To meet the energy and voltage requirements of lithium-ion batteries, hundreds or even thousands of individual cells need to be combined into battery packs by connecting them in series and parallel, which theoretically should have the same characteristics, but in practice, due to fluctuations in manufacturing and production process parameters, even the same batch of lithium-ion batteries still have certain differences in performance (such as capacity, internal resistance, and decay rate). Although we will screen and match before the group is formed, we still can't guarantee 100% consistency, so after the group is formed, these differences will accumulate in the process of use with the increase in the number of cycles, resulting in the performance differences between the single cells continue to expand, and at the same time, due to the huge number of cells in the battery group, there is bound to be a certain amount of internal battery group in the process of use. The existence of temperature gradient will also lead to the inconsistency of the internal resistance and current distribution of the battery, which will lead to the inconsistency of the decay rate of the single battery, all these factors will lead to the cycle performance of the battery pack is much lower than the cycle life of the single battery, for example, the bus running on the Beijing bus demonstration line, in the absence of equalizer protection, although the life of the single battery up to 1000 times This is mainly because the small differences in Coulomb efficiency, decay rate and internal resistance of the single cells continue to accumulate in the cycle, which eventually leads to the decay rate of some single cells being too fast.
The inconsistency between single-cell Li-ion batteries mainly contains indicators such as temperature, voltage, SoC, capacity and internal resistance, etc. If we take the time factor into account, the inconsistency of Li-ion batteries should also contain self-discharge, Coulomb efficiency, capacity decay rate, etc. These inconsistency factors are divided into three categories: the first category is the initial factors, such as the capacity of the battery, internal resistance, etc. They determine the basic Li-ion battery The second is the present factors, such as capacity, voltage, SoC, etc., and these indicators determine the current capability of Li-ion batteries; the third is the time accumulation factors, such as the rate of capacity decay, the rate of increase of internal resistance and the charge/discharge Coulomb efficiency, and these factors determine the future capability of Li-ion batteries. Once the lithium-ion battery is formed, the "initial state" and "present state" of the battery pack has been determined, we need to address is the "time accumulation factor" on the battery pack performance caused by We need to address the impact of the "time accumulation factor" on the performance of the battery pack.
The impact of the "time accumulation factor" on the performance of lithium-ion battery pack is mainly through the accumulation of repeated cycles, we take the "capacity decay rate" as an example, if two batteries in series A and B, assuming that each cycle of the A battery The average reversible capacity decay rate of battery A is 0.005%, while battery B is 0.008%, the inconsistency of 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 battery Breaches 4%, if there is no equalization protection conditions, battery B because of the reversible capacity decay rate is faster, so in the charging time when battery A After fully charged, the B battery has undergone a significant overcharge, resulting in accelerated capacity decay of the B battery and even triggering thermal runaway of the B battery. The decay rate of Li-ion batteries is significantly higher at the beginning of the cycle than at the end, so the difference in capacity decay rate between A and B may be even greater, and the capacity decay rate will be further accelerated after B battery is overcharged and over-discharged.
The equalization strategies for Li-ion batteries can be divided into two main categories: 1) dissipative equalization; 2) non-dissipative equalization.
The main difference between the two is where the energy of the battery goes during the equalization process. Dissipative equalization restores the balance between single cells by directly discharging all cells to a fixed voltage value, which has the advantage of simple structure, but wastes more energy and has the problem of heat generation. Non-dissipative equalization equalizes the single cells by transferring the power from the higher voltage cells to the lower voltage cells, with the advantage of less energy waste and the disadvantage of more complex structure and higher cost.
Voltage is the most common parameter used in the equalization of Li-ion batteries. By measuring the voltage of different cells in the battery pack, once the voltage difference between the cells reaches a certain standard, the equalizer starts to work to equalize the cells, and the use of the equalizer greatly reduces the deviation between cells in the cycle and improves the cycle performance of the battery pack.
Battery pack life is increased by more than five times, why is the balance so important?