How to Improve the Consistency of Individual Batteries?
Consistency of Individual Batteries
The inconsistency among individual batteries often leads to issues such as rapid capacity decay and shortened lifespan during the cycling process of battery packs.
Selecting batteries with as consistent performance as possible for grouping is crucial for the widespread application of lithium-ion batteries.
This article will analyze the manifestations and causes of the inconsistency in lithium-ion batteries and summarize methods to improve the consistency of lithium-ion batteries.
1. Inconsistency Analysis
1.1 Definition of Inconsistency
The inconsistency in a lithium-ion battery pack refers to the differences in parameters such as voltage, capacity, internal resistance, lifespan, temperature impact, self-discharge rate, etc., when individual batteries of the same specification and model are assembled into a battery pack.
After the individual cells are manufactured, there are inherent differences in their initial performance. As the batteries are used, these performance differences continue to accumulate.
Additionally, because each individual cell experiences a slightly different operating environment within the battery pack, the inconsistency of individual cells gradually amplifies.
This accelerates the performance degradation of the battery and ultimately leads to premature failure of the battery pack.
1.2 Manifestation of Inconsistency
The inconsistency in lithium-ion batteries primarily manifests in two aspects:
① Differences in performance parameters of individual batteries (such as battery capacity, internal resistance, and self-discharge rate).
② Differences in State of Charge (SOC) of batteries.
The research and others revealed that the distribution of capacity differences between individual batteries approximates a Weibull distribution. The dispersion of internal resistance is more significant than that of capacity.
Moreover, the internal resistance of batteries from the same batch generally follows a normal distribution, and the self-discharge rate shows an approximate normal distribution.
SOC characterizes the state of charge of a battery, representing the ratio of the remaining capacity to the rated capacity. Due to the inconsistency in batteries, the capacity decay rates vary, resulting in differences in the maximum available capacity between batteries.
Batteries with smaller capacity experience a faster rate of SOC change compared to those with larger capacity, reaching the cutoff voltage more quickly during charging and discharging.
1.3 Causes of Inconsistency
There are many reasons for the inconsistency in lithium-ion batteries, mainly arising during the manufacturing and usage processes.
Each step in the manufacturing process, such as the uniformity of the slurry during ingredient mixing, control of face density and surface tension during coating, can lead to differences in the performance of individual batteries.
The Research explored the impact of the lithium-ion battery production process, particularly the process of producing lithium-ion batteries using a water-based binder system, on battery consistency.
During the battery’s usage, the connection method, structural components/devices, operating conditions, and environment all influence the consistency of the battery pack.
Because the energy consumption at each connection point is inconsistent. The performance and aging rates of each component or structural part are also inconsistent, leading to varied effects on the battery.
Additionally, the fact that each individual battery is positioned differently in the battery pack, experiencing varying temperatures and undergoing distinct performance degradation, contributes to the amplification of the inconsistency among individual batteries.
2. Methods to Improve Battery Consistency
2.1 Control of the Production Process
Control of the production process primarily involves two aspects: raw materials and production technology.
– Regarding raw materials, it is advisable to select materials from the same batch to ensure consistency in particle size and performance.
– In terms of production technology, strict control should be applied to the entire production process, including:
- Ensuring uniform mixing of slurry
- Avoiding prolonged standing time
- Controlling the speed of the coating machine to ensure thickness and uniformity
- Visual inspection and weight sorting of electrode sheets
- Controlling the injection volume
- Managing formation, sorting, storage conditions, etc.
Through research on the preparation technology of lithium-ion batteries, key processes with a significant impact on the consistency of lithium-ion batteries. These processes include mixing of ingredients, coating, roll pressing, winding/stacking, injection, and formation.
In-depth research and analysis were conducted on the relationship between various key process parameters and battery performance.
2.2 Control of the Assembly Process
Control of the assembly process primarily refers to the sorting of batteries.
In the assembly of battery packs, batteries of uniform specifications and models are used. The batteries’ voltage, capacity, internal resistance, etc., are measured to ensure the consistency of the initial performance of the batteries.
Through research that during the assembly of battery packs, the voltage difference between individual batteries is a crucial factor affecting the consistency of individual batteries in the later stages of charge and discharge.
The variation in internal resistance of individual batteries results in significant differences in the voltage plateau of individual batteries during the charge and discharge processes of the battery pack.
A study on the inconsistency of individual cells in lithium-ion series-parallel combined batteries analyzed the main influencing factor, DC resistance (DCR), on the impact of battery packs in parallel configurations and the main influencing factor, capacity, on the impact of battery packs in series configurations. This research provides essential evidence for the development of combined battery packs.
The impact of discharge rate on the consistency of battery packs, it was found that as the discharge rate increased, the inconsistency of batteries was amplified, leading to the effect of eliminating defective batteries.
2.3 Control during Use and Maintenance Processes
2.3.1 Real-time Monitoring of Batteries
Performing consistency screening during battery assembly ensures consistency during the initial use of the battery pack.
Real-time monitoring of batteries during usage allows for the immediate identification of consistency issues during the operation.
However, in cases of poor consistency, the monitoring circuit may cut off the charge and discharge circuits, leading to reduced performance.
A balance must be found between these considerations. Real-time monitoring can also facilitate timely adjustments or replacements for batteries with extreme parameters, ensuring that the inconsistency of the battery pack does not expand over time.
2.3.2 Implementation of a Balancing Management System
Implement intelligent management of batteries through appropriate balancing strategies and balancing circuits.
Common balancing strategies include those based on external voltage, state of charge (SOC), and capacity.
Balancing circuits can be categorized into passive balancing and active balancing based on energy consumption.
Active balancing enables energy flow between batteries without losses and has become a research focus globally.
Common methods in active balancing include battery bypass, switch capacitor, switch inductor, DC/DC conversion, etc.
2.3.3 Thermal Management of Batteries
In addition to maintaining the working temperature of the battery pack within the optimal range, thermal management of batteries also involves ensuring consistency in temperature conditions among the batteries. This effectively guarantees performance consistency among the batteries.
Adopting a rational control strategy is crucial. Within the permissible output power, efforts should be made to minimize the depth of battery discharge while avoiding overcharging, which can extend the cycle life of the battery pack.
Regular maintenance of the battery pack is essential. Periodic low-current maintenance charging of the battery pack should be carried out, and attention should be paid to cleanliness.
Summary
(1)The causes of battery inconsistency mainly stem from both the manufacturing and usage aspects of the battery.
(2)Measures to enhance battery consistency primarily focus on the following three areas:
- Strict control over the production process from both raw materials and production techniques.
- Adoption of more scientific sorting methods, selecting batteries with consistent initial performance for grouping.
- During battery usage and maintenance, real-time monitoring, implementation of a balance management system, utilization of rational control strategies, thermal management of batteries, and enhanced maintenance of the battery pack are crucial.