How Does Lithium Cell Consistency Affect the Battery Pack?

The amount of water a wooden barrel can hold is determined by its shortest stave. This is known as "the barrel principle."

If we compare a lithium-ion battery pack to a water-holding barrel, then the individual lithium cells that make up the battery pack are the wooden staves. The performance of the single worst cell determines the overall performance of the entire battery pack.

In lithium-ion battery pack assembly (PACK), cell consistency is critically important. A pack with poor cell consistency will see its performance—including capacity, cycle life, and charge/discharge characteristics—negatively impacted. As the number of charge/discharge cycles increases, this impact grows larger. But just how significant is the effect of this "barrel principle"?

Lithium-Ion Battery Pack PACK Follows the "Barrel Principle"

A significant reason for this is the management and control by the Battery Management System (BMS). A lithium battery pack's BMS includes protection against overcharging and over-discharging of individual cells. This ensures the lithium-ion cells operate within a safe and reliable voltage range during charge and discharge, preventing performance degradation, reduced lifespan, or even safety hazards caused by overcharging or over-discharging.

Impact of Consistency During Battery Pack Discharge

Take a ternary lithium (NMC/NCA) cell as an example. Its single-cell discharge cut-off voltage is typically 2.5V, and the BMS usually sets the single-cell over-discharge protection at 2.8V. The BMS protection logic is such that when the voltage of any cell in the pack reaches 2.8V, over-discharge protection is triggered. The discharge MOSFETs or relays open, and the entire pack stops discharging.

Consider a 10-series, 1-parallel (10S1P) ternary lithium battery pack. Suppose among the 10 cells, the lowest capacity cell is 2000mAh, while the other 9 cells all have a capacity of 2500mAh. When the pack discharges, all 10 cells discharge simultaneously. The cell voltage drops as its capacity decreases. When the lower-capacity 2000mAh cell is fully discharged, its voltage will reach 2.8V. At this moment, the other 9 cells (2500mAh) still have remaining capacity, with voltages above 3.0V. However, because the BMS has detected that one cell string's voltage has reached 2.8V, it triggers pack over-discharge protection and stops the discharge for the entire pack. Consequently, the usable discharge capacity of the entire pack is limited to 2000mAh.

The impact of consistency during charging is similar to the discharge scenario.

A ternary lithium cell is fully charged at 4.2V. The BMS typically sets single-cell overcharge protection at 4.25V. As cells charge, their voltage rises with increasing capacity. When the lower-capacity 2000mAh cell becomes fully charged and its voltage reaches 4.25V, the other 9 cells are not yet fully charged (likely below 4.1V). Nevertheless, the BMS detects one cell string exceeding 4.25V and triggers pack overcharge protection, stopping the charge for the entire pack. This results in the pack only accepting a charge of 2000mAh.

This is how the "barrel effect" manifests: the cell with the lowest capacity dictates both the charge and discharge capacity of the entire battery pack.

The influence of consistency on the pack's cycle life and charge/discharge characteristics also follows the "barrel principle." The overall pack performance is determined by the single cell with the poorest cycle life and charge/discharge characteristics.

How Can Battery Pack Consistency Be Controlled?

Battery pack consistency involves parameters like open-circuit voltage, capacity, and internal resistance. On a deeper level, it also includes factors like the K-value (self-discharge rate), cycle life, and charge/discharge curve characteristics.

Before PACK assembly, cells undergo sorting and matching. Common matching criteria are:

• Capacity deviation: Controlled within 1%

• Voltage difference: Within 3mV

• Internal Resistance (IR) difference: Within 2mΩ

As for factors like K-value, cycle life, and charge/discharge characteristics, consistency needs to be controlled at the cell manufacturing source. This includes:

• Using identical material systems for cells within the same production batch.

• Controlling the defect rate in mass production.

• Standardizing and automating the mass production process for cells.

In Summary:

By rigorously controlling the consistency of cells within a lithium battery pack, we can optimize the pack's performance to its fullest potential, even under the governing influence of the "Barrel Principle."