In the lithium-ion battery pack, the consistency of the cells is particularly important.

Let’s start the explanation by quoting the bucket effect, to make it more clear and easier to be understood.

The core content of the bucket effect is that the amount of water a bucket can hold does not depend on the tallest piece, but on the shortest piece of wood of the bucket.

Let’s assume a lithium battery pack is a bucket containing water, then the lithium battery cell that makes up the battery pack is like the wooden board that makes up the bucket, thus, the cell with the worst performance determines the overall performance of the whole lithium battery pack.

Öyleyse, if the lithium-ion battery packs with poor cell consistency, performance such as capacity, cycle life, and charge-discharge characteristics will be affected to a certain extent. As the number of charges and discharges used in lithium battery packs increases, this poor performance will also increase.

How does it happen? What’s the logic of it? Let’s explain below:

We all know that BMS is used for protection in most lithium battery packs. BMS has single-cell overcharge and single-cell over-discharge protection to protect lithium-ion cells from charging and discharging within a safe and reliable voltage range. When the single cell is overcharged or over-discharged, it will affect the performance life and even cause safety hazards.

The ternary lithium battery has a single discharge cut-off voltage of 2.5V, and BMS generally sets the single-cell over-discharge protection to 2.8V. The protection management mechanism of the lithium battery BMS is that when the voltage of any series of the batteries in the lithium battery pack reaches 2.8V, the over-discharge protection will be turned on, the discharge MOS tube or relay will be disconnected, causing the entire battery pack to stop discharging.

Take a ternary lithium battery pack of 10 in series and 1 in parallel as an example.

Suppose that among the 10pcs ternary lithium battery cells, the one with the lowest capacity is 2000mAh, while the other 9 battery cells have a capacity of 2500mAh. When the lithium battery pack is discharged, the 10 cells are discharging at the same time, and the voltage of the cells decreases as the capacity decreases.

When the 2000mAh cell with the lower capacity is discharged, the cell voltage will reach 2.8V, while the other 9 cells in the battery pack still have a capacity of 2500mAh and the voltage is above 3.0V. The BMS will detect when the battery pack has a series of voltages reaching 2.8V, the over-discharge protection is turned on, and the entire battery pack stops discharging.

This results in a discharge capacity of only 2000mAh for the entire battery pack.

The effect is similar to that during discharging

The full voltage of the ternary lithium battery cell is 4.2V, and the BMS of the lithium battery pack sets the single overcharge protection at 4.25V. The voltage of the battery cell increases with the increase of the capacity.

When a battery with a lower capacity of 2000mAh is fully charged, the battery voltage reaches 4.25V, and the other 9 cells are not fully charged and the voltage is below 4.1V, but when the BMS detects a series of voltages higher than 4.25V, it starts the battery pack overcharge protection, the entire battery pack stops charging.

This results in the battery pack can only charge 2000mAh, not 2500mAh.

This is how the bucket effect does to the lithium battery pack, that is the cell with the lowest capacity determines the charge capacity and discharge capacity of the whole lithium battery pack.

The same influence of the poor consistency of battery cells happens to the battery pack cycle life.

The lithium battery cell with the worst cycle life in the battery pack determines the overall performance of the lithium battery pack.

The consistency of lithium battery packs includes open circuit voltage, capacity, and internal resistance. More deeply, it also includes the K value (self-discharge rate), cycle life, charge and discharge characteristics, vesaire.

Before the lithium battery pack is assembled, the cells will be sorted and assembled. The standard of matching is generally: the battery capacity difference is controlled within 1%; the voltage difference is within 3mV; the internal resistance difference is within 2mΩ.

As for the K value, cycle life, and charge-discharge characteristics, these consistency factors need to be controlled at the source of the battery cell manufacturers/factories. Standardization of core mass production, automated control, vesaire.