LiFePO4 batteries, with their advantages of high safety and long cycle life, have become the core energy storage components of portable power stations. The cycle life data claimed by manufacturers (such as 80% capacity retention after 2000 cycles) is often obtained under standard test environments with specific depth of discharge (DOD), while in actual outdoor use, the impact of differences in charge-discharge depth on battery life is far beyond expectations.
In standard laboratory tests, 100% DOD (full discharge from a fully charged state to the cut-off voltage) is a common benchmark for measuring cycle life. Taking a LiFePO4 battery adapted to a portable power station as an example, under 100% DOD conditions, after 2000 charge-discharge cycles, its capacity retention rate drops to 80%, reaching the retirement threshold; if the charge-discharge depth is controlled at 50% DOD (that is, only half of the total battery capacity is used each time), the cycle life can be extended to more than 4000 times, and some optimized batteries can even exceed 6000 times. This difference stems from the chemical mechanism inside the battery: during shallow cycling, the structural loss rate of electrode materials slows down significantly.
Shallow charge-discharge (such as 20%-80% DOD) is an ideal mode to extend battery life. Within this range, the battery voltage fluctuates slightly, the lattice structure of the positive electrode material is not easy to break due to excessive expansion or contraction, and the precipitation of lithium dendrites on the negative electrode surface can also be reduced. For portable power stations used outdoors, if users develop the habit of "charging when the power is below 20% and stopping when charged to 80%", the battery cycle life can be increased by 2-3 times compared with the 100% DOD mode. Especially in high or low temperature environments, shallow charge-discharge can also reduce the intensity of chemical reactions inside the battery and reduce the generation of by-products.
Deep charge-discharge (such as 0%-100% DOD) will accelerate battery aging. During each full charge and discharge, the lithium iron phosphate in the positive electrode material will undergo more severe phase changes, and the structure is prone to collapse after long-term cycling; in the fully charged state of the negative electrode, excessive intercalation of lithium ions may lead to the deposition of metallic lithium, and the formed lithium dendrites will not only consume active materials but also may pierce the separator and cause safety hazards. Practical cases show that if the portable power station is frequently charged only when the power is less than 5%, the original 2000-cycle life may be shortened to 1200-1500 cycles, and the later capacity attenuation rate will be significantly accelerated.
Extreme deep discharge (such as discharging the power to 0V) is even more harmful. At this time, the internal voltage of the battery drops sharply, which may cause the dissolution of the copper current collector of the negative electrode. After the generated copper ions are precipitated at the positive electrode, they will cause permanent capacity loss. Even if only one extreme deep discharge occurs, the battery capacity may drop by more than 10%, and cannot be recovered through subsequent charge-discharge.
In addition, the stability of charge-discharge depth will also affect the life. In outdoor use, if frequently switching between deep discharge and shallow discharge (such as using the power to 10% one day and only to 70% the next day), the battery management system (BMS) needs to constantly adjust the charge-discharge strategy, which may lead to a decline in cell consistency and indirectly shorten the overall life.
Therefore, in order to maximize the service life of LiFePO4 batteries in portable power stations, it is recommended to maintain the power in the 10%-90% range for daily use, avoid long-term storage at full power (the risk of lithium dendrite growth is higher in full power state), and try to eliminate deep discharge, especially to prevent the power from being exhausted.
