Why is Consistency of Lithium Battery Cells So Important?

Lithium batteries cannot be made as a single large unit; instead, many small cells are organized together to work in unison, much like how teamwork can drive an electric vehicle forward. This brings us to a crucial issue: consistency. In our daily experience, two AA batteries connected in series can light up a flashlight, and nobody worries about whether they are consistent. However, when it comes to the large-scale application of lithium batteries, the situation is not so simple.

The inconsistency in lithium battery parameters mainly refers to differences in capacity, internal resistance, and open-circuit voltage. Using cells with these inconsistencies in a series or parallel configuration can lead to the following problems:

1. Capacity Loss: The capacity of a battery pack made up of individual cells follows the "bucket principle," where the capacity of the entire battery pack is determined by the cell with the lowest capacity.

To prevent overcharging or over-discharging of the battery, the logic of the Battery Management System (BMS) is set as follows: during discharge, when the voltage of the lowest cell reaches the discharge cut-off voltage, the entire battery pack stops discharging; during charging, when the voltage of the highest cell reaches the charging cut-off voltage, charging stops.

For example, consider two batteries connected in series, one with a capacity of 1C and the other with only 0.9C. In a series connection, both batteries carry the same current.

During charging, the battery with the smaller capacity will inevitably be fully charged first, reaching the charging cut-off condition, and the system will stop charging. During discharging, the smaller capacity battery will also discharge all its available energy first, and the system will immediately stop discharging. As a result, the smaller capacity cell is always fully charged and discharged, while the larger capacity cell only uses part of its capacity. Thus, a portion of the total capacity of the battery pack remains unused.

2. Reduced Lifespan: Similarly, the lifespan of the battery pack is determined by the cell with the shortest lifespan. It is highly likely that the cell with the shortest lifespan is the one with the smallest capacity. This smaller capacity cell is always fully charged and discharged, working harder than the others, and is more likely to reach the end of its lifespan first. Once one cell reaches the end of its life, the entire battery pack, welded together, will also "die."

3. Increased Internal Resistance: Different internal resistances mean that when the same current flows through the cells, the one with the higher internal resistance will generate more heat. High temperatures accelerate degradation, which further increases internal resistance. This forms a negative feedback loop between internal resistance and temperature rise, causing the high-resistance cell to degrade faster.

These three parameters are not entirely independent; cells that have undergone deeper aging generally have higher internal resistance and more capacity loss.

These are explained separately to clarify their respective impacts.

How to Address Inconsistencies:

Inconsistencies in cell performance are formed during production and deepen during use. Within the same battery pack, weaker cells become even weaker, and this weakening accelerates. The degree of parameter dispersion among individual cells increases with the level of aging.

Currently, engineers address inconsistencies in individual cells by considering three main aspects: cell sorting, thermal management after grouping, and providing balancing functionality through the Battery Management System (BMS) when minor inconsistencies occur.

1. Cell Sorting:

Theoretically, cells from different batches should not be used together. Even cells from the same batch need to be sorted, with cells that have relatively similar parameters grouped into the same battery pack.

The purpose of sorting is to select cells with similar parameters. Sorting methods have been studied for many years and are mainly divided into two categories: static sorting and dynamic sorting.

- Static Sorting: This involves screening cells based on their open-circuit voltage, internal resistance, capacity, and other characteristic parameters. Target parameters are selected, statistical algorithms are introduced, and screening standards are set, ultimately dividing cells from the same batch into several groups.

- Dynamic Sorting: This method screens cells based on their characteristics during the charge and discharge process. Some methods use constant current and constant voltage charging processes, some select pulse impact charge and discharge processes, and others compare the relationship between the cell’s charging and discharging curves.

- Combined Static and Dynamic Sorting: First, static sorting is used for preliminary grouping, followed by dynamic sorting. This method results in more groups and higher sorting accuracy, but it also increases costs.

Here, the importance of large-scale production of power lithium batteries is evident. Large-scale production allows manufacturers to perform more refined sorting, resulting in battery packs with more closely matched performance. If production is too small, too many groups may be formed, and a single batch may not be enough to equip a battery pack, making it difficult to implement even the best methods.

2. Thermal Management:

Inconsistencies in internal resistance lead to different amounts of heat generation among cells. The introduction of a thermal management system can regulate the temperature difference across the battery pack, keeping it within a small range. Cells that generate more heat will still have a slightly higher temperature, but not to the extent that it creates a significant gap with other cells, preventing noticeable differences in degradation levels.

3. Balancing:

For cells with slight inconsistencies, certain cells may reach the cutoff voltage earlier than others, leading to reduced system capacity. To address this, the BMS is designed with a balancing function.

When a cell reaches the charging cut-off voltage earlier while the voltages of the other cells lag behind, the BMS initiates the charging balancing function, either by connecting a resistor to dissipate some of the energy from the high-voltage cell or by transferring energy to the low-voltage cells. This removes the charging cut-off condition, allowing the charging process to resume, and enabling the battery pack to store more energy.