KYC (Know Your Cell) – Various ways to understand lithium-ion cell quality

Determining Lithium-ion cell quality

As a battery pack or an electric vehicle company, it is common to work with various types of cells procured from different vendors. It leaves us questioning which supplier is providing better quality cells. The answer lies in how much voltage is dropped in unit time. Also known as the “K” value, it is the main factor used while grading cells during manufacturing. Low K-value cells are considered A-grade cells. Higher K-value cells are labelled under A (minus) and B grades accordingly.

An excellent way to determine the cell quality is by measuring its self-discharge in terms of voltage drop at high temperatures. It is a known fact that a Lithium-ion cell will discharge by itself faster at high temperatures. Hence, various cells from different suppliers with similar characteristics in terms of form factor (cylindrical, pouch or prismatic), capacity (Ah) and cathode material type (for similar voltage) can be put under observation to measure the drop in voltage for a few weeks.

Comparing cells from different sample sets for voltage drop

On the next page, I provide the data of the cell test conducted for two types of similar LFP cells having the same form factor and capacity. The cells were observed at 60°C (environmental chamber), and their voltage was measured after every week. Samples of 7 cells were observed for each type.

• The first set of cells registered a voltage drop of up to 4mV (0.004V) after the first week, and the voltage dropped by 1mV (0.001V) in the second week. Most of the cells in this set did not lose any voltage in the second week. This is considered a very good cell.

• The second set of cells registered a voltage drop of up to 8mV (0.008V) after the first week, and the voltage dropped by 3mV (0.003V) in the second week.

The reference table shows that the first type of cells has a lower voltage drop than the second type of cells.

Additional factors

In addition to voltage drop, the following factors help to identify good-quality cell characteristics:

• Good LFP cells must deliver an average voltage of a minimum 3.2V at a 0.5C discharge rate. I have seen many cases of B-grade cells that can provide similar capacity (Ah) but deliver a lower average voltage. This translates to lower energy storage capacity. Average voltage is one common factor overlooked when analysing the quality of cells.

• Wh efficiency [(Wh energy discharged/Wh energy charged)*100] of LFP cells must be over 91% when charged and discharged at a 0.5C rate.

• Ah efficiency [(Ah capacity discharged/Ah capacity charged)*100] of LFP cells must be over 95% when charged and discharged at a 0.5C rate.

Wh efficiency and Ah efficiency of the cell have to be up to the mark, which is not the case in old (due to calendar ageing) or used cells because the cell’s internal resistance (IR) would have gone up from its original value and thereby making the cells heat up more than usual during charge and discharge. Cell manufacturers suggest that the end-of-life of the cell is considered when its internal resistance value reaches 1.5x of its original value (when it was brand new). It is not considered safe to operate such cells for electric vehicle applications.

Author

Rahul Bollini is an R&D expert in Lithium-ion cells with 8 years of experience. He founded Bollini Energy to assist in deep understanding of the characteristics of Lithium-ion cells to EV, BESS, BMS and battery data analytics companies across the globe. Rahul can be reached at +91-7204957389 and bollinienergy@gmail.com.

This article was first published in EVreporter Nov 2023 magazine.

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