An important parameter of lithium-ion batteries is the self-discharge ratio, which is generally explained by the industry in accordance with the monthly consumption capacity ratio under normal static conditions. This parameter is a key parameter to examine the consistency of lithium-ion batteries, and it is also a parameter that needs to be considered in the assembly and replacement of lithium-ion batteries. The reason is very simple. In the actual application process, the environment and use conditions are far more difficult than laboratory conditions. Being complex and harsh, this magnifies the effect of self-discharge parameters.

　　As a supplier of customized lithium-ion batteries for 20 years, SES Power has developed and produced many lithium-ion batteries for different applications, such as the lead-acid replacement product 12V100Ah made of square aluminum-shell lithium iron phosphate batteries. , 24V100Ah, 12V200Ah, 3kwh, 5kwh, 10kwh wall-mounted smart home energy storage system, rack-mounted energy storage system, split combined high-voltage power lithium battery system, polar lithium battery that can withstand -60 degrees Celsius, starting current up to 2000CCA starter batteries, customized metal shell lithium iron phosphate batteries, etc., these are very high capacity and high voltage lithium ion battery pack products, so we have more strict research and requirements on self-discharge parameters than others. The following is a brief introduction by our senior engineers on how to measure the self-discharge of lithium-ion batteries.

　　The measurement methods are mainly divided into two categories: 1) the stationary measurement method, which obtains the self-discharge rate by standing the battery for a long time; 2) the dynamic measurement method, which realizes the parameter identification of the battery in the dynamic process.

　　The current mainstream lithium-ion battery self-discharge measurement method is static measurement: under certain environmental conditions, the battery is left to stand for a long time, and the change of battery parameters before and after standing is measured to indicate the self-discharge degree of the lithium-ion battery. . According to the different measurement parameters, static measurement is mainly divided into three categories: capacity measurement, open circuit voltage measurement and current measurement.

　　1. Volume measurement

　　Before the battery is left standing for a long time, the battery is charged and discharged once, and the discharge capacity Q0 before standing is recorded. After standing, the battery was discharged in the same way, and the discharge capacity Q after standing was recorded.

　　According to formula (7), the self-discharge rate η of the battery can be calculated. The battery is charged and discharged again in the same way, and the discharge capacity Q1 of the battery after the cycle is recorded. According to equations (8) and (9), the reversible self-discharge amount Qrev and the irreversible self-discharge amount Qirr of the battery can be calculated respectively. This method is shown in the figure below.

　　The International Electrotechnical Commission (IEC) "Accumulators and accumulators containing alkaline or other non-acid electrolytes: Portable secondary lithium batteries and accumulators" (IEC 61960) stipulates that batteries in a 50% SOC state will be Stored at ambient temperature (20±5)℃ for 90d, the discharge capacity of the battery after recharging should not be less than 85% of the rated capacity. The electric vehicle battery test manual issued by the US Automotive Research Council (USCAR) stipulates that the actual power corresponding to the battery working range should be measured before the measurement: after the battery is discharged at a C/3 rate of 50% of the available power, the ambient temperature is 30%. Store at ℃ for 30 d, and measure the discharge capacity of the battery after recharging.

　　The Performance Requirements and Test Methods for Power Batteries for Electric Vehicles (GB/T 31486) issued by the Standardization Administration of China (GB/T 31486) is similar to the IEC standard, and specifies the test procedures for the measurement of charge retention and capacity recovery capabilities. Taking the room temperature test as an example, the battery is stored at room temperature for 8 days, and the charge retention rate is required to be no less than 85% of the initial capacity, and the capacity recovery is no less than 90% of the initial capacity.

　　2. Open circuit voltage measurement

　　Open circuit voltage measurement is to express the degree of self-discharge of lithium-ion batteries by directly measuring the change of open circuit voltage during battery standing. The advantage of this method is that it is simpler and less time-consuming than measuring capacity; the disadvantage is that for lithium-ion batteries (such as LFP batteries) with a long voltage plateau on the open circuit voltage-SOC curve, in a large SOC range, The battery voltage change is small, and it is difficult to characterize the degree of self-discharge by measuring the open-circuit voltage, that is, the method has a certain scope of application.

　　3. Current measurement

　　The lithium-ion battery is charged with a small current to keep the voltage of the battery unchanged, and the charging current value when it is stable is the self-discharge current. According to research, the tiny current may not stabilize for several months, and the stabilization time of different designs of batteries is not the same. Generally, the recommended measurement time is at least one week.

　　This method has similar problems to the method of measuring open-circuit voltage, namely, the validity of this method is challenged for lithium-ion batteries (mainly lithium iron phosphate batteries) with a long voltage plateau. In addition, since the self-discharge current of lithium-ion batteries is extremely small, generally C/50000 or lower, to apply and measure this tiny level of current requires higher requirements for experimental instruments.

　　Taken together, the above three methods are very time-consuming, and the experimental time span ranges from one day to dozens of days. The shortening of the measurement time in the current measurement scenario requires high equipment costs.

　　In order to shorten the measurement time, save space resources and human resources, researchers have introduced dynamic testing methods.

　　Change conditions such as ambient temperature and battery SOC

　　Such changes can greatly speed up the self-discharge rate, allowing relatively large changes in the measured parameters in a relatively short period of time. Although this method saves the experiment time, it also accelerates the aging of the battery and increases the damage to the battery.

　　Modeling the equivalent circuit of a lithium battery

　　On the basis of the existing relatively mature equivalent circuit model of lithium-ion battery, the self-discharge resistance is introduced, and the self-discharge rate of lithium-ion battery is measured in the dynamic process through different parameter identification methods.

　　We can simplify the lithium-ion battery into a first-order resistor-capacitor (R-C) equivalent circuit, apply the same charge and discharge current to the lithium-ion battery and the equivalent circuit, and adjust the parameters of the equivalent circuit according to the difference in output voltage until the two the difference approaches zero, and the self-discharge resistance value of the lithium-ion battery is obtained.

　　For example, we can simplify the battery to the equivalent circuit shown in the figure below. Among them: Rp,i is the electrochemical reaction resistance, Cp,i is the electric double layer capacitance, Rself is the self-discharge resistance, and C is the battery equivalent capacitance. By applying a short-time current pulse to the lithium-ion battery, measuring the voltage change during the subsequent resting process, and further analyzing the self-discharge resistance value. This method only considers the dominant reaction in each stage during standing, which decouples the complex reaction mechanism, reduces the amount of calculation and shortens the measurement time.

　　Specifically, the overvoltage recovery plays a leading role in the initial period of standing, and the self-discharge of the battery at the end of the standing period plays a leading role. The time constant of self-discharge can be analyzed by the data at the end of the static period, and then the voltage drop caused by the self-discharge in the overvoltage recovery period can be compensated, and the equivalent capacitance of the battery can be solved, and finally the self-discharge resistance value can be obtained.

　　This method can obtain the self-discharge resistance of lithium-ion batteries within 10-48 hours, which saves a lot of time compared with the traditional method, but still needs to consume a lot of standing time in order to observe the stage where self-discharge plays a leading role.

　　Statistical methods for outlier identification of parametric and depletion effects

　　The parameter effect means that due to the existence of the short-circuit resistance, the measured open-circuit voltage and internal resistance have a certain deviation from the actual value; the consumption effect means that due to the existence of the short-circuit resistance, the energy stored in the battery is continuously consumed, and the battery SOC continues to decrease. It will cause the actual value of the battery open circuit voltage and internal resistance to have a certain deviation from the normal value.

　　In the battery difference model shown in equations (10) and (11): Ei is the open circuit voltage of the battery, Ri is the internal resistance of the battery, and Ui and I are the measured battery voltage and current, respectively. The values of ΔEi and ΔRi are obtained by the recursive least square method, and finally the abnormal parameters exceeding the threshold are identified by statistical methods, so as to judge whether the battery has an internal short circuit. When the short-circuit resistance is 100Ω, the method can realize the identification of the internal short circuit within 4h43min.

　　SES Power finally summarizes for you: By analyzing the measurement methods of static measurement and dynamic measurement of the self-discharge rate of two types of lithium-ion batteries, the main conclusions drawn include the following three points:

　　1. Side reactions occurring at the interface of negative electrode/electrolyte and positive electrode/electrolyte are the main sources of self-discharge of lithium-ion batteries. Self-discharge can be suppressed by modifying the surface of positive electrode and adding additives to negative electrode and electrolyte. happened.

　　2. During the storage process of the battery, it should be avoided to be in a too high or too low SOC state, and the ambient temperature and humidity should be kept within a relatively low range.

　　The current mainstream self-discharge measurement method is static measurement based on long-term static experiments. The biggest problem of this type of method is that the measurement time is too long, resulting in a huge waste of space and human resources. Researchers have proposed some dynamic measurement methods combined with equivalent circuit models for parameter identification, and these methods have made some progress in shortening the measurement time. Through the innovative experimental design, the decoupling and identification of self-discharge in the dynamic process is completed, which is the key path and development direction to realize the rapid measurement of self-discharge in the future.