Cell balancing method and system therefor
Patent Information
- Application Number
- JP2022119849
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2022-07-27
- Publication Date
- 2025-07-22
- Estimated Expiration
- 2042-07-27
AI Technical Summary
Existing lithium-ion battery cell balancing methods in electric vehicles fail to address individual cell deterioration variations, leading to uneven capacity retention rates and overall battery degradation.
A method and system that adjusts the state of charge (SOC) of lithium-ion battery cells in a battery pack by predicting stoppage times and estimating capacity retention rates, allowing for targeted SOC adjustments to minimize differences between cells, using temperature and SOC data to set appropriate adjustment values.
This approach effectively balances cell capacities, maintaining battery performance over time by reducing variations in capacity retention rates and suppressing deterioration, ensuring consistent battery function.
Smart Images
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Abstract
Description
[Technical field]
[0001] The present invention relates to a cell balancing method and system, and more particularly to a cell balancing method and system for adjusting the charge rate of cells of lithium ion batteries that constitute a battery pack mounted on an electric vehicle. [Background technology]
[0002] The characteristics of lithium-ion batteries, which are the power source installed in electric vehicles, deteriorate depending on the usage environment. For example, the battery capacity maintained initially deteriorates more the longer it is left at high temperatures, and the battery capacity decreases over time. In addition, the longer it is left at a high charging rate (SOC: State of Charge), the more deterioration progresses, and the battery capacity decreases over time. In this way, lithium-ion batteries have a tendency to deteriorate more rapidly the longer they are left at high temperatures and high SOC.
[0003] Furthermore, this degradation problem also occurs in each cell of the lithium ion batteries that make up the battery pack, and the degree of degradation varies depending on the location of the cells, and this variation in degradation adversely affects the operation of the lithium ion batteries.
[0004] In Patent Document 1, when the battery is detected to be in a high temperature and high SOC state a predetermined number of times within a predetermined time while the electric vehicle is stopped, the electric vehicle's auxiliary equipment is operated to discharge the battery and lower the SOC. In this way, the battery is prevented from being exposed to a high temperature and high SOC state, and deterioration of the battery is suppressed. [Prior art documents] [Patent documents]
[0005] [Patent Document 1] JP 2015-37011 A Summary of the Invention [Problem to be solved by the invention]
[0006] In the method of Patent Document 1, an operation to suppress deterioration is performed only when the battery is in a high temperature and high SOC state where deterioration is accelerated. However, since the operation is performed uniformly for the entire battery, it is not possible to suppress the acceleration of deterioration by controlling each individual cell constituting the battery. As described above, the cells constituting the battery are not all in the same deterioration state, and each cell has different capacity, SOC, "capacity maintenance rate" which indicates the rate at which the SOC decreases over time, and "deterioration coefficient" which indicates the degree of acceleration of deterioration related to this capacity maintenance rate, and therefore the progress of deterioration varies from cell to cell. In particular, when the variation in capacity maintenance rate between cells is large, the deterioration of the characteristics is significant when viewed as the battery as a whole. For example, it is known that various problems caused by early discharge of cells with low capacity maintenance rate have a negative effect on the entire battery.
[0007] The present invention has been made in consideration of the above problems, and an object of the present invention is to provide a cell balancing method and system that can accurately suppress the accelerated deterioration of each cell of multiple lithium-ion batteries that make up a battery pack installed in an electric vehicle. [Means for solving the problem]
[0008] In order to achieve the above object, the cell balancing method according to claim 1 comprises: A cell balancing method for adjusting the SOC of a lithium-ion battery composed of a plurality of cells mounted on an electric vehicle as a power source, comprising: a predicted stop time setting step of predicting a stop time from when the electric vehicle is stopped until the next start based on statistics of the stop time collected over a predetermined period of time; a capacity maintenance rate calculation and estimation step of calculating and estimating a capacity maintenance rate of each cell of the lithium ion battery at the next start time after the predicted predicted stop time has elapsed, based on a measured temperature or estimated temperature, a SOC, and a deterioration coefficient of each cell while the vehicle is stopped; a SOC adjustment value setting step of setting one or both of the maximum and minimum cell SOC adjustment values so that the difference between the maximum and minimum values of the estimated capacity maintenance ratio is within the predetermined value when the difference between the maximum and minimum values of the estimated capacity maintenance ratio is equal to or greater than a predetermined value; an SOC adjustment process for adjusting one or both of the maximum and minimum SOC values to the SOC adjustment value set in the SOC adjustment value setting process by using an SOC adjustment device capable of increasing or decreasing the SOC of the cell while the electric vehicle is stopped; The present invention is characterized by having the following.
[0009] According to this method, the capacity maintenance rate of each cell at the next start after the electric vehicle is stopped is estimated by the capacity maintenance rate calculation and estimation step, and when the difference between the maximum and minimum values of the capacity maintenance rate is equal to or greater than a predetermined value (threshold value), the SOC adjustment value of either or both of the maximum and minimum cells is set so that the difference between the maximum and minimum values is within a predetermined value at the next start, and the SOC of either or both of the maximum and minimum cells is adjusted to the set SOC adjustment value while the electric vehicle is stopped. That is, while the electric vehicle is stopped, the SOC of the cells is adjusted so that the difference in the capacity maintenance rate of the cells at the next start falls within a predetermined range, for example, the SOC of the cell with the minimum value is increased or the SOC of the cell with the maximum value is decreased, or both are performed. This makes it possible to reduce the difference in SOC between the cells, and to achieve a good balance of the functions of the cells in the entire battery. Therefore, it is possible to maintain good performance of the lithium-ion battery for a long period of time.
[0010] The cell balancing method according to claim 2 further comprises the steps of: The predicted stopping time of the electric vehicle in the predicted stopping time setting step is predicted based on actual measured values of stopping time for each day of the week collected in advance over a predetermined period of time.
[0011] Therefore, the parking time predicted in the predicted parking time setting step can reflect the trend for each day of the week, making it more accurate, and the capacity maintenance ratio of each cell calculated and estimated in the capacity maintenance ratio calculation and estimation step based on this can be highly reliable, making it possible to perform effective cell balancing.
[0012] The cell balancing method according to claim 3 is the cell balancing method according to claim 1 or 2, The capacity maintenance rate F of each cell calculated and estimated in the capacity maintenance rate calculation and estimation step is: A SOC corresponding to the temperature of each cell while the vehicle is parked; A deterioration coefficient α corresponding to the temperature and SOC of each cell while the vehicle is parked; The stop time T (year (1 / 2) ) Based on, F=F initial -α·T (Finitial: Capacity maintenance rate when the vehicle is stopped) It is characterized in that it is calculated by:
[0013] As a result, the capacity maintenance ratio, which indicates the deterioration state of each cell, calculated and estimated in the capacity maintenance ratio calculation and estimation step can be accurately calculated and highly reliable, thereby enabling effective cell balancing.
[0014] In order to achieve the above object, the cell balancing system according to claim 4 comprises: A temperature determination device that measures or estimates the temperature of a cell of a lithium ion battery that is configured with a plurality of cells and is mounted on an electric vehicle as a power source thereof; A SOC determination device for measuring or estimating the SOC of the cell; A SOC adjusting device capable of adjusting the SOC of the cell to increase or decrease; an information processing device that transmits and receives data to and from the temperature determining device, the SOC determining device, and the SOC adjusting device; The information processing device includes: a predicted vehicle stop time setting unit that predicts a vehicle stop time until the next start based on statistics of the vehicle stop time collected over a predetermined period of time; a capacity maintenance rate calculation / estimation unit that calculates and estimates a capacity maintenance rate of each cell of the lithium ion battery at the next start time after the predicted predicted stop time has elapsed, based on the temperature or estimated temperature of each cell during the stop time determined by the temperature determination device, the SOC determined by the SOC determination device, and a deterioration coefficient; and a SOC adjustment value setting unit that sets a cell SOC adjustment value of either one or both of the maximum and minimum values so that the difference between the maximum and minimum values of the estimated capacity maintenance ratio is within the predetermined value when the difference between the maximum and minimum values of the estimated capacity maintenance ratio is equal to or greater than a predetermined value, The SOC adjusting device adjusts the SOC of the cell having either or both of the maximum and minimum values based on the SOC adjustment value from the SOC adjustment value setting unit.
[0015] With this configuration, the capacity maintenance ratio calculation and estimation unit estimates the capacity maintenance ratio of each cell at the next start after the electric vehicle stops, and when the difference between the maximum and minimum values of the capacity maintenance ratio is equal to or greater than a predetermined value (threshold value), the SOC adjustment value of either or both of the maximum and minimum cells is set so that the difference between the maximum and minimum values is within a predetermined value at the next start, and the SOC of either or both of the maximum and minimum cells is adjusted to the set SOC adjustment value while the electric vehicle is stopped. That is, while the electric vehicle is stopped, the SOC of the cells is adjusted so that the difference in the capacity maintenance ratio of the cells at the next start falls within a predetermined range, for example, the SOC of the cell with the minimum value is increased or the SOC of the cell with the maximum value is decreased, or both are performed. This makes it possible to reduce the difference in SOC between the cells, and to achieve a good balance of the functions of the cells in the entire battery. Therefore, it is possible to maintain good performance of the lithium-ion battery over a long period of time. Effect of the Invention
[0016] According to the cell balancing method and system of the present invention, while the electric vehicle is stopped, the SOC of the cells is adjusted so that the difference between the maximum and minimum capacity retention rates between the cells at the next start falls within a predetermined range. Therefore, the deterioration of each cell of the lithium-ion batteries constituting the battery pack can be appropriately suppressed, so that the performance of the battery pack is maintained for a long period of time and the electric vehicle can be used comfortably at all times. [Brief description of the drawings]
[0017] [Figure 1] 1 is a schematic configuration diagram of an embodiment of a cell balancing system of the present invention; [Figure 2A] FIG. 2 is a flow diagram (first half) of one embodiment of a cell balancing method of the present invention. [Figure 2B] FIG. 2 is a flow diagram of one embodiment of the cell balancing method of the present invention (second half). [Diagram 3] FIG. 4 is an explanatory diagram of a method for calculating a capacity maintenance rate. [Figure 4] FIG. 4 is an explanatory diagram of a method for calculating a capacity maintenance rate. [Diagram 5] FIG. 4 is an explanatory diagram of a method for calculating a capacity maintenance rate. [Figure 6] FIG. 4 is an explanatory diagram of a deterioration coefficient used in calculating a capacity maintenance rate. [Figure 7] FIG. 2 is a schematic diagram illustrating cell balancing of each cell. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] A cell balancing method and system according to an embodiment of the present invention will be described in detail below with reference to the drawings. As described in detail below, the present invention is characterized in that while an electric vehicle is stopped, the SOC of the cells of the multiple lithium-ion batteries that make up the battery pack is adjusted by the time the electric vehicle is next started, and the variation in the capacity maintenance rate of the multiple cells is controlled to within a predetermined value at the next start.
[0019] 1 is a schematic diagram (block diagram) of an embodiment of a cell balancing system of the present invention. The cell balancing system 10 of this embodiment includes a temperature determination device 14 that measures or estimates the temperature of each cell of a lithium-ion battery 12, a SOC determination device 16 that measures or estimates the SOC of each cell, a SOC adjustment device 18 that increases or decreases the SOC of each cell, and an information processing device 20 that transmits and receives data to and from the temperature determination device 14, the SOC determination device 16, and the SOC adjustment device 18.
[0020] The information processing device 20 has a CPU, a program unit 22, and a memory unit 24 (not shown). The program unit 22 includes a predicted stopping time setting unit 22-1 that predicts the stopping time from when the electric vehicle is stopped to when it is next started based on statistics of stopping times collected over a predetermined period of time, a capacity maintenance ratio calculation and estimation unit 22-2 that calculates and estimates the capacity maintenance ratio of each cell of the lithium-ion battery at the time of the next start, which is after the predicted stopping time has elapsed, based on the temperature or estimated temperature of each cell while the vehicle is stopped as determined by the temperature determination device 14, the SOC as determined by the SOC determination device 16, and the deterioration coefficient, and a SOC adjustment value setting unit 22-3 that sets the maximum and minimum SOC adjustment values of either or both of the cells so that the difference between the maximum and minimum values of the capacity maintenance ratio is within a predetermined value when the difference between the maximum and minimum values of the estimated capacity maintenance ratio is equal to or greater than a predetermined value.
[0021] The memory unit 24 stores data such as statistical values 24-1 of the electric vehicle's parking time for each day of the week, which are necessary for setting the predicted parking time, deterioration coefficients (maps) 24-2 of each cell, which are necessary for calculating and estimating the capacity maintenance rate, and a correspondence table 24-3 between the temperature and SOC of each cell.
[0022] As described above, a feature of the present invention is that while the electric vehicle is stopped, the SOC of the cells of the multiple lithium-ion batteries that make up the battery pack is adjusted by the time the electric vehicle is next started, and at the next start, the variation in the capacity maintenance rates of the multiple cells is controlled to within a predetermined value. The steps of the cell balancing method described below are performed by the predicted stoppage time setting unit 22-1, the capacity maintenance rate calculation / estimation unit 22-2, and the SOC adjustment value setting unit 22-3 in the program unit 22 of the information processing device 20 shown in FIG. 1, and the increase / decrease adjustment of the SOC of each cell is performed by the SOC adjustment device 18.
[0023] 2A and 2B show a flow diagram of the cell balancing method. First, it is determined whether the electric vehicle is parked or not (step S1). If the electric vehicle is not parked, the system waits until the electric vehicle is parked. Once it is confirmed that the electric vehicle is parked, the system calculates the capacity maintenance rate at the next start from the measured or estimated temperature of each cell while the vehicle is parked, the corresponding SOC value, the estimated parked time, and the deterioration coefficient (step S2, estimated parked time setting step, capacity maintenance rate calculation and estimation step).
[0024] The capacity maintenance rate is calculated in the capacity maintenance rate calculation estimation step by using the measured temperature or estimated temperature and SOC of each cell while the vehicle is parked, and the predicted parking time T (year (1 / 2) ) and the deterioration coefficient α corresponding to the measured temperature or estimated temperature and SOC of each cell while the vehicle is parked. F=F initial -α·T (Finitial: Capacity maintenance rate when the vehicle is stopped) This allows the capacity maintenance ratio, which indicates the deterioration state of each cell, to be calculated and estimated in the capacity maintenance ratio calculation and estimation step, to be accurately calculated and highly reliable. Here, the temperature of each cell may be an actual measurement value, or an estimated temperature estimated from the actual measurement value of the temperature of an adjacent battery cell, etc.
[0025] The corresponding values of the measured or estimated temperature and the SOC while the vehicle is stopped are obtained by associating the temperature of each cell with the SOC at that temperature, and are compiled, for example, in a table by measuring the temperature of each cell of the lithium ion battery and the SOC at that temperature over a predetermined period of time. For example, if data is obtained that the most frequently measured SOC at a temperature of 20° C. for cell A is 40%, then 60%, then 20%, etc., then cell A is associated with an SOC of 40% at a temperature of 20° C. All cells are associated in the same way. These data are stored in a correspondence table 24-3 of the temperature and SOC of each cell in the memory unit 24 of the information processing device 20.
[0026] The predicted stop time in the predicted stop time setting step is, for example, predicted for a specific day of the week by aggregating the actual values of the stop time for each day of the week over a specific period of time and compiling each day of the week and the typical stop time for that day in a table. For example, the stop time on Monday is predicted to be 8 hours, and the stop time on Sunday is predicted to be 9 hours, etc. This allows the stop time predicted in the predicted stop time setting step to reflect the trends for each day of the week, making it more accurate, and the capacity maintenance rate of each cell calculated and estimated in the capacity maintenance rate calculation and estimation step based on this is highly reliable.
[0027] The deterioration coefficient is calculated by actual measurement of the cell temperature and SOC, and can be calculated immediately if the measured or estimated cell temperature and SOC are known. The higher the SOC and the higher the temperature, the larger the deterioration coefficient becomes. For example, if the temperature is -30°C and the SOC is 20%, the deterioration coefficient is calculated as 0.1, and if the temperature is 40°C and the SOC is 90%, the deterioration coefficient is calculated as 0.9. Data related to this deterioration coefficient is stored in the deterioration coefficient (map) 24-2 in the memory unit 24 of the information processing device 20.
[0028] The method of calculating the capacity retention rate in step S2 will be explained with reference to FIG. 3. FIG. 3 shows how the capacity retention rate is calculated when the cell state (measured temperature or estimated temperature, SOC) is different across four sections. FIG. 3(a) shows the relationship between the capacity retention rate and time for a certain cell in four sections. Time is divided into sections 1 to 4, and if the measured or estimated temperature in section 1 is 20°C and the corresponding SOC is 80%, the deterioration coefficient α1 is calculated to be 0.8 from the table shown in FIG. 3(b). Next, section 1 is the parking time or left unused time ((year) 1 / 2 )T1 is 0.1, so the formula for capacity retention is (F1=F initial -α1·T1), the capacity retention rate is calculated as 0.92. Here, F initial is 1.
[0029] 3(c) is obtained. That is, in section 2, the capacity maintenance ratio is obtained as 0.88, in section 3, 0.86, and in section 4, 0.77.
[0030] Next, with reference to Fig. 4, in this embodiment, when it is estimated that the difference between the maximum and minimum capacity retention rates will exceed the threshold value at the next start, how to keep the difference within the threshold value will be described. The battery pack is composed of many lithium ion battery cells, and the battery cell with the maximum capacity retention rate is cell A, and the battery cell with the minimum capacity retention rate is cell B. The case where the difference between the capacity retention rates of cell A and cell B is kept within a threshold value (0.02) will be described. The threshold value of 0.02 is shown as an example.
[0031] First, the capacity retention rate for cells A and B while the vehicle is stopped is calculated. Figure 5 shows the temperatures (measured or estimated temperatures), SOCs, and leaving times (times the vehicle is stopped) of cells A and B in each section. Based on this, the temperature of battery cell A in section A is 20°C, the SOC is 80%, and the deterioration coefficient is calculated to be 0.7 from the deterioration coefficient table in Figure 6. Using the capacity retention rate calculation formula, the capacity retention rate at the end point of section A is calculated to be 0.93. Similarly, the capacity retention rate at the end point of section B (when the vehicle is stopped) is calculated to be 0.82.
[0032] The capacity retention rate is calculated similarly for battery cell B, and is found to be 0.925 at the end point of section A and 0.805 at the end point of section B (when the vehicle is stopped).
[0033] Next, the capacity maintenance rate at the next start is calculated. Using the above-mentioned formula for calculating the capacity maintenance rate, the capacity maintenance rate for battery cell A is calculated to be 0.71, and the capacity maintenance rate for battery cell B is calculated to be 0.685. These are the calculated and estimated capacity maintenance rates for cell A and cell B at the next start, and the difference between the capacity maintenance rates is 0.025. Here, if the threshold value is set to 0.02, for example, this will exceed the threshold value.
[0034] Returning to the flow chart, in step S3, it is determined whether the predicted stop time is equal to or greater than a threshold. The threshold may be, for example, 8 hours. If the predicted stop time is not equal to or greater than the threshold, the control is terminated. If the predicted stop time is equal to or greater than the threshold, it is determined whether the maximum deviation of the capacity maintenance rate at the next start is equal to or greater than a threshold (step S4).
[0035] If the maximum deviation is not greater than the threshold, the SOC of each cell is adjusted in step S10 (SOC adjustment process). The SOC adjustment here, so-called cell balancing, is a control to make the SOC of all cells the same, as shown in FIG. 7(b).
[0036] Here, a typical cell balancing device (passive type) for cell balancing has resistors connected in parallel to each battery cell via switches, and by turning on the switches, the energy of the battery cell is converted into heat by the resistor, making it possible to lower the SOC of the battery cell. Of course, cell balancing can also be performed using an active cell balancing method.
[0037] In step 4, if the maximum deviation of the estimated capacity maintenance rate at the next start is equal to or greater than the threshold value, in step S5, "the SOC of each cell is calculated from the corresponding values of temperature and SOC, the predicted stop time, and the deterioration coefficient so that the maximum deviation of the capacity maintenance rate at the next start falls within the threshold value" (SOC adjustment value setting process).
[0038] 3 and 4, the calculation method of the SOC (SOC adjustment value) in the SOC adjustment value setting process will be described. In the previous explanation, it was explained that the estimated capacity maintenance rate at the next start is 0.71 for battery cell A and 0.685 for battery cell B, both of which exceed the threshold value of 0.02. Below, the target SOC (SOC adjustment value) for battery cell B to make this deviation 0.02 will be explained.
[0039] Since the estimated capacity retention rate at the next start-up of battery cell A is 0.71, the estimated capacity retention rate of battery cell B must be set to 0.69. Using the capacity retention rate calculation formula, the deterioration coefficient of battery cell B to achieve this is found to be 0.575. From the deterioration coefficient table in Figure 6, the SOC adjustment value to set the deterioration coefficient to 0.575 is found to be 0.75 (75%) by linear interpolation of the row for battery temperature 0°C.
[0040] After setting the target SOC for cell B (SOC adjustment value), the SOC of cell B is adjusted (cell balancing) to the set SOC adjustment value while the electric vehicle is stopped (SOC adjustment process). In this cell balancing, as shown in FIG. 7(a), each cell is individually adjusted to its target SOC. Note that the values shown are examples. Generally, cell balancing at this stage is controlled so that the SOC of cells with high SOC is lowered and the SOC of cells with low SOC is raised.
[0041] Next, the process waits for the predicted stop time to elapse (step S7). Then, the maximum deviation (difference between the maximum and minimum values) of the current capacity maintenance rate is calculated from the corresponding values of temperature and SOC, the actual elapsed stop time, and the deterioration coefficient (step S8). If the values used for the calculation estimation (temperature, deterioration coefficient, predicted stop time, etc.) are the same as the actual temperature, deterioration coefficient, elapsed stop time, etc., between the time the vehicle is stopped and the time it is next started, the maximum deviation falls within the threshold value.
[0042] Next, it is determined whether the maximum deviation of the capacity maintenance rate at the current time (next start) is equal to or less than a threshold value (step S9). If it is not equal to or less than the threshold value (if it is outside the threshold value), the flow ends. If it is equal to or less than the threshold value (within the threshold value), the SOC of each cell is adjusted (cell balancing) in step 10. This cell balancing is a control that makes the SOC of each cell the same, as shown in FIG. 7(b). In this way, the capacity of the lithium-ion battery can be increased, and the driving distance of the electric vehicle can be extended.
[0043] According to the cell balancing method and system of the present embodiment, while the electric vehicle is stopped, the SOC of the cells is adjusted so that the maximum deviation (difference between the maximum and minimum values) of the capacity maintenance rate of the cells at the next start falls within a predetermined range. Therefore, since the accelerated deterioration of each cell constituting the lithium-ion battery is appropriately suppressed, the performance of the lithium-ion battery is maintained and the electric vehicle can be used comfortably at all times.
[0044] The present invention is not limited to the above embodiment, and various modifications are possible without departing from the spirit of the present invention. For example, in FIG. 4, the capacity maintenance rate when the vehicle is stopped is calculated using the past history of two sections (section A and section B), but this is also an example and is not limited to this. In addition, the threshold value of the difference in the capacity maintenance rate is set to 0.02, but this is also an example and may be an exception. [Explanation of symbols]
[0045] 10 Cell Balancing System 12 Lithium Ion Battery 14 Temperature determination device 16 SOC determination device 18 SOC adjustment device 20 Information processing device 22 Program Section 24 Memory section 22-1 Predicted stop time setting unit 22-2 Capacity maintenance rate calculation estimation section 22-3 SOC adjustment value setting section 24-1 Stop Time Statistics 24-2 Degradation coefficient map 24-3 Temperature and SOC Correspondence Table SOC charging rate F1~F4, F, F initial Capacity retention rate T1~T4, T stop time (year (1 / 2) ) α Deterioration coefficient
Claims
1. A cell balancing method for adjusting the SOC of a lithium ion battery composed of a plurality of cells mounted on an electric vehicle as a power source thereof, comprising: a predicted stop time setting step of predicting a stop time from when the electric vehicle is stopped until the next start based on statistics of the stop time collected over a predetermined period of time; a capacity maintenance rate calculation and estimation step of calculating and estimating a capacity maintenance rate of each cell of the lithium ion battery at the next start time after the predicted predicted stop time has elapsed, based on a measured temperature or estimated temperature, an SOC, and a deterioration coefficient of each cell while the vehicle is stopped; an SOC adjustment value setting step of setting an SOC adjustment value of one or both of the maximum and minimum values of the cell when a difference between the estimated maximum and minimum values of the capacity maintenance ratio is equal to or greater than a predetermined value so that the difference between the maximum and minimum values of the capacity maintenance ratio is within the predetermined value; an SOC adjustment process for adjusting, while the electric vehicle is stopped, one or both of the maximum and minimum SOC values to the SOC adjustment value set in the SOC adjustment value setting process by an SOC adjustment device capable of increasing or decreasing the SOC of the cell; 13. A cell balancing method comprising:
2. 2. The cell balancing method according to claim 1, wherein the predicted stopping time of the electric vehicle in the predicted stopping time setting process is predicted based on actual measured stopping times for each day of the week collected in advance over a predetermined period of time.
3. The capacity maintenance rate F of each cell calculated and estimated in the capacity maintenance rate calculation and estimation step is an SOC corresponding to the temperature of each cell while the vehicle is parked; A deterioration coefficient α corresponding to the temperature and SOC of each cell while the vehicle is parked; The stop time T (year (1 / 2) ) and, based on F=F initial -α・T (Finitial: Capacity maintenance rate when the vehicle is stopped) 3. The cell balancing method according to claim 1, wherein the cell balancing time is calculated by:
4. A temperature determination device that measures or estimates the temperature of a cell of a lithium ion battery that is configured with a plurality of cells and is mounted on an electric vehicle as a power source thereof; An SOC determination device that measures or estimates the SOC of the cell; An SOC adjusting device capable of adjusting the SOC of the cell to increase or decrease; An information processing device that transmits and receives data to and from the temperature determination device, the SOC determination device, and the SOC adjustment device, The information processing device includes: a predicted vehicle stop time setting unit that predicts a vehicle stop time until the next start based on statistics of the vehicle stop time collected over a predetermined period of time; a capacity maintenance rate calculation / estimation unit that calculates and estimates a capacity maintenance rate of each cell of the lithium ion battery at the next start time after the predicted vehicle stop time has elapsed, based on the temperature or estimated temperature of each cell during the stop time determined by the temperature determination device, the SOC determined by the SOC determination device, and a deterioration coefficient; an SOC adjustment value setting unit that, when a difference between the estimated maximum value and the minimum value of the capacity maintenance ratio is equal to or greater than a predetermined value, sets an SOC adjustment value of one or both of the maximum value and the minimum value of the cell so that the difference between the maximum value and the minimum value of the capacity maintenance ratio is within the predetermined value; The cell balancing system according to claim 1, wherein the SOC adjustment device adjusts the SOC of one or both of the maximum and minimum cells based on the SOC adjustment value from the SOC adjustment value setting unit.