# Lithium battery related parameter curve types

by：CTECHi     2021-07-10

1. Capacity decay curve

In the cycle test, the number of cycles is taken as the abscissa and the discharge capacity per discharge is taken as the ordinate to create a capacity decay curve. The following is a capacity decay curve made with raw data:

A careful friend will find that the raw data in the cycle test often contains individual points where the capacity suddenly decreases. These points are generally caused by interruptions and connections in the cyclic test, and abnormal data read from the cabinet points. In order to give customers a good 'visual experienceAfter that, the shape of the curve is more acceptable. Of course, the editor wants to remind everyone that this kind of change is best not used for highly professional customers, because they can fully understand this kind of abnormality.

The ordinate of the capacity decay curve is capacity, so it is helpful for customers to evaluate the discharge time of their products at different times of use, but it is not suitable for us to compare the cycle differences of batteries with different capacities, so we still need The following curve:

2, capacity retention rate curve

Take the number of cycles as the abscissa, and the capacity retention rate (current discharge capacity/first discharge Capacity) is the ordinate, so we get the capacity retention rate curve:

For the same cell, the shape of the capacity retention rate curve and the capacity attenuation curve are the same. The main value of the capacity retention rate curve is: for cells with different capacities, the same initial state (the initial capacity retention rate is 100%) is given to facilitate the comparison of subsequent trends.

We know that the capacity of the battery cell should continue to decay as the number of cycles increases, and the cycle curve should basically be a straight line that continues downward, but why some cycle curves are irregular , What about small fluctuations up and down? Let’s take a look at the capacity retention curve of a group of cells tested at the same time:

As can be seen from the above figure, the fluctuation trend of the initial capacity increase or decrease of the two cells is basically Consistent, with the increase of the number of cycles, the trend becomes 'half a beat'. The reason at this time is that the difference in capacity between the cells causes the cycles to cycle to the same time node, the cycle times are not exactly the same, if the cycle at the same time If the number of times is the same, the trend is still in sync. It is possible to continuously make different batteries have the same small fluctuations in capacity up and down beats, and the reason is only the test temperature change. Of course, as long as the cycle capacity fluctuations are not very obvious, such subtle cycle curve fluctuations do not need special treatment.

Under normal circumstances, we only need to provide customers with capacity decay curve and capacity retention curve for the cycle curve, but for our own analysis, we need to make another curve:

3. Charge-discharge capacity ratio curve

Take the number of cycles as the abscissa, and the charge capacity/discharge capacity of each cycle as the ordinate, you can get the following charge-discharge capacity ratio curve:< /p>

The charge-discharge capacity ratio curve is generally a scatter plot with 1.00 as the average value and randomly distributed up and down. According to my personal experience, the upper and lower limits of the ordinate are set to 0.97 ~1.03 is more appropriate.

For an ideal battery, each discharge capacity should be exactly equal to the previous charge capacity, but due to the capacity attenuation and test errors during the cycle, the charge-discharge capacity ratio curve will not be a single A straight line that happens to be 1.00, but there will be many scattered points. And by observing the state of these scattered points, we can analyze the cycle state of the cell in more depth.

This article introduces three cycle curves: capacity decay curve, capacity retention rate curve, and charge-discharge capacity ratio curve. It also explains that the interruption of the test equipment and abnormal readings will cause abnormal points in the capacity curve, and the test temperature The change will cause irregular up and down fluctuations of the capacity curve.

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