The inconsistency in cell performance is formed during production and deepens during use. Within the same battery pack, weaker cells consistently weaken and deteriorate faster. The dispersion of parameters between individual cells increases with aging.
Lithium-ion batteries, already firmly occupying the dominant position in the electric vehicle power industry, boast long service life, high energy density, and significant potential for improvement. Safety can be enhanced, and energy density can continue to increase. In the foreseeable future (around 2020), they are expected to match the range and cost-effectiveness of gasoline cars, entering the first mature stage of electric vehicles. However, lithium batteries also come with their own concerns.
1,Why Are Most Lithium Batteries Small?
The lithium batteries we see—cylindrical cells, pouch cells, square cells—are generally compact, lacking the bulkiness of traditional lead-acid batteries. Why is this?
With high energy density, lithium batteries are often not designed for large capacities. The energy density of lead-acid batteries is around 40Wh/kg, while lithium batteries have surpassed 150Wh/kg. Increased energy density raises safety requirements.
Firstly, an excessively high-energy lithium battery can lead to thermal runaway in case of accidents, causing a rapid and dangerous internal reaction due to the inability to dissipate excess energy quickly. Particularly when safety technologies and control capabilities are not fully developed, each battery’s capacity should be restrained.
Secondly, once the energy enveloped by the lithium battery’s casing is released in an accident, firefighters and extinguishing agents cannot intervene effectively. They can only isolate the scene and let the battery react until its energy is depleted.
Of course, lithium batteries have multiple safety measures in place. Taking cylindrical cells as an example:
Safety valves automatically release pressure when internal reactions and gas generation exceed normal limits and pressure reaches a preset value. When the safety valve opens, the battery becomes completely ineffective.
Thermistors installed in some cells increase resistance dramatically at a certain temperature due to overheating, reducing current flow and preventing further temperature rise.
Fuses equipped with overcurrent protection function disconnect circuits when overcurrent risks occur, preventing catastrophic accidents.
2,Lithium Battery Consistency Issues
Due to the inability to create large single cells, lithium batteries are often made up of numerous smaller cells that work together. However, this leads to a challenge: consistency.
In our daily experience, two dry cells connected together can power a flashlight, regardless of their consistency. However, in large-scale lithium battery applications, the situation is not so simple.
Inconsistency in lithium battery parameters mainly refers to variations in capacity, internal resistance, and open-circuit voltage. Using cells with inconsistent parameters together can lead to the following problems:
（1），Capacity Loss: When cells with different capacities are connected in series, the capacity of the entire battery pack is determined by the lowest-capacity cell. To prevent overcharging and over-discharging, the battery management system stops discharging the pack when the voltage of the weakest cell reaches the cutoff voltage during discharge or when the voltage of the highest cell reaches the cutoff voltage during charging. As a result, smaller cells are always fully charged and discharged, while larger cells are only partially utilized, leading to underutilization of the pack’s capacity.
（2），Reduced Lifespan: The lifespan of the battery pack is determined by the shortest lifespan of the cells, which is likely to be the cell with the smallest capacity. Smaller cells, being fully charged and discharged every time, are subject to more stress and are more likely to reach the end of their lifespan first, leading to premature failure of the entire pack.
（3），Increased Internal Resistance: Cells with different internal resistances dissipate different amounts of heat when carrying the same current. High internal resistance leads to higher temperatures, accelerating degradation. The relationship between internal resistance and temperature forms a negative feedback loop, accelerating the degradation of cells with higher internal resistance.
These three parameters are not completely independent. Cells with higher internal resistance also tend to experience greater capacity degradation. Presently, engineers address cell inconsistency primarily from three aspects: cell sorting, thermal management after grouping, and balance function when a small number of cells exhibit inconsistency.
3,How to Address Inconsistencies
The inconsistency in cell performance is formed during production and deepens during use. Within the same battery pack, weaker cells consistently weaken and deteriorate faster. The dispersion of parameters between individual cells increases with aging. Currently, engineers address cell inconsistency primarily from three aspects: cell sorting, thermal management after grouping, and balance function when a small number of cells exhibit inconsistency.
(1),Sorting
Cells from different batches ideally should not be used together. Even cells from the same batch need to be screened to group cells with relatively similar parameters 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 involves screening cells based on characteristics such as open-circuit voltage, internal resistance, and capacity, using statistical algorithms to set sorting criteria and dividing cells of the same batch into several groups.
Dynamic sorting involves screening cells based on their performance during charge and discharge processes. Some methods use constant current-constant voltage charging, while others use pulse discharge-charge cycles or compare the charge and discharge curves.
Combining dynamic and static sorting involves initially grouping cells using static sorting and then using dynamic sorting to further refine the groups. This approach results in more groups with higher accuracy but also increases costs.
This illustrates the importance of large-scale production in the lithium battery industry. Mass production allows manufacturers to conduct more refined sorting, resulting in battery packs with more consistent performance. If production volumes are too small and too many groups are needed, even the best methods will be ineffective.
(2),Thermal Management
To address the issue of cells with inconsistent internal resistance generating different amounts of heat, a thermal management system is introduced to regulate the temperature difference of the entire battery pack, keeping it within a small range. Cells that generate more heat still experience higher temperatures, but the temperature difference between cells is minimized, preventing significant differences in degradation levels.
(3),Balancing
In cases where individual cells consistently reach charging cutoff voltage before others, reducing the pack’s effective capacity, battery management systems (BMS) incorporate balancing functions. When one cell reaches the cutoff voltage earlier than others, indicating a full charge, while others lag behind, the BMS activates the charging balancing function. This can involve discharging the overcharged cell or transferring energy to lower-voltage cells, allowing the charging process to continue and increasing the amount of energy stored in the pack.
Until now, cell inconsistency remains a significant area of research in the industry. Despite the high energy density of lithium batteries, inconsistency can significantly reduce the capacity and performance of battery packs.
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