We all know that assembling a battery pack is to weld multiple cells together in series and parallel.
So do you know that the cells of the battery pack are sorted? The cells in the lithium battery pack all require similar voltages and internal resistance to be assembled. Simply put, the battery cell needs to achieve consistency.
What is consistency?
Lithium battery consistency refers to the consistency of the initial performance indicators of single cells used for groups, including: capacity, impedance, electrical characteristics of electrodes, electrical connection, temperature characteristics, decay speed, etc. The inconsistency of the above factors will directly affect the difference in output electrical parameters in operation.
Why achieve consistency?
In pursuit of consistency, in addition to exerting the maximum capacity of the battery pack (including maximum power, maximum current, and maximum usable capacity) in the current state, we also want to maintain the life of the battery pack as much as possible. The difference in the consistency of the cells is like mixing old and new batteries. The cells with large differences, in addition to dragging down the normal cells, are also the weak points of the entire battery pack, and overcharge, overdischarge, heating, etc. all start from them.
Why are the cells in the same batch inconsistent?
The inconsistency of lithium battery packs is a continuous accumulation process, and the longer the time, the greater the difference between individual cells; And the lithium battery pack will also be affected by the use environment, and the inconsistency of the single battery will be gradually amplified during the future use, resulting in accelerated attenuation of the performance of some single batteries, and eventually resulting in the battery not using the function.
The inconsistency of lithium battery packs is mainly affected by time, and the reasons mainly include two aspects:
1. First of all, there are problems in the process and uneven materials in the manufacturing process, so that there are very slight differences in the materials and so on of lithium batteries; After the lithium battery pack is put into use, the electrolyte density, temperature and ventilation conditions, self-discharge degree and charging and discharging process of each battery in the battery pack may be affected by the difference, and the capacity and internal resistance of the same model of battery leaving the same batch may be different.
2. When loading and using, the difference in electrolyte density, temperature and ventilation conditions, self-discharge degree and charging and discharging process of each battery in the lithium battery pack is affected.
What harm and problems can inconsistencies cause?
(1) Capacity loss, the cell monomer constitutes a lithium battery pack, the capacity is in line with the "barrel principle", the capacity of the worst cell determines the capacity of the entire battery pack.
(2) Life loss, small capacity cells, every time is full of discharge, the output is too strong, it is likely to reach the focus of life first. At the end of the life of a cell, a group of cells welded together will end of life.
(3) The internal resistance increases, different internal resistance, flowing through the same current, the cell with large internal resistance generates relatively more heat. The battery temperature is too high, resulting in faster deterioration and further increase in internal resistance. The internal resistance and temperature rise form a pair of negative feedback, which accelerates the deterioration of the high internal resistance cell.
Sorting
Different batches of cells are theoretically not used together. Even the same batch of cells needs to be screened, and the cells with relatively concentrated parameters are placed in a battery pack and in the same battery pack. The lithium battery cell automatic sorting machine can directly sort out the set open circuit voltage, AC internal resistance and other parameters. And the sorting efficiency is high, 10 channels are sorted together, 9 OK channels, 1 NG channel. The consistency of the lithium battery pack is guaranteed.
