Charging method of high voltage lithium battery -Lithium - Ion Battery Equipment

Charging method of high voltage lithium battery pack -Lithium - Ion Battery Equipment

General series charging

At present, lithium ion battery packs are generally charged in series, which is critical because of its simple structure, low cost and easy implementation. However, due to the differences in capacity, internal resistance, attenuation characteristics, self discharge and other performances between single lithium-ion batteries, when the lithium-ion battery pack is charged in series, the single lithium-ion battery with the smallest capacity in the battery pack will be the first to overflow. At this time, other batteries have no overflow. If the series charging is continued, the single lithium-ion battery that has overflowed may be overcharged.

However, overcharging of the lithium ion battery will seriously damage the battery performance, and may even cause personal injury due to explosion. Therefore, in order to guard against overcharging of the single lithium ion battery, the lithium ion battery pack is generally equipped with a battery management system (bMS) to protect each single lithium ion battery from overcharging through the battery management system. During series charging, 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 in case that this single battery is overcharged, which will cause other lithium ion batteries to be unable to overflow.(Lithium - Ion Battery Equipment)

After years of development, lithium iron phosphate power lithium-ion battery has basically met the requirements of electric vehicles, especially pure electric cars, due to its high safety, good cycle performance and other advantages, and the process has basically met the conditions for large-scale processing. However, the performance of lithium ion iron phosphate battery is different from that of other lithium ion batteries, especially its voltage characteristics are different from that of lithium manganate battery and lithium cobalate battery. The following is a comparison between the charging curve of lithium iron phosphate and lithium manganate lithium-ion batteries and the corresponding relationship of lithium ion deinking:

It is not difficult to see from the curve of the corresponding relationship between the lithium ion de embedding and the charging curve of the lithium iron phosphate battery that when the lithium ion of the lithium iron phosphate battery is rapidly overflowing, the lithium ion is almost completely de embedded from the positive pole to the negative pole, the battery terminal voltage will rise rapidly, and the charging curve will rise, which will cause the battery to 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 the equalization function, the equalization current of the battery management system is generally far less than the current of series charging in terms of cost, heat dissipation, reliability and other aspects, so the equalization effect is not very clear, and some single batteries may not be fully charged. This is more clear for lithium-ion battery packs that require high current charging, such as lithium-ion battery packs for electric vehicles.

For example, 100 lithium-ion batteries with a discharge capacity of 100Ah are connected in series to form a battery pack. However, if 99 of the single lithium-ion batteries are charged with 80Ah, and the other single lithium-ion battery is charged with 100Ah, when the battery pack is charged in series, the single lithium-ion battery with a charge of 100Ah will overflow 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 cut off the whole series charging circuit, which will prevent the other 99 batteries from overflowing, so the discharge capacity of the entire battery pack is only 80Ah

Generally, when the battery manufacturer detects the capacity when leaving the factory, the single battery is charged at constant current, then charged at constant voltage, and then discharged at 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. However, there is generally no constant voltage charging process for single battery in the actual series charging process of battery pack, so there will be no constant voltage charging capacity, and the battery pack capacity will be less than the single battery capacity. 



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