Analysis of the cell consistency in the lithium battery pack


1. What is lithium battery consistency

The consistency of lithium batteries refers to the convergence of important characteristic parameters of a group of lithium batteries. It is a relative concept. For multiple cells in the same lithium ion battery pack, it is best for each parameter to be within a smaller range for better consistency.

Adding the time dimension, consistency refers to the consistency of all characteristic parameters of all cells in the battery pack throughout their life cycle, taking into account inconsistencies in capacity attenuation, internal resistance growth, and aging rates. The life of the entire battery pack is our ultimate focus on consistency.

The purpose of pursuing consistency is not only to exert the maximum capacity of the battery pack (including maximum power, maximum current, and maximum available capacity) in the current state, but also to maintain it for as long as possible.

Parameters of all cells in the battery pack

2. What is lithium battery inconsistency

Inconsistencies in lithium battery parameters mainly refer to inconsistencies in capacity, internal resistance, and open-circuit voltage. The voltage is the initial voltage of the battery when assembled. The internal resistance is the AC internal resistance when fully charged, and the capacity is the discharge capacity of the battery cell after it is fully charged.

Accumulation occurs with the continuous charge and discharge cycles of the battery during use, resulting in greater differences in the status (SOC, voltage, etc.) of each individual battery.

AC internal resistance

The usage environment in the lithium battery pack is also different for each single cell. This leads to the inconsistency of single cells gradually amplifying during use, which in some cases accelerates the degradation of the performance of some single cells, and ultimately causes premature failure of the lithium battery pack.

Supplement: SOC is used to describe the remaining power of the battery and is one of the important parameters during battery use. SOC estimation is the basis for judging whether the battery is overcharged or discharged.

The open circuit voltage of lithium batteries has a clear and monotonic correspondence with the battery's charge. As long as the accurate open circuit voltage is obtained, the battery's charge can be calculated.

3. Causes of inconsistency

Uneven materials in the manufacturing process

The inconsistency of the lithium battery pack is a continuous accumulation process. The longer the time, the greater the difference between the single cells; and the lithium battery pack will also be affected by the use environment, and the single cells will be inconsistent in the future use process. The characteristics will be gradually amplified, causing the performance of some individual batteries to decline at an accelerated rate, and ultimately causing the battery to become useless.

The inconsistency of lithium battery packs is mainly affected by time. The reasons mainly include two aspects:

  • First of all, there are process problems and uneven materials in the manufacturing process, which make the materials and materials of lithium batteries slightly different. After the lithium battery pack is put into use, the electrolyte density of each battery in the battery pack will change temperature and ventilation conditions, self-discharge degree and charging and discharging processes, etc., there may be differences in capacity and internal resistance of batteries of the same model shipped from the same batch.
  • When used in a vehicle, the electrolyte density, temperature and ventilation conditions, self-discharge degree and charge and discharge process of each battery in the lithium battery pack are affected by differences.

4. Scope of evaluation of consistency

  • Parallel connection situation

The battery cell with low discharge capacity (cell B) is connected in parallel with other normal battery cells to form a parallel module D.

For example, this is a module with 10 batteries connected in parallel. To discharge the system, each parallel module needs to provide the same current, such as 100A. For other normal parallel modules, each battery discharges 10A; B can only discharge a maximum current of 1A, and the other 9 batteries each need to discharge 11A.

Generally speaking, these cells will age faster than other parallel modules due to long-term overloading. One day, the overall maximum battery discharge capacity of this parallel module cannot reach the designed maximum capacity. This parallel battery pack has become a bottleneck in the discharge capacity of the entire battery pack.

Parallel connection

  • Series connection situation

According to the general situation, the series connection relationship is mainly between modules. Continuing the previous the parallel connection situation, there is a battery pack D that is more aged than other battery packs in the entire battery pack.

D has a small capacity and a large internal resistance. Reflected on the curve between battery SOC and open circuit voltage, the open-circuit voltage corresponding to the same SOC has a higher voltage at D terminal. When the entire battery pack is charged, D reaches the charging cut-off voltage first, and the battery pack stops charging.

Therefore, cell consistency is not a matter within a module that is welded together, but is a requirement for all batteries.

Series connection

5. What problems will occur if the lithium battery packs are inconsistent

Poor consistency may lead to uneven real-time voltage distribution of each battery cell during charging and discharging, causing overvoltage charging or undervoltage discharging, causing safety issues.

Capacity loss

The cells form a lithium battery pack, and the capacity conforms to the "barrel principle". The capacity of the worst cell determines the capacity of the entire battery pack.

Loss of life

If a small-capacity battery is fully discharged every time and the output is too strong, it is likely to reach the end of its life first. When the life of the battery cell ends, a group of battery cells welded together will also end their life.

The internal resistance increases

With different internal resistances, the same current flows, and the battery cell with large internal resistance generates relatively more heat. If the battery temperature is too high, the degradation rate will be accelerated, and the internal resistance will further increase. Internal resistance and temperature rise form a pair of negative feedback, which accelerates the degradation of high internal resistance cells.

Internal resistance generates relatively more heat

6. Conclusion

Lithium batteries use protective circuit systems during use to ensure safety. The intuitive performance of the consistency of lithium batteries during use is the difference in voltage consistency (voltage difference), and the detection of the protection system is based on voltage monitoring.

When the voltage of one of the single cells reaches the protection condition, the battery circuit will be cut off, regardless of whether the other single cells are fully charged or discharged. After continuous charging and discharging, this difference will become larger and larger until the battery pack loses its use value. Safety issues can occur when combined with factors such as individual protection system failures or failures.


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