A method for determining the relative self-discharge rate of a lithium-ion battery cell

Abstract

The present invention discloses a method for measuring the relative self-discharge rate of a lithium-ion battery cell, which includes: adjusting the open-circuit voltage of the battery under test and the reference battery to the same value through the formation process; placing the reference battery in a first thermostat at temperature T1, and placing the battery under test in a second thermostat at temperature T2, where T1 < T2; connecting the reference battery and the aged battery under test in parallel, and connecting an ammeter in the circuit; the real-time current value shown by the ammeter is the relative self-discharge rate of the battery under test with respect to the reference battery. According to the characteristic that the higher the temperature, the faster the self-discharge rate of the battery, the present invention places the battery under test and the reference battery in thermostats at different temperatures and connects them in parallel, and connects an ammeter in the parallel circuit, and uses the real-time current value of the ammeter to represent the relative self-discharge rate, with a short detection period and high test efficiency.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a method for measuring the relative self-discharge rate of a single lithium-ion battery cell. Background Technology

[0002] Self-discharge rate is a key parameter in battery selection and grouping, affecting the calendar life and capacity performance of the battery system. Currently, commonly used self-discharge detection methods include the voltage drop method, capacity decay method, and self-discharge current method. The voltage drop method often involves the cell's K-value. This method calculates the difference in open-circuit voltage per unit time by recording the open-circuit voltage (OCV) at two time points; the unit is the K-value, expressed in mV / h. Due to the relaxation phenomenon in electrochemistry, the battery voltage must be allowed to rest for a period of time after charging or discharging before returning to its equilibrium voltage. This resting time is generally related to factors such as temperature, battery state of charge, battery material system, and charging / discharging conditions. The relaxation phenomenon affects both the efficiency of K-value measurement and the accuracy of open-circuit voltage measurement, thus affecting the accuracy of K-value measurement. The capacity decay method measures the capacity loss ratio of the cell after it has been left to rest under specified conditions for a period of time, obtaining the percentage reduction in cell capacity per unit time. This method has been incorporated into national standards for measuring the capacity retention and charge recovery capability of cells after resting. The self-discharge current method assumes that the battery's self-discharge is a constant current discharge and calculates the battery's self-discharge current by measuring the capacity loss and time during the cell's resting period. As can be seen from the detection principle, the existing voltage drop method, capacity decay method, and self-discharge current method are all process detection methods, which require detecting parameters (voltage or capacity) in two states before and after the battery is placed, resulting in problems such as long detection cycles and high equipment and site investment costs.

[0003] The "Battery Self-Discharge Detection Method, Apparatus, Computer Equipment, and Storage Medium" disclosed in Chinese patent literature, publication number CN114355212A, published on 2022-04-15, includes: acquiring discharge data of n batteries; calculating the voltage decay rate based on the time interval between the second and first moments, the first open-circuit voltage value, and the second open-circuit voltage value; calculating the slope of the capacity-voltage curve based on a preset charge value, the second open-circuit voltage value, the third open-circuit voltage value, the voltage decay rate, the time interval between the fourth and second moments; and calculating the battery's self-discharge current based on the voltage decay rate and the slope. This technology corrects the magnitude of the battery's self-discharge current based on the dQ / dV value, directly obtaining the battery's self-discharge current, which can more intuitively characterize the magnitude of the battery's self-discharge current, improves testing efficiency, avoids interference from external factors, and reduces the occurrence of missed and false detections. However, this technology is still a process detection method, requiring multiple resting of the test battery and detection of parameters after each resting, resulting in a long detection cycle and low detection efficiency. Summary of the Invention

[0004] In order to overcome the problem that the detection method for the self-discharge of batteries in the prior art belongs to a process detection method, with a long detection period and low detection efficiency, the present invention provides a method for measuring the relative self-discharge rate of a single lithium-ion battery. The battery under test and the reference battery are connected in parallel in constant-temperature boxes at different temperatures, and an ammeter is connected in the parallel circuit. The real-time current value of the ammeter represents the relative self-discharge rate, with a short detection period and high test efficiency.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for measuring the relative self-discharge rate of a single lithium-ion battery, comprising:

[0007] Adjust the battery under test and the reference battery to the same open-circuit voltage through the formation process;

[0008] Place the reference battery in the first constant-temperature box at temperature T1, and place the battery under test in the second constant-temperature box at temperature T2, where T1 < T2; connect the reference battery and the aged battery under test in parallel, and connect an ammeter in the circuit;

[0009] The real-time current value shown by the ammeter is the relative self-discharge rate of the battery under test relative to the reference battery.

[0010] In the present invention, if the open-circuit voltages of two batteries (with the same material system) are equal, there will be no current in the parallel circuit after the two batteries are connected in parallel. However, when the self-discharge rates of the two batteries are different, the battery with a slower self-discharge rate will have a smaller discharge amount, and the battery with a faster self-discharge rate will have a larger discharge amount. After parallel connection, the battery with a slower self-discharge rate will charge the battery with a faster self-discharge rate. At this time, the current in the parallel circuit reflects the relative self-discharge rate of the battery under test relative to the reference battery; the purpose of measuring the relative self-discharge rate is for the screening and matching of single lithium-ion batteries to improve the screening and matching efficiency; the result obtained by the measurement is a relative value, and the magnitude of the relative value can be used as a basis for screening or grading the single battery. For example, set a relative self-discharge rate of 40 microamperes for each same-grade single battery to grade all the produced lithium-ion batteries.

[0011] Preferably, the method for obtaining the reference battery is:

[0012] Randomly select two formed batteries under test and place them in the first constant-temperature box at temperature T1 and the second constant-temperature box at temperature T2 respectively. Connect the two batteries under test in parallel. After turning on the circuit, let it stand for a set time length and then disconnect the circuit. Select the battery under test placed in the first constant-temperature box as the reference battery.

[0013] In this invention, the set time for resting is 2 to 4 hours. Since the battery under test and the reference battery are placed in constant temperature chambers at different temperatures during the test, in order to eliminate the voltage difference between the battery under test and the reference battery caused by the temperature difference before the test, they need to be placed in constant temperature chambers at corresponding temperatures and left to rest for a period of time to make the open circuit voltage the same.

[0014] Preferably, the final stage of the formation process employs a staged constant current and voltage limiting method for charging or discharging:

[0015] During staged constant current and voltage-limited charging, the charging rate decreases step by step, and the charging cut-off voltage is less than the upper limit of the battery's operating voltage. During staged constant current and voltage-limited discharging, the discharging rate decreases step by step, and the discharging cut-off voltage is greater than the lower limit of the battery's operating voltage.

[0016] The purpose of this invention is to ensure that the cells under test have the same open-circuit voltage after formation and to eliminate the influence of electrode relaxation on the open-circuit voltage as soon as possible. At the end of the formation process, charging can be performed using a staged constant current and voltage limiting method, or discharging can be performed using a staged constant current and voltage limiting method.

[0017] Preferably, the current ratio of the staged constant current limited voltage charging in the battery formation process is 0.05 to 0.5*Cr; the current ratio of the staged constant current limited voltage discharging in the battery formation process is 0.05 to 1.0*Cr, where Cr is the rated capacity of the battery.

[0018] In this invention, Cr is the rated capacity value of the battery. During charging, the unit of 0.05 to 0.5*Cr is the current unit, and during discharging, the unit of 0.05 to 1.0*Cr is the current unit.

[0019] As a preferred option, before conducting the relative self-discharge rate test on the battery under test and the reference battery connected in parallel, the battery under test is placed in the second constant temperature chamber for 2 to 4 hours, and the reference battery is placed in the first constant temperature chamber for 2 to 4 hours.

[0020] In this invention, the purpose of placing the battery under test in the second constant temperature chamber for 2 to 4 hours and the reference battery in the first constant temperature chamber for 2 to 4 hours is to make the temperature of the battery under test consistent with the temperature in the second constant temperature chamber and the temperature of the reference battery consistent with the temperature in the first constant temperature chamber. In order to improve the testing production efficiency, this step can be combined with the aging process of the battery under test, that is, after placing the battery under test in the second constant temperature chamber for 2 to 4 hours, continue to place it for aging, so that the aging time is greater than 4 hours.

[0021] Preferably, the temperature T1 is in the range of less than or equal to 30 degrees Celsius, and the temperature T2 is in the range of greater than or equal to 45 degrees Celsius and less than or equal to 55 degrees Celsius. In this invention, temperature T1 simulates the battery condition under normal ambient temperature, where the battery self-discharge rate is relatively low; temperature T2 simulates the battery condition under higher ambient temperature, where the battery self-discharge rate is relatively high.

[0022] Preferably, the reference battery is replaced after every 200 to 1000 test batteries have undergone relative self-discharge rate measurements.

[0023] In this invention, since the reference battery continues to discharge to the battery under test for a long time during the test, in order to ensure the reference battery's comparability, a new reference battery needs to be replaced after a certain number of tests are performed. However, in order to improve test efficiency and continuity, when using the previous reference battery for testing, the selection and placement of subsequent reference batteries can be completed, thereby enabling direct replacement of the reference battery.

[0024] Preferably, the positive electrode material of the battery to be tested is lithium manganese oxide or lithium manganese oxide plus other positive electrode materials. The other positive electrode materials are one or two of Li(Ni,Co,Mn)O2, Li(Ni,Co,Al)O2, Li(Mn,Fe)PO4, xLi2MnO3·(1-x)LiMO2, and Li(Mn,M)2O4, where M = Ni,Co,Mn.

[0025] The present invention uses common positive electrode materials for lithium-ion batteries. As the temperature increases, the self-discharge rate of the battery will increase. M can be not only Ni, Co or Mn, but also other transition metal elements such as Fe.

[0026] The present invention has the following advantages: the battery under test and the reference battery are placed in constant temperature chambers at different temperatures and connected in parallel, and an ammeter is connected in the parallel circuit. The real-time current value of the ammeter represents the relative self-discharge rate, resulting in a short detection cycle and high testing efficiency. Attached Figure Description

[0027] Figure 1 This is a flowchart of the relative self-discharge rate measurement method in this invention;

[0028] Figure 2 It is the equivalent GNL circuit model of lithium-ion batteries in the existing technology;

[0029] Figure 3 It is the capacity retention rate of lithium-ion batteries after being stored at different temperatures for 7 days.

[0030] Figure 4 This is a schematic diagram of the relative self-discharge rate test of the battery under test in this invention;

[0031] Figure 5 This is a schematic diagram of the selection of the reference battery in this invention;

[0032] Figure 6 This is a schematic diagram illustrating the continuous measurement of the relative self-discharge rate of the battery in an embodiment of the present invention;

[0033] In the diagram: 1. First constant temperature chamber; 2. Second constant temperature chamber; 3. Reference battery; 4. Battery under test; 5. Wires; 6. Ammeter. Detailed Implementation

[0034] The present invention will now be further described with reference to the accompanying drawings and specific embodiments.

[0035] like Figure 1 and Figure 4 As shown, a method for determining the relative self-discharge rate of a lithium-ion battery cell includes:

[0036] The test cell 4 and the reference cell 3 are adjusted to the same open-circuit voltage through a formation process;

[0037] Reference battery 3 is placed in a first constant temperature chamber 1 at temperature T1, and battery 4 to be tested is placed in a second constant temperature chamber 2 at temperature T2. <T2;

[0038] The reference battery 3 and the aged test battery 4 are connected in parallel via wire 5, and an ammeter 6 is connected in the circuit; the real-time current value displayed by the ammeter 6 is the relative self-discharge rate of the test battery 4 relative to the reference battery 3.

[0039] like Figure 5 As shown, the reference battery is obtained as follows:

[0040] Two formed batteries were randomly selected and placed in a first constant temperature chamber 1 at temperature T1 and a second constant temperature chamber 2 at temperature T2, respectively. The two batteries were connected in parallel by wire 5. After the circuit was turned on, the batteries were left to stand for a set time (2 to 4 hours) and then the circuit was turned off. The battery placed in the first constant temperature chamber was selected as the reference battery.

[0041] At the end of the formation process, charging or discharging is performed using a staged constant current and limited voltage method:

[0042] During staged constant current and voltage-limited charging, the charging rate decreases step by step, and the charging cut-off voltage is less than the upper limit of the battery's operating voltage. During staged constant current and voltage-limited discharging, the discharging rate decreases step by step, and the discharging cut-off voltage is greater than the lower limit of the battery's operating voltage.

[0043] The current ratio of the staged constant current limited voltage charging in the formation process of the battery under test is 0.05 to 0.5*Cr; the current ratio of the staged constant current limited voltage discharging in the formation process of the battery under test is 0.05 to 1.0*Cr, where Cr is the rated capacity of the battery.

[0044] Before conducting the relative self-discharge rate test on the parallel-connected battery under test and reference battery, the battery under test is placed in the second constant temperature chamber for 2 to 4 hours, and the reference battery is placed in the first constant temperature chamber for 2 to 4 hours.

[0045] The temperature range T1 is less than or equal to 30 degrees Celsius, and the temperature range T2 is greater than or equal to 45 degrees Celsius and less than or equal to 55 degrees Celsius.

[0046] After each 200 to 1000 test batteries have had their relative self-discharge rates measured, the reference battery is replaced.

[0047] The positive electrode material of the battery under test is lithium manganese oxide or lithium manganese oxide + other positive electrode materials. The other positive electrode materials are one or two of Li(Ni,Co,Mn)O2, Li(Ni,Co,Al)O2, Li(Mn,Fe)PO4, xLi2MnO3·(1-x)LiMO2, and Li(Mn,M)2O4, where M = Ni, Co, Mn.

[0048] like Figure 2 The figure shows the equivalent GNL circuit model of a lithium-ion battery, where U oc As an ideal voltage source, R represents the open-circuit voltage of the battery. s This represents the self-discharge resistance. Because the equivalent circuit model is inside the battery, current technology cannot determine R using Ohm's law. s The size cannot be determined by R. s Self-discharge current I s .

[0049] There is currently no clear academic understanding of the self-discharge mechanism of lithium-ion batteries. Self-discharge can be broadly categorized into chemical self-discharge and physical self-discharge. Chemical self-discharge can be further divided into reversible and irreversible chemical self-discharge. Reversible chemical self-discharge is caused by the deintercalation of lithium ions embedded in graphite to form metastable substances or their embedding in the cathode material. During subsequent discharge, the metastable substances generated will convert into stable substances, releasing active lithium ions, thus preventing irreversible capacity loss. Irreversible chemical self-discharge, on the other hand, is caused by the deintercalation of lithium ions embedded in graphite to form stable substances, leading to irreversible capacity loss. Physical self-discharge is caused by internal micro-short circuits resulting from metal particles, conductive microparticles, etc., leading to a reduction in battery capacity during idle periods.

[0050] Temperature affects the activity of materials and the rate of chemical reactions; increasing battery temperature will accelerate self-discharge. Figure 3The graph shows the capacity loss of lithium manganese oxide-graphite batteries due to self-discharge versus temperature at different temperatures. The test method involves placing fully charged batteries in a 55°C constant temperature chamber for 7 days. After 7 days, the remaining capacity is measured, and the capacity retention rate is calculated by comparing the remaining capacity with the initial capacity. The graph shows that the capacity loss caused by storage at 25°C is only 1.90%, which is relatively small, and the self-discharge current I can be considered relatively low. s Approaching 0A, the capacity loss caused by storage at 45°C is nearly 2.5 times that caused by storage at 25°C, and the capacity loss caused by storage at 55°C is nearly 6 times that caused by storage at 25°C. Based on this known information, the relative self-discharge rate determination method of the present invention was designed.

[0051] In this invention, if two batteries (with the same material system) have equal open-circuit voltages, there is no current in the parallel circuit after connecting the two batteries in parallel. However, when the self-discharge rates of the two batteries are different, the battery with a slower self-discharge rate will discharge less, while the battery with a faster self-discharge rate will discharge more. After being connected in parallel, the battery with a slower self-discharge rate charges the battery with a faster self-discharge rate. At this time, the current in the parallel circuit reflects the relative self-discharge rate of the battery under test relative to the reference battery. The purpose of relative self-discharge rate measurement is to screen and group lithium-ion battery cells, thereby improving the efficiency of screening and grouping. The measured result is a relative value, and the magnitude of the relative value can be used as the basis for screening or classifying battery cells. For example, setting a relative self-discharge rate of 40 microamps per cell as the same class of battery cells allows for the classification of all produced lithium-ion batteries.

[0052] In this invention, the set time for resting is 2 to 4 hours. Since the battery under test and the reference battery are placed in constant temperature chambers at different temperatures during the test, in order to eliminate the voltage difference between the battery under test and the reference battery caused by the temperature difference before the test, they need to be placed in constant temperature chambers at corresponding temperatures and left to rest for a period of time to make the open circuit voltage the same.

[0053] The purpose of this invention is to ensure that the cells under test have the same open-circuit voltage after formation and to eliminate the influence of electrode relaxation on the open-circuit voltage as soon as possible. At the end of the formation process, charging can be performed using a staged constant current and voltage limiting method, or discharging can be performed using a staged constant current and voltage limiting method.

[0054] In this invention, Cr is the rated capacity value of the battery. During charging, the unit of 0.05 to 0.5*Cr is the current unit, and during discharging, the unit of 0.05 to 1.0*Cr is the current unit.

[0055] In this invention, the purpose of placing the battery under test in the second constant temperature chamber for 2 to 4 hours and the reference battery in the first constant temperature chamber for 2 to 4 hours is to make the temperature of the battery under test consistent with the temperature inside the second constant temperature chamber, and to make the temperature of the reference battery consistent with the temperature inside the first constant temperature chamber. In order to improve the testing production efficiency, this step can be combined with the aging process of the battery under test, that is, after placing the battery under test in the second constant temperature chamber for 2 to 4 hours, continue to place it in the second constant temperature chamber for aging, so that the aging time is greater than 4 hours.

[0056] In this invention, temperature T1 simulates the battery condition under normal ambient temperature, where the battery self-discharge rate is relatively low; temperature T2 simulates the battery condition under higher ambient temperature, where the battery self-discharge rate is relatively high.

[0057] In this invention, since the reference battery continues to discharge to the battery under test for a long time during the test, in order to ensure the reference battery's comparability, a new reference battery needs to be replaced after a certain number of tests are performed. However, in order to improve test efficiency and continuity, when using the previous reference battery for testing, the selection and placement of subsequent reference batteries can be completed, thereby enabling direct replacement of the reference battery.

[0058] The present invention uses common positive electrode materials for lithium-ion batteries. As the temperature increases, the self-discharge rate of the battery will increase. M can be not only Ni, Co or Mn, but also other transition metal elements such as Fe.

[0059] In embodiments of the present invention, such as Figure 6 As shown, in order to continuously measure the relative self-discharge rate of the batteries under test in batches, a long constant temperature chamber is selected as the second constant temperature chamber, and a conveyor belt is set inside to transport the batteries under test. The relationship between the length of the second constant temperature chamber (the length of the conveyor belt) and the conveyor belt speed needs to ensure that the batteries under test are placed in the second constant temperature chamber for at least 4 hours. The first constant temperature chamber and the reference battery inside it are set parallel to the conveyor belt conveying direction. The batteries under test transported by the conveyor belt pass through the reference battery in sequence to measure the relative self-discharge rate. After the measurement is completed, the batteries under test are transported off the line by the conveyor belt.

[0060] Example 1: Formation of a 12000mAh soft-pack lithium-ion battery cell using lithium manganese oxide-graphite.

[0061] Step 1: After the liquid injection and aging processes are completed, the batch of lithium-ion battery cells are subjected to pressure formation. The formation process parameters are as follows:

[0062] Step 2: Randomly select the battery to be tested after the process is completed, and press... Figure 5The circuit diagram shown connects the positive and negative terminals of the two batteries to be tested. The temperature of the first constant temperature chamber is preferably 25°C, and the temperature of the second constant temperature chamber is preferably 55°C. After standing for 2 hours, the battery in the first constant temperature chamber is taken out, which is the reference battery.

[0063] Step 3: The aging process for the battery under test involves placing it in a second constant-temperature chamber at 55°C for 4 hours. To align with the relative self-discharge test, a conveyor belt is installed in the second constant-temperature chamber. The length of the second constant-temperature chamber and the conveyor belt speed are adjusted to ensure that the battery under test remains stationary in the second constant-temperature chamber for at least 4 hours. The temperature of the first constant-temperature chamber is 25°C.

[0064] Step 4: The conveyor belt sequentially transports the batteries under test through the reference batteries, connects them in parallel, and measures the relative self-discharge rate. The batteries under test that have completed the measurement are then transported off the production line by the conveyor belt.

[0065] The final test results are shown in Table 1:

[0066] Table 1 Relative self-discharge test results

[0067]

[0068] The measured results show that battery cells with a relative self-discharge rate of less than 40 microamps can be considered to be in the same category, and they are classified into categories with 40 microamp intervals. That is, they are classified into categories [0,40) and [40,80).

[0069] Example 2: Formation of a 24000mAh soft-pack lithium-ion battery cell using lithium manganese oxide-graphite.

[0070] Step 1: After the liquid injection and aging processes are completed, the batch of lithium-ion battery cells are subjected to pressure formation. The formation process parameters are as follows:

[0071]

[0072]

[0073] Step 2: Randomly select the battery to be tested after the process is completed, and press... Figure 5 The circuit diagram shown connects the positive and negative terminals of the two batteries to be tested. The temperature of the first constant temperature chamber is preferably 25°C, and the temperature of the second constant temperature chamber is preferably 55°C. After standing for 2 hours, the battery in the first constant temperature chamber is taken out, which is the reference battery.

[0074] Step 3: The aging process for the battery under test involves placing it in a second constant-temperature chamber at 55°C for 4 hours. To align with the relative self-discharge test, a conveyor belt is installed in the second constant-temperature chamber. The length of the second constant-temperature chamber and the conveyor belt speed are adjusted to ensure that the battery under test remains stationary in the second constant-temperature chamber for at least 4 hours. The temperature of the first constant-temperature chamber is 25°C.

[0075] Step 4: The conveyor belt sequentially transports the batteries under test through the reference batteries, connects them in parallel, and measures the relative self-discharge rate. The batteries under test that have completed the measurement are then transported off the production line by the conveyor belt.

[0076] The final test results are shown in Table 2:

[0077] Table 2 Relative Self-Discharge Test Results

[0078]

[0079] The measured results show that battery cells with a relative self-discharge rate of less than 40 microamps can be considered to be in the same category, and they are classified into categories with 40 microamp intervals. That is, they are classified into categories [0,40), [40,80), and [80,120).

[0080] The above embodiments are further elaborations and descriptions of the present invention to facilitate understanding, and are not intended to limit the present invention in any way. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for determining the relative self-discharge rate of a lithium-ion battery cell, characterized in that, include: The test cell and the reference cell are adjusted to the same open-circuit voltage through a formation process; The reference battery is placed in a first constant temperature chamber at temperature T1, and the battery under test is placed in a second constant temperature chamber at temperature T2. <T2; The reference battery and the aged battery under test are connected in parallel, and an ammeter is connected in the circuit. The real-time current value displayed by the ammeter is the relative self-discharge rate of the battery under test relative to the reference battery.

2. The method for determining the relative self-discharge rate of a lithium-ion battery cell according to claim 1, characterized in that, The reference battery is obtained as follows: Two batteries to be tested are randomly selected and placed in a first constant temperature chamber at temperature T1 and a second constant temperature chamber at temperature T2, respectively. The two batteries to be tested are connected in parallel. After the circuit is turned on, the batteries are left to stand for a set time and then the circuit is turned off. The battery to be tested placed in the first constant temperature chamber is selected as the reference battery.

3. A method for determining the relative self-discharge rate of a lithium-ion battery cell according to claim 1 or 2, characterized in that, The final stage of the formation process employs a staged constant current and voltage limiting method for charging or discharging. During staged constant current and voltage-limited charging, the charging rate decreases step by step, and the charging cut-off voltage is less than the upper limit of the battery's operating voltage. During staged constant current and voltage-limited discharge, the discharge rate decreases step by step, and the discharge cutoff voltage is greater than the lower limit of the battery's operating voltage.

4. The method for determining the relative self-discharge rate of a lithium-ion battery cell according to claim 3, characterized in that, The current ratio of the staged constant current limited voltage charging in the formation process of the battery under test is 0.05 to 0.5*Cr; the current ratio of the staged constant current limited voltage discharging in the formation process of the battery under test is 0.05 to 1.0*Cr, where Cr is the rated capacity of the battery.

5. A method for determining the relative self-discharge rate of a lithium-ion battery cell according to claim 1, 2, or 4, characterized in that, Before conducting the relative self-discharge rate test on the parallel-connected battery under test and reference battery, the battery under test is placed in the second constant temperature chamber for 2 to 4 hours, and the reference battery is placed in the first constant temperature chamber for 2 to 4 hours.

6. The method for determining the relative self-discharge rate of a lithium-ion battery cell according to claim 5, characterized in that, The temperature T1 is in the range of less than or equal to 30 degrees Celsius, and the temperature T2 is in the range of greater than or equal to 45 degrees Celsius and less than or equal to 55 degrees Celsius.

7. A method for determining the relative self-discharge rate of a lithium-ion battery cell according to claim 1, 2, 4, or 6, characterized in that, After each 200 to 1000 test batteries have had their relative self-discharge rates measured, the reference battery is replaced.

8. A method for determining the relative self-discharge rate of a lithium-ion battery cell according to claim 1, 2, 4, or 6, characterized in that, The positive electrode material of the battery under test is lithium manganese oxide or lithium manganese oxide + other positive electrode materials. The other positive electrode materials are one or two of Li(Ni,Co,Mn)O2, Li(Ni,Co,Al)O2, Li(Mn,Fe)PO4, xLi2MnO3·(1-x)LiMO2, and Li(Mn,M)2O4, where M = Ni, Co, Mn.

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