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Under normal temperature conditions, a lithium-ion battery undergoes a certain charge and discharge cycle, how does it perform during and after the process? This is a problem related to the direction of lithium battery process improvement. To this end, certain test parameters need to be explained. Due to the acceleration of the popularity of new energy vehicles in China, the selection of large-capacity lithium batteries for test data collection is helpful to understand the power The performance and characteristics of lithium batteries.

The following general conclusions can be drawn from the test of lithium batteries: According to the constant current and constant voltage charging stage, the ratio of the constant current charging capacity to the charging capacity decreases with the increase of the number of cycles; at the discharge platform of 3.7V~4.2V The discharged electricity is greater than 90% of the total discharge capacity, and the charge and discharge efficiency is not affected by the number of cycles. A detailed description is given below.

Before describing the data, it is necessary to explain the test environment: a BYD 80Ah lithium cobalt oxide battery should be selected first, and the charge and discharge test should be performed at room temperature (10℃~250℃).

Charging and discharging system design: 1. The charging is a constant current and constant voltage method. First, charge to 4.2V with 1C or 80A constant current, then turn to 4.2V constant voltage charging until the current is less than 4A (ie 0.05C) and stop; 2. Leave for 10 minutes Then use 80A constant current to discharge to 2.75V; 3. After constant current discharge, let it stand for 10 minutes and then perform a new round of charge-discharge cycle, and repeat it for 500 times.

In this process, relevant data needs to be collected to form the corresponding chart: 1. Constant current/constant voltage charging characteristic curve; 2. The relationship between the ratio of constant current charging capacity to the total charging capacity and the number of cycles; 3. Discharge curve; 4. Charge and discharge efficiency curve.

It can be seen from the above chart:

1. From the perspective of constant current charging stage, the charging platform of lithium battery is between 3.8V and 4.1V, and the charging capacity in this stage reaches more than 80% of the total charging capacity. As the number of cycles increases, the voltage rise speed increases, the charging time decreases, and the charged power decreases gradually.

2. From the perspective of the constant current charging capacity as a percentage of the total charging capacity, as the number of cycles increases, the constant current charging capacity as a percentage of the total charging capacity decreases, while the constant voltage charging capacity as a percentage of the total charging capacity increases. This shows that as the number of charge and discharge cycles of lithium batteries increases, the smaller the current, the better the charging effect.

3. From the perspective of the discharge curve, the discharge platform (the discharge curve is stable in a certain voltage interval, close to a straight line, instead of the rising and falling slopes before the interval) decreases with the increase of the number of cycles, at 4.2V~3.7 The electricity discharged on the discharge platform of V accounts for 90% of the total electricity.

4. Charging and discharging efficiency: that is, the percentage of electricity discharged each time to the charged electricity. It represents the discharge capacity of a battery. From the charging and discharging efficiency curve, this value remains basically unchanged, reaching more than 99%.

We know that the capacity of lithium batteries will decrease as the number of charge and discharge cycles increases. This can be answered from the above data. The specific manifestations are as follows: the discharge platform decreases, the charging time of lithium batteries decreases, and the proportion of constant current charging decreases. These are finally manifested as the rechargeable power declines with the increase in the number of cycles, and the decline rate is getting faster and faster. After 500 cycles, the capacity must not be less than 80% to be considered qualified.