How are lithium batteries graded?

How are lithium batteries graded?

Inconsistency between lithium cells can cause rapid capacity degradation and reduce battery life in lithium battery packs.

It is important to select cells with the same capacity, voltage and internal resistance to form a battery pack, which is important for the capacity and life of lithium-ion batteries.

Why inconsistency? (Cask Effect)

Just as there are no two leaves the same in the world, the same batch of cells have some differences after production.

After combining into a battery pack and cycling for a period of time, these differences continue to get bigger and bigger.

With the use of the battery, these performance differences continue to accumulate, leading to a gradual increase in the inconsistency between the cells,

just like the barrel effect, where the capacity of the battery pack depends on the capacity of the smallest cell. Making the battery pack performance and capacity decay.Li-ion battery inconsistency mainly refers to the inconsistency of Li-ion cell parameters (mainly cell capacity,

internal resistance, voltage and self-discharge) and the difference of battery SOC.

What are the causes of inconsistency?

There are many reasons for the inconsistency of Li-ion battery cells. Each step in the production process,such as the uniformity of the slurry during batching, the control of surface density and surface tension during coating, etc., can cause differences in the performance of single cells. In the battery combination process, the spot welding method and structure, the use of the environment will have an impact on the consistency of the battery pack. Because the energy consumed by each connection point is inconsistent, the performance of each component or structural part and the aging rate are also inconsistent, so the impact on the battery is also inconsistent. In addition, due to the different location of each individual cell in the battery, the temperature is different, and the performance degradation is also different, which will make the inconsistency of the single cell is amplified.

How to improve the consistency of the battery cell? There are three main ways

1, production process control

The control of the production process is mainly from the raw materials and production process. In terms of raw materials, try to select the same batch of raw materials to ensure the consistency of raw material particle size and performance. Production process should be strictly controlled throughout the production process, such as to ensure that the slurry mixing is uniform, not to be left for a long time, to control the speed of the coating machine to ensure that the thickness and uniformity of the coating, the appearance of the pole piece to check, weighing and grading, to control the amount of liquid injection, and the formation of the chemical, capacity, storage conditions, and so on.

 2, selection process control

Selection process refers to the sorting of battery cells, selecting the same grade of cells for the combination of battery packs (measuring the capacity/voltage/internal resistance of the cells, selecting the same grade of cells for assembly) to ensure the consistency of the initial performance of the battery. The voltage difference of single cell is an important factor affecting the consistency of each single cell in the battery pack, while the difference in internal resistance of single cell causes a big difference in the voltage plateau of each single cell in the process of charging and discharging the battery pack. In addition, with the increase of the discharge rate, the inconsistency of the battery will be greater.

3, maintenance process control

Real-time monitoring of lithium battery cells during the charging and discharging process can detect the consistency of the use of the process in real time, through real-time monitoring of the bad cells for timely adjustment or replacement, to ensure that the inconsistency of the battery pack will not expand over time. Alternatively, a balancing management system (BMS) can be used, and common balancing methods can be divided into passive balancing and active balancing.

In order to meet the use of the situation, try to reduce the depth of battery discharge, at the same time, to avoid overcharging the battery, can extend the cycle life of the battery pack.

Lithium-ion battery matching method:

Considering the capacity, internal resistance, voltage, self-discharge rate and other external conditions of the battery comprehensive evaluation, you can sort out the capacity, internal resistance and voltage closer to the battery cell, measures to improve the consistency of the battery are mainly in the following three areas: from the raw materials and the production process in two aspects of strict control of the production process; the use of a more scientific approach to sorting, as far as possible, select the initial performance of the same battery for the group; in the use of the battery and the maintenance process, the battery real-time control. In the process of battery use and maintenance, real-time monitoring of the battery, the introduction of balanced management system, the use of reasonable control strategies, thermal management of the battery, but also to strengthen the maintenance of the battery pack.