What is the charging method based on high-voltage lithium-ion battery pack?

　　Overcharging of lithium ion batteries will seriously damage the performance of the battery, and may even cause an explosion to cause personal injury. Therefore, in order to prevent overcharging of single lithium ion batteries, lithium ion battery packs are generally equipped with a battery management system (Battery Management System, referred to as BMS), through the battery management system to protect each single lithium-ion battery from overcharging.

　　At present, the charging of lithium-ion battery packs generally adopts series charging, which is mainly because the series charging method is simple in structure, low in cost, and easier to implement. However, due to the differences in capacity, internal resistance, attenuation characteristics, self-discharge and other performance between single lithium ion batteries, when charging lithium ion battery packs in series, the single lithium ion battery with the smallest capacity in the battery pack will Fully charged first, and at this time, other batteries are not fully charged, if you continue to charge in series, the fully charged single lithium-ion battery may be overcharged.

　　The overcharging of lithium-ion batteries will seriously damage the performance of the battery and may even cause an explosion to cause personal injury. Therefore, in order to prevent overcharging of single lithium-ion batteries, lithium-ion battery packs are generally equipped with a battery management system (BatteryManagementSystem, Referred to as BMS), each single lithium ion battery is protected by overcharging through the battery management system. When charging in series, if the voltage of a single lithium-ion battery reaches the overcharge protection voltage, the battery management system will cut off the entire series charging circuit and stop charging to prevent the single battery from being overcharged, which will cause other The lithium ion battery cannot be fully charged.

　　After years of development, lithium iron phosphate power batteries have basically met the requirements of electric vehicles, especially pure electric cars, due to their high safety and good cycle performance. The process is basically equipped with large-scale production. condition. However, the performance of lithium iron phosphate batteries is somewhat different from other lithium-ion batteries, especially their voltage characteristics are different from those of lithium manganese oxide batteries and lithium cobalt oxide batteries. The following is a comparison between the charging curves of lithium iron phosphate and lithium manganate batteries and the corresponding relationship between lithium ion deintercalation:

　　Figure 1 Corresponding relationship between lithium ion deintercalation and charging curve of lithium manganate battery

　　Figure 2 Corresponding relationship between lithium ion deintercalation and charging curve of lithium iron phosphate battery

　　It is not difficult to see from the curve in the figure above that when the lithium iron phosphate battery is almost fully charged, the lithium ions are almost completely extracted from the positive electrode to the negative electrode, and the battery terminal voltage will rise rapidly, and the charging curve will be upturned. Easily reach the overcharge protection voltage. Therefore, the phenomenon that some batteries in the lithium iron phosphate battery pack are not fully charged is more obvious than that of the lithium manganate battery pack.

　　In addition, although some battery management systems have equalization functions, due to cost, heat dissipation, reliability and other considerations, the equalization current of the battery management system is generally much smaller than the current of series charging, so the equalization effect is not very obvious, and it will appear. Some single cells are not fully charged, which is more obvious for lithium-ion battery packs that require high-current charging, such as lithium-ion battery packs for electric vehicles.

　　For example, if 100 lithium-ion batteries with a discharge capacity of 100Ah are connected in series to form a battery pack, if 99 single lithium-ion batteries are charged 80Ah before the group is formed, and the other single lithium-ion battery is charged 100Ah, the When this battery pack is charged in series, the single lithium ion battery with a charge of 100Ah will be fully charged first, so as to reach the overcharge protection voltage. In order to prevent this single lithium ion battery from being overcharged, the battery management system will The entire series charging circuit is cut off, which makes the other 99 batteries unable to be fully charged, so the discharge capacity of the entire battery pack is only 80Ah.

　　When testing the capacity when the battery factory leaves the factory, the single battery is first charged with a constant current, then charged with a constant voltage, and then discharged at a constant current to measure the discharge capacity. Generally, the discharge capacity is approximately equal to the constant current charging capacity plus the constant voltage charging capacity. In the actual battery pack series charging process, there is generally no constant voltage charging process for the single battery, so the constant voltage charging capacity will be lost, and the battery pack capacity will be less than the single battery capacity. Generally, the smaller the charging current, the smaller the constant voltage charging capacity ratio, and the smaller the loss of the battery pack. Therefore, a battery management system and a charger coordinated series charging mode has been developed.