Comparison of Several 18650 lithium ion battery cellOnline Testing Technologies

　　1. Reasons for the backwardness of batteries and the importance of detection

　　Valve-regulated lead-acid batteries (VRLA) have been called maintenance-free batteries from the beginning, and the manufacturer promises that the 18650 lithium ion battery cellhas a service life of 10 to 20 years (at least 8 years), thus giving us power system maintenance personnel A misunderstanding, it seems that this kind of 18650 lithium ion battery cellis both durable and completely maintenance-free. Many users have basically not performed maintenance and management since installing the battery. In the early 1990s, as the use time increased, the VRLA batteries used appeared. Many new problems that have not been encountered before, such as 18650 lithium ion battery cellcase deformation, electrolyte leakage, electrode corrosion, insufficient capacity, uneven 18650 lithium ion battery cellterminal voltage, etc., are caused by corrosion of the internal terminals, connecting pieces of the same pole and electrode connectors of VRLA batteries. Breakage also occurs more often than open batteries. These failures lead to capacity loss, but there is no correlation between VRLA 18650 lithium ion battery cellterminal voltage and discharge capacity, which makes it difficult for users to grasp the durability and failure issues of VRLA batteries. Our maintenance department used to only pay attention to the maintenance and management of the equipment part of the backup power supply, and ignored the important role of the 18650 lithium ion battery cellpack. Little did we know that the danger of power outage lurked in the 18650 lithium ion battery cellpack to a large extent. The characteristic of charging the entire 18650 lithium ion battery cellpack is that if there is one or several aging batteries in the 18650 lithium ion battery cellpack, its capacity will inevitably become smaller. When the charger charges the 18650 lithium ion battery cellpack, the aging batteries will be fully charged quickly due to their small capacity. The charger will mistakenly think that the entire 18650 lithium ion battery cellpack is fully charged and will switch to a float charge state, charging the 18650 lithium ion battery cellpack with a constant voltage and small current. The rest of the batteries in good condition are unlikely to be fully charged. The 18650 lithium ion battery cellpack will be charged and discharged based on the capacity of the aging battery. After repeated cycles of float charge - discharge - equal charge - discharge - float charge, the capacity will continue to decrease and the 18650 lithium ion battery cellbackup time will be shortened.

　　Therefore, if there is no regular testing, regular testing and online monitoring of batteries and 18650 lithium ion battery cellpacks are very important and necessary. They are already a very important link in the power supply system. However, judging from the results of many years of operation and maintenance, 18650 lithium ion battery cellvoltage detection can no longer fully reflect 18650 lithium ion battery cellproblems. It is poor in early warning and forward-looking, and cannot accurately and timely identify aging batteries. Because there is no way to distinguish and process the small-amplitude difference monitoring of float voltage, that is, for accurate judgment of aging batteries whose 18650 lithium ion battery cellperformance has deteriorated and whose 18650 lithium ion battery cellcapacity has dropped significantly, the voltage parameters cannot do anything. Instead, it is found that a certain battery's There will be a warning only when the discharge voltage (or curve) is abnormal, but it is usually too late.

　　If the internal performance parameters of the 18650 lithium ion battery cellcannot be clearly understood, such as the internal resistance of the 18650 lithium ion battery celland the current remaining capacity, and if there are outdated batteries in the 18650 lithium ion battery cellpack, it cannot be accurately judged and maintained in advance. Therefore, the internal resistance of the 18650 lithium ion battery celland the current remaining capacity cannot be accurately determined. Monitoring can be an effective means for us. During the operation of VRLA batteries and 18650 lithium ion battery cellpacks, as the use time increases, some or some batteries will inevitably become degenerative and aging due to increased internal resistance. Practice has proven that the entire 18650 lithium ion battery cellpack will The capacity is based on the capacity value of the 18650 lithium ion battery cellwith the worst condition, rather than the average or rated value (initial value). When the actual capacity of the 18650 lithium ion battery celldrops below 90% of its rated capacity, the 18650 lithium ion battery cellwill Entering the decline period, when the 18650 lithium ion battery cellcapacity drops below 80% of the original value, the 18650 lithium ion battery cellwill enter a sharp decline state. The decline period is very short. At this time, the 18650 lithium ion battery cellpack already has great accident risks.

　　The working principle of lead-acid batteries is the "double sulfation theory". Its structural components are mainly positive plates (pbO2), negative plates (heavy metal alloys such as lead), grids, etc. Lead-acid batteries will form crystals during discharge and charge. When the lead ions are reduced to metallic lead. If we use or maintain improperly, such as frequent undercharging or overdischarging, a thick and hard pbSO4 crystal will gradually form on the positive and negative plate terminals of the battery. This phenomenon is called "irreversible sulfation". Referred to as "vulcanization". "Sulfation" increases the internal resistance of the 18650 lithium ion battery celland decreases its capacity. The reason for this irreversible sulfation is the recrystallization of lead sulfate. After the formation of coarse crystals, the solubility decreases. The recrystallization of lead sulfate makes the crystal volume larger because Polycrystals tend to be the result of their surface free energy. Therefore, the sulfation of the 18650 lithium ion battery cellis the main reason for the increase in the internal resistance of the battery, which has a great impact on the service life of the battery. The method used to test the internal resistance parameters will affect the final effect of online monitoring.

　　Against the above background, we conducted in-depth research on 18650 lithium ion battery cellonline monitoring equipment some time ago, and participated in product testing of six online monitoring equipment manufacturers in a pilot laboratory. After the research, we found that each has its own different testing methods. Summary There are mainly three categories. There are obvious differences in the effects of these categories: 2. Technical analysis of communication testing method

　　The AC method is to inject a low-frequency AC signal into the battery. Due to the impedance existing inside the battery, after the signal is injected, the feedback current signal is measured, signal processing is performed, and the difference between the injected signal and the feedback signal is compared to measure the internal resistance of the battery. The AC method for measuring 18650 lithium ion battery cellimpedance relies on high-speed digital signal processing technology, but the signal interference caused by the charger and external noise composed of high-frequency modules in the system cannot be completely eliminated.

　　Its principle can be expressed by the following simple formula: R=△V/△I

　　Based on the above description and the analysis of the test results seen in the pilot test, we have the following opinions on the communication method:

　　1. For the AC injection method, it can actually only be defined as the impedance of the battery.

　　2. As shown in Figure 1 on the right, the capacity of a single cell is formed by the positive and negative plates, which are two parallel parts of the conduction path. When passing an AC signal or doing impedance testing, any increase in resistance in any part of the path will be masked by the capacitor. At the same time, the measured values of signals of different frequencies are also different, which requires high requirements for the injected signal. For example, the stability of the frequency and the purity of the sine wave directly affect the test results. In addition, it is also susceptible to ripple and harmonic interference from the charger during online testing.

　　3. Since the internal resistance of a small-capacity 18650 lithium ion battery cellis in the milliohm range, while the internal resistance of a large-capacity 18650 lithium ion battery cellwill be in the micro-ohm range, we know from the testing principle of the AC method that the signal is measured by measuring the current signal fed back. Process, compare the difference between the injection signal and the feedback signal to get the internal resistance, but the current amplitude of the AC method is very small, generally 1A (in order to avoid the impact on the system, it cannot be too large), such a small current must be in the micro-ohm level The differential changes are measured on microresistors, so the requirements for signal processing accuracy are very demanding and strict. It is very susceptible to interference from the charger and external noise on the signal, resulting in large discreteness in the test results.

　　4. Injecting an AC signal source into the DC system essentially pollutes the system. Although manufacturers are constantly studying to reduce the frequency (actually approaching DC), the amplitude has to be increased, but as a signal source Not the load, but the increase in amplitude will have a serious impact on the normal operation of the system equipment, because we know that the redundant parallel connection of high-frequency charging modules has strict requirements on the output characteristics of each module.

　　3. Technical analysis of a company’s offline internal resistance testing method

　　According to a company's introduction, its online 18650 lithium ion battery cellmonitoring is based on the analysis of the discharge curve, multiple tests, and comprehensive judgment, including the following three points:

　　1. Dynamic large current (>100A) impact load discharge, obtain the instantaneous discharge curve of the 18650 lithium ion battery cellin a short period of time, and measure the internal resistance: internal resistance = (18650 lithium ion battery cellelectromotive force - 18650 lithium ion battery cellvoltage) / discharge current.

　　2. Discharge with static small current and constant current, and measure the 18650 lithium ion battery cellcapacity: 18650 lithium ion battery cellcapacity = discharge current x time.

　　3. Comprehensive calculation and analysis of the above parameters through computer to obtain an accurate assessment of 18650 lithium ion battery cellperformance.

　　Based on the above description and the analysis of the test results seen in the pilot test, we have the following opinions on a certain company's testing method:

　　1. Although the discharge curve is a traditional and effective method for testing 18650 lithium ion battery cellperformance, in the actual online operation process, considering the cycle life of the battery, it is impossible to perform frequent capacity tests on the battery. Although it is a static small current constant current discharge process, Because our power system does not allow batteries running online to be discharged for a long time, so according to the above calculation formula of 18650 lithium ion battery cellcapacity = discharge current x time, the capacity measured under the condition of insufficient discharge time is not the actual capacity of the battery.

　　The online test results of the static small current constant current discharge test capacity method are not credible. First, when the 18650 lithium ion battery cellpack is running online, the charger is constantly providing charging current for the battery. This current is automatically adjusted as the 18650 lithium ion battery cellload changes. Therefore, how to ensure static small current and constant current discharge is a problem that cannot be solved online.

　　2. From the internal resistance calculation formula, the 18650 lithium ion battery cellelectromotive force refers to the open circuit voltage of the battery, and the 18650 lithium ion battery cellvoltage refers to the closed circuit voltage after the 18650 lithium ion battery cellis loaded. From this, we can know that there are two problems in the process of testing internal resistance:

　　During testing, the 18650 lithium ion battery cellpack must be disconnected to obtain the electromotive force. This process must leave the 18650 lithium ion battery cellpack in an open circuit state. It is not allowed for the 18650 lithium ion battery cellpack to be in an open circuit state during operation, even for a short period of time. A major safety hazard.

　　The voltage difference △U is calculated by comparing the two electromotive forces with the voltage after high-current discharge. This voltage difference is very unstable and discrete. △V will be completely different under different float charging voltages. In addition, the discharge time is extremely short, and the time value and error will fundamentally affect the voltage value. Once the time is greater than the starting polarization time of a battery, its internal resistance is no longer the true internal resistance of a battery. It is far from the actual internal resistance value.

　　This method is an estimation algorithm, and it is difficult to accurately test the internal resistance of the 18650 lithium ion battery cellwith trace characteristics. The resolution is low. In the test of the pilot laboratory, when the connecting piece was consciously loosened, the tester could not find the connection. Abnormal changes, this test verified the above analysis. 4. Technical analysis of the pure DC instantaneous high current discharge method of the American Albér Company

　　As shown in Figure 2, the 18650 lithium ion battery cellunder test discharges a large current (50-70A) to the load for about 3.25 seconds. After the discharge voltage stabilizes, the instantaneous interruption voltage difference ΔV (V2-V1) and the current value (I) are measured. Calculate the internal resistance of the 18650 lithium ion battery cellR internal resistance = △V/I using the ratio of

　　The pure DC instantaneous high-current discharge method adopts the pure resistive load method, which uses zero-moment technology (ZEROTIME). Taking into account the characteristic that the potential of the 18650 lithium ion battery cellwill rise immediately after the load is disconnected, the on-circuit and off-circuit voltages are read simultaneously at the moment of disconnection. and the on-off current difference to calculate the accurate internal resistance.

　　Based on the above description and the analysis of the test results seen in the pilot test, we have the following opinions on the testing method of Albér in the United States:

　　1. The testing accuracy and stability of Albér in the United States are very good. Albér measures the instantaneous interruption voltage difference after the discharge voltage stabilizes, △V=(V2-V1); the method of taking the value must be specific and effective. This method The voltage difference taken has good stability, and the test time is enough to enter a stable period. The time error will have nothing to do with the value of the voltage. At the same time, it also skips the decreasing period during the initial discharge process, eliminating the voltage and current decreasing process. AC component, thereby ensuring that the calculated value is a DC resistance value so that it can truly reflect the 18650 lithium ion battery cellcharacteristics. At the same time, because the test current is large and is not affected by the 18650 lithium ion battery cellcapacitance (shown in Figure 3), it not only makes the pressure difference reading accurate, but also has good anti-interference ability, the reading is stable, and is not affected by the ripple voltage of the charging circuit, which can effectively It can fully discover the subtle changes in the internal structure of its resistor, and its test current is large, so ripples, harmonics and spikes in the loop will not affect its test error value.

　　2. There is no need to take the 18650 lithium ion battery celloffline, which avoids potential risks to system security; it can be seen from the test results that the static and dynamic test results are consistent.

　　3. The resolution of the test data of Albér Company is very high. Although the internal resistance of large-capacity batteries was not tested in the test of the Central Laboratory, a very valuable test was carried out, which was to detect the connection status between batteries. , the connection screws between the batteries were intentionally loosened. Only the instrument of the American Albér company can accurately detect subtle changes. This shows that the resolution of the test data is very high, which is very important for large-capacity batteries. , because the batteries used in power systems are all larger than 300AH, which is far different from batteries in the automobile industry and electric vehicles. Their internal resistance is only a few hundred microohms (a few tenths of a milliohm). It cannot be accurately found without a high-resolution instrument. The trend of trace internal resistance changes to identify lagging batteries.

　　4. Regarding the question of whether such a large current is harmful to the battery, according to the calculation method of the corresponding relationship between the current and the test time in the test procedure, assuming that the discharge rate is 70A for one hour, it is normal for the 18650 lithium ion battery cellampere-hours to be above 137AH. A test, let alone an instant? It can be seen that the large current instantaneous discharge of 70A is completely within the allowable operating current range and time of the battery, and will not cause any damage to the battery. The purpose of our online monitoring of batteries is to accumulate the operating internal resistance data of the battery, obtain its operating trend, discover the turning point of 18650 lithium ion battery celldeterioration in advance, and eliminate faults before they occur. Therefore, I think the use of large currents is in line with the power system's impact on batteries. The testing requirements for load capacity truly reflect the load-bearing capacity and performance of the battery.

　　5. Product intelligence - internal resistance data analysis is another key technology of this product. According to the information provided by Albér and his understanding of Albér, Albér summarized based on 30 years of testing experience and hundreds of thousands of experimental tests, At the same time, after 20 years of practical work summary of the use of this product by AT&T Telecommunications Company in the United States, it was concluded that when the internal resistance of the 18650 lithium ion battery cellis greater than 25% of the initial value, the 18650 lithium ion battery cellcapacity will be less than 80%.

　　5. Conclusion

　　According to the recent "Technical Specifications for DC Systems of Guangdong Power Grid Company Substations" issued by the provincial company, the internal resistance of the 18650 lithium ion battery cellmust not be greater than 50% of the nominal value, and the internal resistance consistency between batteries must not be greater than 10%. Combined with the 18650 lithium ion battery cellcapacity we use, Most of them are 300AH, 500AH, and 800AH. The internal resistance of these batteries is several hundred microohms, which belongs to the category of microresistance. Even if it is a 50% standard, its absolute value will not be very large, so the 18650 lithium ion battery cellmust be effectively and accurately monitored. To determine the internal resistance, the connection status between batteries, and the changing trend of 18650 lithium ion battery cellinternal resistance during operation, only test instruments with high resolution, high accuracy, stable testing, and strong consistency can be selected.

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