The inconsistency of lithium battery parameters mainly refers to the inconsistency of capacity, internal resistance and open circuit voltage. The inconsistency of the performance of the battery cells are formed in the production process and deepened in the process of use. Today, we will take you to understand the consistency of lithium battery packs.
Definition of Consistency
Consistency of lithium batteries, in front of the eyes, refers to a group of lithium batteries important characteristic parameters of convergence, is a relative concept, there is no most consistent, only more consistent. The same battery pack within the multiple strings of cells, each parameter, the best all in a smaller range, is good consistency.
Adding the time dimension, consistency refers to the consistency of all the characteristic parameters of all the cells in the battery pack during the whole life cycle, adding the inconsistency of capacity degradation, the inconsistency of internal resistance growth, and the inconsistency of aging rate. The life of the entire battery pack is the ultimate focus of our attention to consistency.
The purpose of pursuing consistency is not only to maximize the capacity of the battery pack (including maximum power, maximum current, and maximum usable capacity) under the current state, but also to maintain such capacity for as long as possible.
The Concept of Inconsistency
The inconsistency of lithium battery parameters mainly refers to the inconsistency of capacity, internal resistance, and open circuit voltage. Where voltage is the initial voltage of the battery at the time of assembly. The internal resistance is the AC internal resistance when fully charged, and the capacity is the discharging capacity of the cell when fully charged.
Accumulated with the continuous charging and discharging cycles of the battery during use, resulting in greater differences in the state of each individual cell (SOC, voltage, etc.);.
The usage environment within the lithium battery pack is also different for each single cell. This leads to the inconsistency of the single cell gradually amplified in the process of use, which in some cases accelerates the decline of some single cell performance, and ultimately triggers the premature failure of the lithium battery pack.
Supplementary: SOC is used to describe the remaining battery charge, which is one of the important parameters in the process of battery use, and SOC estimation is the basis for judging the battery overcharge and overdischarge.
There is a clear monotonic correspondence between the open circuit voltage of the lithium battery and the charge of the battery, as long as the accurate open circuit voltage is obtained, the battery charge can be deduced.
Reasons for Inconsistency
Inconsistency of lithium battery pack is an accumulative process, the longer the difference between the single cell is greater; and lithium battery pack will also be affected by the use of the environment, the inconsistency of the single cell will be gradually amplified in the future use of the process, resulting in accelerated degradation of the performance of some single cell, ultimately resulting in the lack of functionality of the battery.
The inconsistency of lithium battery packs is mainly affected by time, the reasons mainly include two aspects:
1, first of all, there are process problems in the manufacturing process and the unevenness of the material and other issues, so that the lithium battery materials and so there are very small differences; in the lithium battery pack into use, the battery pack of each battery electrolyte density, temperature and ventilation conditions, the degree of self-discharge and the charging and discharging process of the difference in the impact of the same batch of the same type of factory battery capacity and internal resistance may be different.
2, when loaded for use, the lithium battery pack in each battery electrolyte density, temperature and ventilation conditions, the degree of self-discharge and the charging and discharging process and other differences in the impact.
Evaluation Scope of Consistency
Personal understanding, the consistency of all the cells as power, regardless of the series or parallel relationship. Simple examples are given.
Parallel Connection
The cell with low discharging capacity (code B) is connected in parallel with other normal cells to become a parallel module D. For example, this is a module of 10 cells connected in parallel. To discharge the system, each parallel module needs to provide the same current, for example 100 A. For other normal parallel modules, each battery discharges 10 A; B can only discharge 1 A maximum current, so the other 9 batteries need to discharge 11 A. Generally, overloaded for a long period of time, the aging rate of these cells will be faster compared to other parallel modules. One day, the overall maximum discharge capacity of this parallel module cannot reach the design maximum capacity. This parallel module becomes the bottleneck of the discharge capacity of the whole battery pack.
Series Connection
According to the general situation, the series connection relationship is mainly between modules and modules. Continuing the drama of the previous parallel connection situation, a battery pack D with deeper aging than all other battery packs appears in the whole battery pack, D has a small capacity and a large internal resistance. Reacting to the curve of SOC versus open circuit voltage, the same SOC corresponds to an open circuit voltage with a high voltage at the D terminal. The whole battery pack is charged, D is the first to reach the charging cutoff voltage, and the pack stops charging. Other modules have not been fed, he has to burst his belly, because it is old, the belly becomes smaller.
Therefore, monomer consistency, not a certain welded together within the module, but the requirements of all power batteries.
What are the hazards and problems associated with inconsistent lithium battery packs?
Poor consistency may lead to uneven distribution of real-time voltage of each cell during charging and discharging, resulting in over-voltage charging or under-voltage discharging, which may cause safety problems.
Specifically as follows:
Loss of capacity, the capacity of single cell of lithium battery pack is in line with the "barrel principle", the capacity of the worst cell determines the capacity of the whole battery pack.
② life loss, small-capacity battery cells, every time is full of full discharge, too much power, it is likely to be the first to reach the focus of life. Has been the end of the life of the battery cell, a group of welding together the battery cell, also followed the end of life.
③ internal resistance increases, different internal resistance, flow through the same current, the internal resistance of the core heat generation is relatively large. Battery temperature is too high, resulting in accelerated deterioration, internal resistance will further increase. Internal resistance and temperature rise, the formation of a pair of negative feedback, so that the high internal resistance of the battery cell to accelerate the deterioration.
Lithium batteries are used in the process of using the protection circuit system to ensure safety. The consistency of lithium batteries in the use of the process of intuitive performance is the consistency of the difference in voltage (voltage difference), and the detection of the protection system is based on voltage monitoring. One of the single battery voltage reaches the protection conditions will cut off the battery circuit, without caring whether the other single battery is fully charged or discharged. After continuous charging and discharging work, this difference will become bigger and bigger until the battery pack loses its use value. If this is compounded with individual protection system malfunctions or failures, a safety problem can occur.
