In lithium-ion battery applications, to meet energy and voltage requirements, hundreds or even thousands of individual cells are combined into a battery pack through series and parallel connections. Theoretically, these individual cells should have identical characteristics. However, in reality, due to fluctuations in manufacturing and processing parameters, even lithium-ion batteries from the same batch can exhibit performance differences (e.g., capacity, internal resistance, and degradation rate). Although screening and matching are performed before assembly, 100% consistency cannot be guaranteed. Therefore, after assembly, these differences accumulate with increasing cycle counts, leading to a widening performance gap between individual cells. Furthermore, due to the large number of cells in the battery pack, internal variations inevitably occur within the pack during use. Under certain temperature gradients, the existence of temperature gradients can also lead to inconsistencies in battery internal resistance and current distribution, resulting in inconsistent degradation rates of individual cells. These factors can cause the cycle performance of the battery pack to be much lower than the cycle life of individual cells. For example, buses operating on the Beijing public transport demonstration line, without equalizer protection, although the lifespan of individual cells can reach more than 1,000 cycles, experienced severe capacity decay after only 150 cycles after being assembled into a battery pack. Spot checks revealed that the capacity of some individual cells was already below 80% of the rated capacity. This is mainly because the small differences in coulombic efficiency, degradation rate, and internal resistance increase of individual cells continue to accumulate during cycling, ultimately leading to the excessively rapid degradation rate of some individual cells.
The inconsistencies between individual lithium-ion batteries mainly involve indicators such as temperature, voltage, system-on-chip (SoC), capacity, and internal resistance. If we also consider the time factor, the inconsistencies of lithium-ion batteries should also include self-discharge, coulombic efficiency, and capacity decay rate. Long Zhou et al. from Shanghai University of Science and Technology categorized these inconsistencies into three types: 1) Initial factors, such as battery capacity and internal resistance, which determine the basic capabilities of lithium-ion batteries; 2) Current factors, such as capacity, voltage, and SoC, which determine the current capabilities of lithium-ion batteries; 3) Time-accumulated factors, such as the rate of capacity decay, the rate of increase in internal resistance, and charge/discharge coulombic efficiency, which determine the future capabilities of lithium-ion batteries. Once lithium-ion batteries are assembled into a battery pack, the "initial state" and "current state" of the battery pack are determined. What we need to address is the impact of "time-accumulated factors" on the performance of the battery pack.
The impact of "time accumulation factors" on the performance of lithium-ion battery packs is mainly manifested through the accumulation of repeated cycles. Taking "capacity decay rate" as an example, if two batteries A and b are connected in series, and the uniform reversible capacity decay rate of battery A in each cycle is 0.005%, while that of battery b is 0.008%, the inconsistency in the capacity decay rate of these two batteries will continue to accumulate in the cycle. After 500 cycles, the capacity decay of battery A is 2.5%, while that of battery b reaches 4%. If there is no equalization protection, battery b, due to its faster reversible capacity decay rate, will actually be significantly overcharged when battery A is fully charged, causing battery b's capacity to decay more rapidly, and even triggering thermal runaway of battery b. In reality, the capacity degradation rate of lithium-ion batteries is significantly higher in the early stages of cycling than in the later stages. Therefore, the difference in capacity degradation rate between A and b may be even greater. Furthermore, after overcharging and over-discharging of battery b, its capacity degradation rate will be further accelerated. Thus, without equalization protection, the performance degradation rate of a lithium-ion battery pack is much faster than that of a single lithium-ion battery cell.
Lithium-ion battery balancing strategies can be broadly categorized into two types: 1) dissipative balancing and 2) non-dissipative balancing. The key difference lies in how the battery's energy is disposed of during the balancing process. Dissipative balancing restores balance between individual cells by directly discharging all cells to a fixed voltage. Its advantage is its simple structure, but it wastes a significant amount of energy and generates heat. Non-dissipative balancing achieves balance by transferring energy from higher-voltage cells to lower-voltage cells. Its advantage is less energy waste, but its disadvantages include a more complex structure and higher cost.
Voltage is the most commonly used parameter in the equalization of lithium-ion batteries. By measuring the voltage of different individual cells in the battery pack, once the voltage difference between individual cells reaches a certain standard, the equalizer starts to work to equalize the individual cells. The use of the equalizer greatly reduces the deviation between individual cells during cycling and improves the cycle performance of the battery pack.
