The measurement method of self-discharge rate
From the above analysis, it is clear that the self-discharge rate of lithium batteries is generally low. The self-discharge rate itself is affected by the temperature, the number of cycles, and SOC, so it is very difficult and time-consuming to achieve accurate measurement of the self-discharge of the battery.
1 Traditional measurement methods of self-discharge rate
At present, there are three traditional self-discharge detection methods as follows：
● Direct measurement method
Firstly, the battery cell is charged to a certain state of charge and maintained in an open circuit for a period of time, and then the battery cell is discharged to determine the capacity loss of the battery cell. The self-discharge rate is.
Lithium battery self-discharge factors and measurement methods
Where: C is the rated capacity of the battery; C1 is the discharge capacity. After the open circuit is set aside, the remaining capacity of the cell can be obtained by discharging the cell. At this time, the cell is operated again for several charge/discharge cycles to determine the full capacity of the electric gar at this time. This method can determine the irreversible capacity loss and reversible capacity loss of the battery.
● Open circuit voltage decay rate measurement method
The open-circuit voltage is directly related to the battery charge state SOC, and it is only necessary to measure the rate of change of the OCV of the battery over a period of time, i.e.
Factors affecting self-discharge of lithium battery and measurement method
This method is simple to operate, and only needs to record the voltage of the battery in any time period, and then according to the correspondence between the voltage and the SOC of the battery, the state of charge of the battery at that moment can be derived. The self-discharge rate of the battery can be obtained by calculating the slope of voltage decay and the decay capacity per unit time.
● Capacity retention method
The self-discharge rate of the battery is obtained by measuring the amount of power required to maintain the desired open-circuit voltage or SOC of the battery. That is, the charging current is measured to maintain the open-circuit voltage of the battery, and the battery self-discharge rate can be considered as the measured charging current.
2 Fast self-discharge rate measurement method
Due to the long time required by traditional measurement methods and the lack of measurement accuracy, the self-discharge rate is mostly used as a screening method for battery qualification in the battery testing process. The emergence of a large number of novel and convenient new measurement methods has saved a lot of time and effort for the measurement of battery self-discharge.
● Digital control technology
The digital control technology is a new self-discharge measurement method derived from the traditional self-discharge measurement method by using a microcontroller and so on. This method has the advantages of short measurement time, high accuracy, and simple equipment.
Equivalent circuit method
The equivalent circuit method is a new self-discharge measurement method, which simulates the battery as an equivalent circuit and can measure the self-discharge rate of lithium-ion batteries quickly and effectively.
The significance of measuring self-discharge rate
As an important performance index of a lithium-ion battery, the self-discharge rate has an important impact on the selection and configuration of the battery, so it is of far-reaching significance to measure the self-discharge rate of lithium batteries.
1 Prediction of problematic cells
The same batch of cells, the materials used and made to control basically the same, when there is an individual battery white discharge is significantly large, the reason is likely to be internal impurities, burrs piercing the diaphragm and a serious micro-short circuit. Because the impact of the micro-short circuit on the battery is slow and irreversible. Therefore, in the short term, the performance of such batteries will not differ much from normal batteries, but after long-term shelving, with the gradual deepening of the internal irreversible reaction, the performance of the battery will be much lower than its factory performance and other normal battery performance. Therefore, in order to ensure the quality of the factory battery, a self-discharge large battery must be removed.
2 to the battery grouping grouping
Lithium batteries need good consistency, including capacity, voltage, internal resistance, and white discharge rate. The impact of self-discharge rate of the battery on the battery pack is mainly manifested as follows: once assembled into a module, the self-discharge rate of each single lithium battery is different, the voltage will drop to different degrees during the shelving or cycling process, and in series charging, the current will be equal, so each time after charging may appear in the lithium battery module overcharged or unfilled single battery, with the increase in the number of charge and discharge, the battery As the number of charges and discharges increases, the battery performance will gradually deteriorate, and the cycle life will be significantly reduced compared to the unpaired single cell. Therefore, the battery configuration requires accurate measurement and screening of the self-discharge rate of Li-ion batteries.
3 Battery SOC estimation correction
The state of charge, also known as the remaining power, represents the ratio of the remaining capacity of the battery after a period of use or long-term shelving to its fully charged capacity, often expressed as a percentage. The self-discharge rate is an important reference value for the SOC estimation of Li-ion batteries. After the self-discharge current correction of the initial value of SOC can improve the accuracy of SOC estimation, on the one hand, the customer can estimate the product useable time or driving distance according to the remaining power; on the other hand, improving the accuracy of SOC prediction of BMS can effectively prevent the battery overcharge and over-discharge, thus prolonging the battery life.
