Battery cell cycle life refers to how many times a single battery cell can go through a full charge and discharge before its capacity drops—usually to 80% of the rated capacity. For example, a lithium iron phosphate (LFP) cell might be rated for 12,000 cycles, but that’s under perfect lab conditions: ideal temperature, relatively low charging current and voltage, etc.

However, battery cell cycles do not fully correspond to system cycles. A battery energy storage system (BESS) consists of hundreds of battery cells. Integrated into a system, each individual battery cell degrades at slightly different rates, and over time, this leads to inconsistent performance (capacity) across each cell. As we know, the least performant battery cell determines the performance of BESS, and the system cycle life will be also in a way determined by the least performant battery cells. From the perspective of a system, battery cell cycle life consumed by internal balancing cannot be ignored. Internal balancing means that the battery management system (BMS) will balance the capacity and voltage among cells in BESS, which is also conducted by charging and discharging, thus depleting the cycle life of cells.

In general, BESS cycle life does not equal cell cycle life. BESS purchasers should not only focus on cell cycle life, but also need to consider other parts of the BESS which could influence the real cycle life for BESS, i.e., cell packaging techniques, temperature management system, and BMS.