What is inconsistency in battery cells?

 What is inconsistency in battery cells?

The inconsistency of battery cells is a common problem in battery packs, mainly referring to the differences in performance parameters of each cell in the battery pack, such as capacity, voltage, internal resistance, SOC, etc. This difference may be caused by process problems, material differences, or environmental changes during the manufacturing process of battery cells. The hazards of inconsistent battery packs include reduced capacity, increased system failure rate, decreased performance, and shortened service life.

SOC determine the low limit of discharge

The inconsistency of battery cells can be metaphorically described as some of the cells in a battery pack being old, weak, or sick, while others are young and strong. During discharge, the old, weak, or sick cells are discharged first and cannot continue to discharge, otherwise they will over discharge and the entire battery pack will have to stop discharging and wait for charging. When charging, the smaller capacity battery cells will be fully charged first, and continuing to charge at this time may pose a risk of overcharging, while other larger capacity battery cells have not yet been fully charged.

In addition to the inconsistency in capacity and voltage, there is also the inconsistency in resistance. Cells with high internal resistance generate relatively more heat, leading to high battery temperature, which in turn accelerates cell aging in a vicious cycle. Refurbished disassembled battery packs pose greater risks due to the greater differences in battery cells, resulting in shorter battery life, lower safety, and shorter lifespan.

Reasons for inconsistency in battery cells:

1. Generated during the production process. After ingredients, coating, rolling, winding, assembly, liquid injection, and volume separation, the cumulative error in each process is formed. It is best to select the same batch of cells, classify and screen them, and combine similar cells together.

2. Generated during the usage process. Due to the different positions of each battery cell in the battery pack, there will inevitably be differences in the environment between the middle and edge cells, such as slight differences in temperature, which can accumulate over time and cause inconsistencies in the battery cells.

To address inconsistency and balance technology, BMS is required to actively or passively balance the battery cells, as well as a good thermal management system.