What causes of inconsistencies in lithium-ion batteries, what methods to improve lithium-ion battery consistency. Let’s find out!

When used as a power source for electric vehicles or large energy storage systems, due to the requirements of high power and large capacity, the single-cell lithium-ion battery cannot meet the requirements, so the lithium-ion battery needs to be used in series and parallel combination.

However, the inconsistencies between the single cells often cause problems such as excessive capacity decay and short life cycles in the battery pack during cycling. Selecting batteries with as consistent performance as possible for grouping is of great significance for the lithium ion battery pack production.

Inconsistency analysis

Definition of Lithium battery inconsistency

The inconsistency of the lithium-ion battery pack means that after the battery cells of the same specification and model form a battery pack, there are certain differences in parameters such as voltage, capacity, internal resistance, life, temperature influence, and self-discharge rate.

After the cell is manufactured, there are some differences in the initial performance itself. With the use of batteries, these performance differences continue to accumulate. At the same time, the use environment of each battery cell in the battery pack is not exactly the same, which also causes the inconsistency of the battery cells to gradually increase, thereby accelerating the degradation of battery performance, and eventually triggering The battery pack has failed prematurely.

Lithium iron phosphate battery Discharge graph
LG 18650 battery Pack

How to know the battery is Inconsistent

The inconsistency of lithium-ion batteries is mainly manifested in two aspects: the difference in battery cell performance parameters (battery capacity, internal resistance, self-discharge rate, etc.) and the difference in battery state of charge (SOC).

CMXbattery have found that the difference in capacity between battery cells is close to the Will distribution, and the degree of dispersion of internal resistance is more significant than the capacity, and the internal resistance of batteries in the same batch generally meets the law of normal distribution, self-discharge The rate also shows an approximately normal distribution. SOC represents the state of charge of the battery, and is the ratio of the remaining capacity of the battery to the rated capacity. Xie Jing et al. Believe that due to the inconsistency of the battery, the capacity decay rate of the battery is different, resulting in differences in the maximum available capacity between the batteries. The SOC change rate of a battery with a small capacity is faster than that of a battery with a large capacity, and the cut-off voltage is reached faster during charging and discharging.

What Causes of Inconsistency

There are many reasons for the inconsistencies of lithium-ion batteries, which are mainly produced during the manufacturing process and the use process. Every step of the manufacturing process, such as the uniformity of the slurry during batching, the control of the areal density and surface tension during coating, will cause differences in the performance of the single cell.

CMXbattery engineer Studied the impact of lithium-ion battery manufacturing process on battery consistency, and focused on the impact of lithium-ion battery production and manufacturing processes on water-based binder system on battery consistency. During the use of the battery, Xie Jiao engineer al. Believed that the connection method and structural components / devices, the operating conditions and the environment will affect the consistency of the battery pack. Because the energy consumed by each connection point is inconsistent, the performance and aging rate of each component or structure are also inconsistent, so the impact on the battery is also inconsistent. In addition, because each cell in the battery is located differently, the temperature is different, and the performance degradation is also different, these will amplify the inconsistencies of the cell.

3 Ways to Improve lithium ion Battery pack Consistency

  • Control of the production process-The control of the production process is mainly carried out from two aspects of raw materials and production processes. As far as the raw materials are concerned, the same batch of raw materials should be selected as far as possible to ensure the consistency of raw material particle size and performance. The production process must be strictly controlled throughout the production process, such as ensuring that the slurry is stirred uniformly and not placed for a long time, the speed of the coating machine is controlled to ensure the thickness and uniformity of the coating, the appearance of the pole pieces, and the weighing bin , To control the amount of injection and the formation, volume, storage conditions and so on.

  • Control of the grouping process-The control of the matching process mainly refers to the sorting of batteries. The battery packs use uniform specifications and models of batteries, and the voltage, capacity, and internal resistance of the batteries are measured to ensure the consistency of the initial performance of the batteries.

  • Control of the use and maintenance process-Real-time monitoring of the battery. The battery is screened for consistency during matching to ensure consistency during the initial use of the battery pack. The battery is monitored in real time during use, and the consistency problem during use can be observed in real time. However, when the consistency is poor, the monitoring circuit will cut off the charge and discharge circuit, so the performance will decrease. A balance must be found between the two. You can also adjust or replace the battery with extreme parameters in real time through real-time monitoring to ensure that the inconsistency of the battery pack does not expand with time.

5 Ways to improve matching and grouping lithium-ion battery

Voltage matching method

Voltage matching methods can be divided into static voltage matching methods and dynamic voltage matching methods. The static voltage matching method is also called no-load matching method, without load, considering only the battery itself, and measuring the self-discharge rate of the single cells being screened when stored for several dozen days in a fully charged state and the different storage periods under a fully charged state. The open circuit voltage of the internal battery. This method is the simplest but not accurate. The dynamic voltage matching method examines the voltage situation with a load, but it does not take into account factors such as load changes, so it is not accurate.

Static capacity allocation method

Charge and discharge the battery under the set conditions, calculate the capacity from the discharge current and discharge time, and arrange the batteries according to the capacity. This method is simple and easy to implement, but it can only reflect that the battery has the same capacity under specific conditions, cannot explain the complete working characteristics of the battery, and has certain limitations.

Internal resistance matching method

The internal resistance of a single battery is mainly considered. This method can achieve rapid measurement, but because the internal resistance of the battery will change with the progress of the discharge process, it is difficult to accurately determine the internal resistance.

Multi-parameter matching method

At the same time, considering multiple external conditions such as capacity, internal resistance, voltage, and self-discharge rate to comprehensively evaluate the battery, a battery pack with better consistency can be selected. However, the premise of this method is that single-parameter sorting must be accurate and it takes too long.

Dynamic characteristics matching method

The dynamic characteristic matching method is to use the charge and discharge characteristic curves of the batteries to sort the batteries for matching. The charge-discharge curve can reflect most of the characteristics of the battery, and the dynamic characteristic matching method can ensure the consistency of various performance indicators of the battery. The dynamic characteristic matching method has a lot of data, which is usually realized by computer programs. In addition, the battery utilization rate of this method is reduced, which is not conducive to the reduction of the battery composition cost. The determination of the standard curve or reference curve is also a difficult point in its implementation.

Conclusion

  1. The reason causes of battery inconsistency are mainly in the manufacture and use of batteries.
  2. The measures to improve the consistency of the battery mainly include the following three aspects:
    strict control of the production process from two aspects of raw materials and production processes;
    using more scientific sorting methods, as far as possible, select batteries with consistent initial performance for matching; In the process of battery use and maintenance, the battery is monitored in real time, an equilibrium management system is introduced, a reasonable control strategy is adopted, the battery is thermally managed, and the maintenance of the battery pack is also strengthened.
  3. When the battery is assembled, the single-parameter allocation method has too few factors to consider, so it has no practical application value. The multi-parameter matching method and the dynamic characteristic matching method are relatively comprehensive. In addition, methods such as the electrochemical impedance spectrum method have also made some progress.