible increased capacity should, therefore, always be avoided.

A depth of discharge (DOD) of more than 50% is considered a deep discharge, and 80% is a typical practical limit to prevent excessive degradation.