KR102013306B1 - Method for predicting life time of battery - Google Patents

Abstract

The present invention provides a method for predicting the life of a battery, comprising: measuring an open circuit voltage (OCV) at a state of charge (SOC) within an operating voltage range of a fresh battery; Dividing the measured open circuit voltage according to the SOC section; Measuring battery life within a divided SOC interval voltage range; Adjusting a voltage to a partition including an OCV of the battery to be predicted; And calculating a state of health (SOH) of the battery by measuring the battery capacity in the divided voltage range.

Description

Method for predicting life time of battery}

The present invention relates to a battery life prediction method.

In general, the state of health (SOH) of a battery refers to a figure of merit expressed by comparing an ideal state of a battery with a current battery state. The unit is expressed as a percentage (%).

If the SOH is 100%, the current state of the battery meets the exact specifications and specifications of the initial battery.Theoretically, the SOH is 100% at the time of battery manufacture, and the SOH decreases with increasing use time or frequency. In practice, since various errors may occur in the manufacturing process, the SOH of all batteries at the time of manufacturing does not satisfy 100%.

The SOH of such a battery determines the current SOH by measuring the SOH of the initial battery state and storing the value, and then comparing the stored SOH value with the stored values based on the values that change as the battery is used. Information such as battery internal resistance, impedance, conductance, capacity, voltage, self discharge, charging performance, and number of charge / discharge cycles is considered.

In the conventional battery life prediction technology, the cycle life was measured in the entire voltage range, and thus there was a problem in that the execution time was long in the life prediction.

An object of the present invention is to provide a battery life prediction method that can reduce the execution time.

The present invention provides a first step of measuring an open circuit voltage (OCV) in a state of charge (SOC) within an operating voltage range of a fresh battery to achieve the above object; A second step of dividing the OCV measured in the first step into a plurality of divided regions according to a plurality of SOC sections; A third step of measuring battery life within a start and end close circuit voltage (CCV) range of each SOC section divided in a second step; A fourth step of adjusting a voltage to a starting point in the SOC section of a second step including measuring the OCV of the battery for which the life expectancy is measured; A battery life prediction method comprising a fifth step of calculating a state of health (SOH) of a battery by measuring a capacity of a target battery in a corresponding region voltage range of a fourth step.

In the first step of the present invention, the step of measuring the open circuit voltage of the SOC is calculated by using a current titration method, and includes repeatedly measuring the voltage when the balance is reached by applying a current for a predetermined time and then stopping the current supply. can do.

In the first step of the present invention, the applied current may include a value corresponding to a current of 1/20 (0.05 C) to 3 times (3 C) of the commercial capacity.

In the first step of the present invention, the step of reaching equilibrium may include a step in which the voltage change amount corresponds to 1 to 10% after the current supply is stopped.

In the second step of the present invention, the partition may include 2 to 20 sections according to the SOC.

Measuring the battery life (deterioration) within the SOC section voltage range divided in the third step of the present invention may include the step of measuring the capacity, energy density, output density, impedance, DC resistance using constant current charge and discharge Can be.

Measuring the battery life within the SOC section voltage range divided in the third step of the present invention is accelerated life at room temperature (20 ℃ to 30 ℃) to high temperature environment (45 ℃ to 60 ℃) to shorten the time of measurement It may include the step comprising a measurement.

In the fourth step of the present invention, moving the voltage of the target battery to the starting point of the partition region may include applying a current to the minimum voltage of the partition region including the measured OCV of the target battery.

In the fifth step of the present invention, calculating the SOH of the battery may include a process of data processing by a correction equation having a correlation between the capacity in the divided voltage range and the capacity in the full region voltage range. have.

After measuring the capacity of the partition in the fifth step of the present invention, after calculating the X value from the equation (2), after calculating the capacity of the remaining partition using the X value, the sum according to equation (1) After confirming the total capacity through, it is possible to calculate the SOH of the target battery according to the equation (3).

[Equation 1]

[Equation 2]

[Equation 3]

In Formula 1, C _total is total capacity; C _i is the capacity of the i-th division section; n is the number of divisions; In Equation 2, X is the number of cycles; a _i , b _i , c _i , d _i e _i , and f _i are constants independently determined, and in Equation 3, C1 is the discharge capacity of the first cycle.

According to the present invention, the cycle capacity can be inferred even in some regions without measuring the cycle life in the entire voltage region. Although the method according to the present invention is a method of direct measurement, it is a method of measuring a part rather than the whole and inferring the total value from the result, and the value obtained through the prediction and the actual capacity show a high agreement rate. Since only a part can be measured to check the life of the whole, it can help to shorten the execution time in predicting the life of the battery.

1 is a current titration graph of a cell according to an embodiment of the present invention.
2 is a correlation graph of OCV and actual voltage of a cell according to an embodiment of the present invention.
3 is a table showing intervals and interval ranges divided according to SOC according to an embodiment of the present invention.
4 is a life graph in the interval range divided by the SOC according to an embodiment of the present invention.
5 is a life graph in the full range according to an embodiment of the present invention.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The battery life prediction method according to the present invention may include the following steps.

The first step includes: measuring an open circuit voltage (OCV) in a state of charge (SOC) within an operating voltage range of a fresh battery;

Second step: OCV measured in the first step is a plurality of SOC intervals (for example, five SOC intervals; specifically SOC 0-20% interval, SOC 21-40% interval, SOC 41-60% interval, SOC Partitioning into a plurality of partitions according to a 61-80% section, and an SOC 81-100% section;

Step 3: measuring battery life within a start and end close circuit voltage (CCV) range of each SOC section divided in a second step;

Step 4: After measuring the OCV of the life prediction target battery, adjusting the voltage by constant current charging or discharging to the starting point in the SOC section of the second step including the measured OCV of the target battery; In this case, it is not full charge or full discharge, so changing the voltage to the starting point of the divided section is possible in a shorter time than moving to the starting point of the whole section. If the target OCV is 3.56 V, measuring the capacity by reducing the range after discharge to 3.48 V can shorten the time rather than measuring the capacity by discharging up to 3.0 V.)

Step 5: calculating the state of health (SOH) of the battery by measuring the capacity of the target battery in the voltage range of the corresponding partition of the fourth step (Life measurement in step 3 is measured for the fresh cell This step is to measure the capacity of the battery to check the SOH).

The first to third steps are for collecting information on the fresh cell, and the fourth to fifth steps are for calculating the SOH for the battery in use.

In the first step, the open circuit voltage measurement step of the SOC is calculated by using a current titration method, and may include repeatedly measuring the voltage when reaching the equilibrium by stopping the current supply after applying a current for a predetermined time. .

In the first step, the applied current may include a value corresponding to 1/20 (0.05 C) to 3 times (3 C) current of the commercial capacity. If the value of the applied current is lower than 1/20, the time required for analysis is longer, and if it is higher than three times, there is a possibility of accelerating deterioration of the battery due to high current charge and discharge.

The step of reaching equilibrium in the first step may include a step in which the voltage change amount corresponds to 1 to 10% after the current supply is stopped. If the voltage change exceeds 10%, the equilibrium cannot be measured because it does not reach equilibrium. If the change in the voltage is less than 1%, the accurate OCV can be measured, but the efficiency may be reduced due to the long time.

In the second step, the partition may include 2 to 20 sections according to the SOC. If the segmentation period is less than 2, the analysis loses meaning, and if the segmentation section exceeds 20 sections, the segmentation section cannot be divided correctly.

Regarding the limitation to the CCV range in the third step, the reason for measuring the OCV in the first and second steps is to accurately distinguish the SOC. In the case of OCV, the voltage at equilibrium can be measured. However, since the current is applied in the process of charging and discharging the battery, the CCV corresponding to each OCV is set to a cut-off range. If the range is set by OCV, it is impossible to select the range for each cycle, and the capacity change in each section due to deterioration cannot be properly measured. In addition, due to the increased resistance due to deterioration, the CCV corresponding to each OCV is changed, so that the first divided SOC may be changed.

Measuring the battery life (deterioration) within the SOC section voltage range divided in the third step may include measuring capacity, energy density, output density, impedance, and DC resistance using constant current charge and discharge.

Measuring the battery life within the SOC section voltage range divided in the third step includes accelerated life measurement at room temperature (20 ° C. to 30 ° C.) to high temperature environment (45 ° C. to 60 ° C.) to shorten the measurement time. It may include the step.

The fourth step is a preparation step for measuring the OCV of the cell under test, moving to the start point of the CCV section defined as the OCV of the fresh cells in the first to third steps to measure the cycle capacity in the corresponding section.

In the fourth step, moving the voltage of the target battery to the start point of the partition region may include applying a current to a minimum voltage of the partition region including the measured OCV of the target battery. The divided region may mean a region obtained by dividing the OCV of the fresh battery into SOC, and may include applying a current for discharging at a minimum voltage that is a charging start voltage in a section including the OCV of the target battery.

The calculating of the SOH of the battery in the fifth step may include a process of data processing by a correction equation having a correlation between the capacity in the divided region voltage range and the capacity in the entire region voltage range. Specifically, based on the capacity actually measured in the divided area, the capacity in another divided section may be calculated as follows and the total capacity may be obtained from the sum thereof. The lifetime graph in each partition can be expressed as 5th order as follows and the constant in the interval can be found. The SOH of the target battery can be confirmed by obtaining the cycle number X from the measured capacity of the divided region, calculating the capacity of the remaining divided regions, and confirming the total capacity through the summation.

For example, the voltage is divided into five sections from the OCV of the fresh battery in advance and the lifetime in each section is measured. Measuring the lifetime over the entire voltage range takes a long time, but dividing into five sections can reduce the time to 1/5. The life graph in each measured interval can be expressed as a fifth order function.

To find the SOH of a battery that knows how much and how much it has been used, capacity must be measured and compared with the initial capacity over the entire voltage range, but this step also takes a long time. Therefore, after measuring the OCV of the target battery, measuring the capacity within the section corresponding to the voltage range of the division from the fresh battery, the X value is inversely calculated from the fifth equation, and substituted into the fifth functional equation of the other four sections. The capacity in each section is measured and summed.

[Equation 1]

[Equation 2]

[Equation 3]

In Formula 1, C _total is total capacity; C _i is the capacity of the i-th division section; n is the number of divisions; In Equation 2, X is the number of cycles; a _i , b _i , c _i , d _i e _i and f _i are constants independently defined, and in Equation 3, C1 is the first cycle capacity. C1 is the discharge capacity of the first cycle when i = 1.

The calculation process of Equations 1 to 3 may be performed through a computer program.

EXAMPLE

Hereinafter, the present invention will be described in more detail based on examples with reference to the accompanying drawings, but the present invention is not limited thereto.

In order to predict the life of the cell, the total charge / discharge range was divided into five regions based on the SOC, and the lifetime in each region was measured. In order to obtain an accurate cut-off depending on the SOC, charging and discharging were performed by a constant current titration method at 3.0 to 4.2 V as shown in FIG. 1. In general, the constant current titration method stops the current supply and applies a rest when a constant current is applied, and a rest is applied to observe the change in OCV, and then the diffusion coefficient of lithium ions using each component such as resistance and time. This method is mainly used for finding back. Compared with continuous CC (constant current) charging, by inducing a voltage in an open circuit state by applying a rest at every interval, it is possible to observe OCV, which is the voltage actually changed by charging when a constant current is applied. Since the rest and the constant current application are repeated in each section, the reversibility of the cell can be determined by observing the IR drop caused by the resistance of the cell through the section where the current is suddenly supplied or interrupted. have.

The applied current was set to a value corresponding to 0.5 C. When the capacity corresponding to 1/50 of the discharge capacity in the first cycle was accumulated, the current supply was stopped to reach OCV. When the current supply is interrupted, the increasing voltage initially decreases rapidly and then gradually decreases to reach a nearly constant voltage. The longer the rest time, the more stable OCV is reached, but in the present invention, it is set to 300 seconds.

2 is a graph showing the relationship between CCV and OCV according to the capacity from the data measured by the constant current titration method. In general, the CCV representing the operating voltage in the battery is indicated by a solid line, and the voltage when the current is stabilized and stabilized is indicated by a dotted line.

The CCV curves of the charge and discharge are caused by the resistance component when the current is applied, and tend to increase in the direction of the current sign compared to the actual voltage, which causes the intersection of the two curves to be shifted to the left. do. However, in the case of OCV, since the voltage is stabilized to the actual potential after reaching CCV, the intersection point moves to the right side than OCV. The internal resistance of the cell can be determined according to the movement of the intersection point as follows. The intersection of the charge and discharge curves at 50% of the voltage and capacity represents the ideal reversibility, and the further away from it, the more irreversible the charge and discharge, i.e., oxidation and reduction reactions. By comparing the intersections of the CCV and OCV curves, the difference can be interpreted in conjunction with the galvanostatic intermittent titration technique (GITT) curves. The internal resistance of the cell determines the size of the IR drop in the GITT experiment. The difference in OCV voltage is determined.

Considering SOC and DOD (Depth of discharge) as shown in FIG. 3, 1 section SOC 0-20, 2 sections SOC 21-40, 3 sections SOC 41-60, 4 sections SOC 61-80, 5 sections SOC 81-100 The cut-off voltage range condition was selected by supplementing the deviation caused by five cells in each section under the same condition. In the 1 ~ 4 section, only CC charging was performed, and in the 5 section, CC / CV (constant voltage) charging was performed in parallel, and in the CV charging section, it was done in one step without giving cut-off condition unlike CC charging. Set to lose.

The cycle life results at the cut-offs divided by SOC sections are shown in FIG. 4, and in some regions, portions that deviate from a predetermined pattern exist and are shown after a simple smoothing process. In SOC 0-20, the reduction rate of discharge capacity was the lowest, and it was high after 97 cycles, at 97.09% of the first capacity. In the SOC 21-40 interval, it has a relatively steep slope after 100 cycles, followed by a gentle slope, and then rapidly decreases again after 300 cycles. On the other hand, SOC 41-60 showed four inflection points. It rapidly decreased to 50 cycles and then increased again, and then rapidly decreased again after 200 cycles to fall to 87% of the initial discharge capacity. In SOC 61-80, it was found that it rapidly decreased to 150 cycles and maintained 95% discharge capacity.In the last SOC 81-100 period, the rate of decrease was slightly increased around 200 cycles after maintaining a gentle slope at first. Decreases to a level near linear. These results indicate that the capacity change in SOC 41-60 is most attributable to the total capacity.

FIG. 5 shows that the above results are consistent with the cycle life results under RPT (Reference Performance Test) conditions. Although the cycle life graph in each SOC region differs from the results in the entire interval, the trend in the discharge capacity reduction in each interval is shown as cumulative capacity, which shows that the tendency is consistent. It can be seen that the cycle life under the RPT condition decreases rapidly to 30 cycles, then the slope becomes gentle, and then decreases again after 230 cycles. In the cycle life graph of each SOC section, it can be confirmed that this generally coincides with the trend of SOC 41-60, and again, the increase is offset with the decrease rate of other regions and coincides with the trend of the RRT section. This result means that there is a possibility of inferring the full capacity even if the cycle life is not measured in the entire voltage region but measured in some cut-off regions.

After measuring the OCV of the cell tested by an arbitrary method by a third party, it was discharged with a current corresponding to 0.5 C at the closest cut-off voltage in the five SOC intervals previously divided (step 4). In this section, charging and discharging were performed at 0.5 C current to confirm the deterioration of the discharge capacity. After the discharge capacity was measured, X was inversely calculated by the C _i equation (Equation 2), and the obtained X value was substituted to obtain Ci in the remaining sections, and then the total capacity was calculated. SOH can be obtained from the equation (3), C _total The discharge capacity value after 100 cycles was calculated using a calculation formula. In order to compare the calculated discharge capacity after 100 cycles with the actual discharge capacity after 100 cycles, charging and discharging was carried out in the entire voltage range with 0.5 C current, and the discharge capacity after 100 cycles was measured and compared (step 5). Table 1 shows.

Cell OCV after blind test
(V) SOC section Discharge capacity
(mAh) SOH
(%) Estimated capacity after 100 cycles
(mAh) Actual capacity after 100 cycles
(mAh) Match rate
(%) Cell1 3.87 41-60 14.61 92.57 70.31 71.39 98.5 Cell2 4.06 61-80 14.29 88.79 65.29 63.10 96.6 Cell3 3.42 0-20 14.58 91.46 68.33 67.24 98.4

Although this method is a direct measurement, the value obtained through the prediction by measuring a part rather than the whole and inferring the total value from the result matches the actual capacity after 100 cycles compared with the value measured by the RPT method. Although the degree was somewhat lower, the coincidence rate was higher than 96.6%. Since the prediction method can measure only a part of the life of the whole, it may help to shorten the execution time in predicting the life of the battery.

Claims (10)

A first step of measuring an open circuit voltage (OCV) in a state of charge (SOC) within an operating voltage range of a fresh battery;
A second step of dividing the OCV measured in the first step into a plurality of divided regions according to a plurality of SOC sections;
A third step of measuring battery life within a start and end close circuit voltage (CCV) range of each SOC section divided in a second step;
A fourth step of adjusting a voltage to a starting point in the SOC section of a second step including measuring the OCV of the battery for which the life expectancy is measured;
Comprising a fifth step of calculating the state of health (SOH) of the battery by measuring the capacity of the target battery in the corresponding region voltage range of the fourth step,
In the first step, the open circuit voltage measurement step of the SOC is calculated by using a current titration method, and the battery life includes repeatedly measuring the voltage at equilibrium by stopping current supply after applying a current for a predetermined time. Prediction Method. delete The method of claim 1,
In the first step, the applied current includes a value corresponding to a current of 1/20 (0.05 C) to 3 times (3 C) of the commercial capacity. The method of claim 1,
The step of reaching the equilibrium in the first step comprises the step of changing the voltage after the current supply stops 1 to 10%. The method of claim 1,
In the second step, the partition region includes 2 to 20 sections according to the SOC. The method of claim 1,
Measuring the battery life (deterioration) within the SOC section voltage range divided in the third step battery life prediction comprising the step of measuring capacity, energy density, output density, impedance, DC resistance using constant current charge and discharge Way. The method of claim 1,
Measuring the battery life within the SOC section voltage range divided in the third step includes accelerated life measurement at room temperature (20 ° C. to 30 ° C.) to high temperature environment (45 ° C. to 60 ° C.) to shorten the measurement time. Battery life prediction method comprising the step of. The method of claim 1,
The shifting of the voltage of the target battery to the start point of the partition in step 4 includes applying a current to the minimum voltage of the partition including the measured OCV of the target battery. The method of claim 1,
The calculating of the SOH of the battery in the fifth step includes a data processing process by a correction equation having a correlation between the capacity in the divided voltage range and the capacity in the entire region voltage range. . The method of claim 9,
After measuring the capacity of the partition in the fifth step, the X value is inversely calculated from Equation 2, the capacity of the remaining partition is calculated using the X value, and then summed according to Equation 1. After checking the capacity of the battery life prediction method for calculating the SOH of the target cell according to Equation 3:
[Equation 1]

[Equation 2]

[Equation 3]

In Formula 1, C _total is total capacity; C _i is the capacity of the i-th division section; n is the number of divisions; In Equation 2, X is the number of cycles; a _i , b _i , c _i , d _i e _i , and f _i are constants independently determined, and in Equation 3, C1 is the discharge capacity of the first cycle.

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