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K-value of lithium-ion batteries

The K value refers to the voltage drop of a battery per unit time, usually expressed in mV/d, used to measure the self discharge rate of lithium-ion batteries.

The commonly used K value measurement methods are as follows:

Measure OCV1 at time t1

Measure OCV2 at t2

K is equal to OCV1-OCV2 divided by t1-t2.

2. The standard deviation feedback of pressure drop difference is often adopted by some Japanese and Korean companies:

△ OCV>Average (△ OCV)+4.5, judged as NG

1 NG for a period of time, retest and screen;

Tip: The average value (△ OCV) is the average value of △ OCV; It is the standard deviation of △ OCV

The K value of a good battery is generally less than 2mV/d or 0.08mv/h

The important factors affecting K value include:

1. Positive and negative electrode materials, electrolyte type, diaphragm thickness type:

Due to the importance of self discharge occurring between materials, the properties of materials have a significant impact on self discharge. Someone has conducted experiments to prove that ternary materials have higher self discharge than lithium cobalt oxide batteries;

2. Storage time:

The longer the storage time, on the one hand, it increases the absolute value of pressure drop; On the other hand, it disguises and reduces the absolute error/pressure drop value of the instrument, making the results more accurate. However, if the storage time is too long, the capacity balance between the positive and negative electrodes will gradually be disrupted and deepened, and the decomposition of the electrolyte will also accumulate some irreversible capacity loss, causing an increase in self discharge.

The longer the time, the smaller the k value

3. Storage conditions:

The addition of storage temperature and humidity will increase the degree of self discharge.

4. It is also related to the self consumption of the battery protection board.

5. Initial voltage of the test:

The K value obtained varies depending on the initial voltage (or primary voltage). The higher the initial voltage, the higher the self discharge rate;

Technically speaking, the attenuation of self discharge capacitance means that in the absence of suitable equipment, the battery should not be discharged and the remaining capacity should not be measured directly. Self discharge can only be indirectly measured by measuring voltage, which is called the K value. If changes in environmental conditions cause a deviation in the test voltage, it may lead to measurement errors in the K value. Even if we actually measure K negative, this is obviously inconsistent with the meaning of self discharge itself. Therefore, it is necessary to regularly check the equipment and measure the environment to be consistent.

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