In general, the self-discharge rate of li ion batteries is low. Also, the self-discharge rate is affected by factors such as temperature, cycle times, and SOC. Therefore, it is very difficult and time-consuming to accurately measure the self-discharge of the battery.
(1) Direct measurement method
First, the measured cell should be charged to a certain degree and keep the open circuit for a period. Then, the cell should be discharged to determine its capacity loss. The self-discharge rate:
In the formula: C is the rated capacity of the battery, and C1 is the discharge capacity. After the open circuit is left aside, the remaining capacity of the cell can be obtained by discharging it. At this time, multiple charge-discharge cycles should be implemented on the cell again to determine the full capacity of cells of li ion batteries. This method can determine the irreversible capacity loss and reversible capacity loss of the battery.
(2) Open circuit voltage attenuation rate measurement method
The open-circuit voltage is directly related to the SOC of the battery. It is only necessary to measure the change rate of the battery's OCV over a period of time, namely:
The method is simple to operate. It only needs to record the voltage of the battery in any period of time, and then the charge state can be obtained based on the corresponding relationship between voltage and the battery SOC. Finally, the self-discharge rate of the battery can be obtained by calculating the attenuation slope of the voltage and the corresponding attenuation capacity per unit time.
(3) Capacity retention method
The self-discharge rate of the battery can be obtained by measuring the expected open-circuit voltage of the battery or the amount of power required by the SOC, which refers to measure the charging current of li ion batteries while maintaining the open-circuit voltage. The battery self-discharge rate can be considered as the measured charging current.
The traditional measurement method takes a long time with insufficient accuracy. Therefore, the self-discharge rate is only used as a method to screen whether the battery is qualified in most cases. A large number of new and convenient measurement methods have saved a lot of time and energy for the measurement of battery self-discharge.
(1) Digital control technology
By using a single-chip microcomputer, digital control technology is a new self-discharge measurement method derived from the traditional self-discharge measurement method. It has the advantages of short measurement time, high accuracy, and simple equipment.
(2) Equivalent circuit method
The equivalent circuit method is a new self-discharge measurement method that simulates the battery as an equivalent circuit. It can quickly and effectively measure the self-discharge rate of li ion batteries.
As an important performance index of li ion batteries, the self-discharge rate plays an important role in battery selection and configuration. Therefore, it is of great significance to measure the self-discharge rate of li ion batteries.
(1) Predict problem cells
The materials and process control used in the same batch of cells are basically the same. When individual battery self-discharge is obviously too large, it is likely to be a serious micro-short circuit caused by internal impurities and burrs piercing the separator. The effect of the micro-short circuit on the battery is slow and irreversible. Therefore, the performance of this type of battery will not be much different from normal batteries in a short term. However, with the gradual deepening of internal irreversible reaction after a long term, the performance of the battery will be far lower than its factory performance and other normal battery. Therefore, the battery with large self-discharge must be eliminated to ensure the quality of the battery.
(2) Configuration of the battery
li ion batteries need better consistency, including capacity, voltage, internal resistance, and self-discharge rate. The influence of the self-discharge rate of the battery is mainly shown as follows: Once assembled into a module, the voltage will drop to varying degrees during use or circulation due to due to the different self-discharge rates of each single li ion battery. In series charging, the current will be equal. Therefore, overcharged or uncharged batteries may appear in li ion batteries module after each charge. With the increase of charging and discharging times, battery performance will gradually deteriorate, and the cycle life will be greatly reduced compared to unmatched single batteries. Therefore, the battery configuration requires accurate measurement and screening of the self-discharge rate of li ion batteries.
(3) Estimation and correction of battery
SOC The state of charge is also called the remaining power, which represents the ratio of the remaining capacity of the battery after a period of use or long-term use to its fully charged capacity. It is often expressed as a percentage. The self-discharge rate has an important reference value for the SOC estimation of li ion batteries. The correction of the initial value of SOC by self-discharge current can improve the estimation accuracy of SOC. On the one hand, customers can estimate the service time or driving distance of the product based on the remaining power. On the other hand, improving the SOC prediction accuracy of BMS can effectively prevent overcharge and over-discharge of batteries to prolong the service life of li ion batteries.