How to detect remaining battery capacity

Abstract

To greatly shorten a stopping time of charge and discharge of a battery, and to detect a residual amount of battery quickly.SOLUTION: A method of detecting a residual amount of a battery is a detection method that detects an open battery of the battery, and identifies a residual amount (%) from the detected open voltage. The detection method includes: a voltage detection step of detecting a plurality of open voltages at prescribed time intervals after stopping the charge / discharge of the battery; a function setting step of computing a prediction function in which the open voltage drops from the plurality of detected open voltages in the voltage detection step; a computation step of computing a prediction open voltage in a convergence timing where the open voltage of the battery drops and converges on the basis of the prediction function to be identified in the function setting step; and a residual amount prediction step of identifying a prediction residual amount (%) from the computed prediction open voltage in the computation step.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a method for temporarily stopping the charge and discharge of a battery, detecting an open-circuit voltage, predicting a remaining capacity (SOC) from the open-circuit voltage, and detecting the remaining capacity of the battery.

Background Art

[0002] After the battery is fully charged, as the remaining capacity (SOC) decreases during discharge, the open-circuit voltage decreases. Also, after discharge, as the remaining capacity (SOC) increases during charging, the open-circuit voltage gradually increases. Therefore, the remaining capacity (SOC) can be specified from the open-circuit voltage. Utilizing this physical property of the battery, a method of detecting the open-circuit voltage and specifying the remaining capacity from the open-circuit voltage has been conventionally adopted as a method for detecting the remaining capacity of the battery. (See Patent Document 1)

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] A method for detecting the remaining capacity (SOC) of a battery by stopping charging and discharging, detecting the open-circuit voltage, and predicting the remaining capacity from the open-circuit voltage required a long time between stopping charging and discharging and detecting the open-circuit voltage, for example, one hour after stopping charging and discharging, in order to accurately predict the remaining capacity from the open-circuit voltage. This is because, after stopping charging, the open-circuit voltage gradually decreases, and it takes a long time for the open-circuit voltage to decrease and converge to an open-circuit voltage at which the remaining capacity can be accurately predicted. Therefore, in order to accurately predict the remaining capacity from the open-circuit voltage, the time during which charging and discharging must be stopped must be considerably long. This resulted in the problem that it takes a long time to detect the remaining capacity of the battery from the open-circuit voltage. For example, a detection method that stops charging for one hour to detect the remaining capacity of the battery not only takes a long time to fully charge the battery, but also hinders the frequent detection and accurate display of the remaining capacity of the battery in real-world usage environments.

[0005] The present invention was further developed with the aim of overcoming the above-mentioned drawbacks, and an important objective of the present invention is to provide a method for detecting the remaining capacity of a battery that can quickly detect the remaining capacity of the battery by significantly shortening the time required to stop charging and discharging the battery. [Means for solving the problem]

[0006] A method for detecting the remaining capacity of a battery according to one embodiment of the present invention involves detecting the open-circuit voltage of the battery and determining the remaining capacity from the detected open-circuit voltage. of A detection method comprising: a voltage detection step of detecting the open-circuit voltage multiple times at predetermined time intervals after stopping the charging and discharging of the battery; a function setting step of calculating a prediction function for the decrease in the open-circuit voltage from the multiple open-circuit voltages detected in the voltage detection step; a calculation step of calculating the predicted open-circuit voltage at the convergence timing when the open-circuit voltage of the battery decreases and converges, based on the prediction function identified in the function setting step; and a remaining capacity prediction step of identifying the predicted remaining capacity (SOC) from the predicted open-circuit voltage calculated in the calculation step. In a voltage detection step that detects open-circuit voltage multiple times, before the first detection of open-circuit voltage, Pre-process asThe process includes a pre-charging step that increases the open-circuit voltage of the battery by pre-charging it. [Effects of the Invention]

[0007] The above method for detecting the remaining battery capacity has the advantage of significantly reducing the time during which the battery charging and discharging is stopped, allowing for rapid detection of the remaining battery capacity. [Brief explanation of the drawing]

[0008] [Figure 1] This graph shows the characteristic of how the remaining capacity (SOC) of a lithium-ion secondary battery changes with respect to its open-circuit voltage. [Figure 2] This graph shows an example of the characteristic of a battery's voltage dropping when charging is stopped. [Figure 3] Figure 2 is a magnified view of a portion of the graph shown. [Figure 4] This figure shows an example of correcting the full charge capacity (FCC) of a battery whose remaining capacity (SOC) has been determined from its open-circuit voltage. [Figure 5] This figure shows another example of correcting the full charge capacity (FCC) of a battery whose state of charge (SOC) is determined from its open-circuit voltage. [Modes for carrying out the invention]

[0009] A method for detecting the remaining capacity of a battery according to one embodiment of the present invention is a method for detecting the remaining capacity (%) by detecting the open-circuit voltage of a battery and identifying the remaining capacity from the detected open-circuit voltage, and includes: a voltage detection step of detecting the open-circuit voltage multiple times at predetermined time intervals after stopping the charging and discharging of the battery; a function setting step of calculating a prediction function for the decrease in the open-circuit voltage from the multiple open-circuit voltages detected in the voltage detection step; a calculation step of calculating a predicted open-circuit voltage at a convergence timing when the open-circuit voltage of the battery decreases and converges, based on the prediction function identified in the function setting step; and a remaining capacity prediction step of identifying a predicted remaining capacity (%) from the predicted open-circuit voltage calculated in the calculation step.

[0010] The above method for detecting the remaining capacity of a battery involves detecting the open-circuit voltage of the battery multiple times after charging and discharging has stopped, calculating a prediction function for voltage decrease from the detected open-circuit voltage, calculating the predicted open-circuit voltage of the battery at a convergence timing after a predetermined time has elapsed based on this prediction function, and further identifying the predicted remaining capacity (SOC) based on the characteristics of the open-circuit voltage and remaining capacity (SOC) using the calculated predicted open-circuit voltage. This significantly reduces the time required to detect the open-circuit voltage of the battery, allowing for rapid detection of the remaining capacity of the battery.

[0011] In another embodiment of the present invention, a method for detecting the remaining capacity of a battery allows the time period for detecting the open-circuit voltage in the voltage detection step to be 1 / 20 or less of the convergence timing.

[0012] The above method for detecting the remaining battery capacity has the advantage of accurately detecting the remaining battery capacity (SOC) while shortening the time period for detecting the open-circuit voltage in the voltage detection step, which is less than 1 / 20 of the convergence timing. This is because, after the voltage detection step, while charging and discharging of the battery is started, the predicted open-circuit voltage, which will be the open-circuit voltage of the battery when charging and discharging stops, is calculated, and the remaining capacity (SOC) is determined from this predicted open-circuit voltage.

[0013] In another embodiment of the present invention, the method for detecting the remaining capacity of a battery allows the convergence timing to be within the range of 1 hour ± 30 minutes.

[0014] The above method for detecting the remaining capacity of a battery involves detecting the open-circuit voltage in the voltage detection step, and then, while starting the charging and discharging of the battery, predicting the open-circuit voltage after a time range of 1 hour ± 30 minutes from the time of detection to determine the remaining capacity (SOC). This method has the advantage of accurately determining the remaining capacity (SOC) from the predicted open-circuit voltage without stopping the charging and discharging of the battery for an extended period of time.

[0015] For the method of detecting the remaining capacity of a battery according to another embodiment of the present invention, in the voltage detection step, the open-circuit voltage can be measured three or more times at a predetermined time interval.

[0016] For the method of detecting the remaining capacity of a battery according to another embodiment of the present invention, the prediction function can be the following power function with time (x) as a variable. Open-circuit voltage (OCV) = ax b However, x is time, and a and b are approximation curve coefficients

[0017] The above method of detecting the remaining capacity of the battery has the feature that by setting the prediction function of the open-circuit voltage decreasing when the charging and discharging of the battery stops as a power function that converges to a predetermined voltage, it is possible to accurately predict the remaining capacity (SOC) of the battery by lengthening the convergence timing without stopping the charging and discharging of the battery for a long time.

[0018] For the method of detecting the remaining capacity of a battery according to another embodiment of the present invention, the battery can be a non-aqueous electrolyte secondary battery.

[0019] For the method of detecting the remaining capacity of a battery according to another embodiment of the present invention, after measuring the open-circuit voltage of the battery in the voltage detection step, the battery is charged to the charge stop voltage, and the charge capacity (Ah) from the start of charging to the charge stop voltage is detected. A full charge capacity correction step of correcting the full charge capacity (FCC) of the battery from the charge capacity (Ah) and the predicted remaining capacity (SOC) can be included.

[0020] For the method of detecting the remaining capacity of a battery according to another embodiment of the present invention, the charge stop voltage in the full charge capacity correction step can be set to a voltage of 80% to 100% of the full charge voltage with the remaining capacity (SOC) being 100%.

[0021] A method for detecting the remaining capacity of a battery according to another embodiment of the present invention may include a voltage detection step in which the open-circuit voltage of the battery is measured, the battery is discharged to a discharge stop voltage, the discharge capacity (Ah) from the start of discharge to the discharge stop voltage is detected, and a full charge capacity correction step in which the full charge capacity (FCC) of the battery is corrected from the discharge capacity (Ah) and the predicted remaining capacity (SOC).

[0022] In another embodiment of the present invention, the method for detecting the remaining capacity of a battery allows the discharge stop voltage in the full charge capacity correction step to be a voltage of 0% to 20% of the minimum voltage at which the remaining capacity (SOC) becomes 0.

[0023] The present invention will be described in detail below with reference to the drawings. In the following description, terms indicating specific directions or positions (for example, "up," "down," and other terms including these) will be used as needed. The use of these terms is for the purpose of facilitating the understanding of the invention with reference to the drawings, and the meaning of these terms does not limit the technical scope of the present invention. Furthermore, the embodiments described below illustrate specific examples of the technical concept of the present invention and do not limit the present invention to those described below. Also, the dimensions, materials, shapes, relative arrangements, etc., of the components described below are intended to be illustrative, and not to limit the scope of the present invention unless otherwise specified. Moreover, the content described in one embodiment or example is applicable to other embodiments and examples. Additionally, the size and positional relationships of the members shown in the drawings may be exaggerated for clarity.

[0024] (Embodiment 1) Batteries can be conveniently used by displaying their remaining usable time by detecting their state of charge (SOC). For example, in a laptop computer environment, accurately displaying the battery usage time is extremely important to prevent the problem of the battery discharging and becoming unusable during work. The remaining capacity of a battery can be detected by integrating the charge and discharge currents, but this method of calculating the remaining capacity (SOC) accumulates errors, so the error gradually increases. Non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries have the advantage of being able to determine the remaining capacity by stopping the charge and discharge and detecting the open-circuit voltage, allowing for repeated detection and accurate display of the remaining capacity.

[0025] Figure 1 shows the characteristic of how the state of charge (SOC) of a lithium-ion secondary battery changes with respect to its open-circuit voltage. In a battery with this electrical characteristic, the open-circuit voltage gradually decreases as it discharges from a fully charged state with a SOC of 100% to a state of 0%. The "SOC-remaining capacity" characteristic shown in this figure is not affected by the degree of battery degradation, so the SOC of a degraded battery can also be determined from its open-circuit voltage. However, the open-circuit voltage used to determine the SOC must be detected after the battery charging and discharging has stopped and the voltage change has converged to a constant value, for example, after 1 hour. In this specification, the time it takes for the battery's open-circuit voltage to decrease and converge to a constant value is referred to as the convergence timing.

[0026] Figure 2 shows an example of the characteristic of a battery voltage dropping when charging is stopped. As shown in this figure, the open-circuit voltage of the battery gradually decreases from the moment charging is stopped, and converges to a constant voltage after about 1 hour. Therefore, the remaining capacity (SOC) can be determined from the open-circuit voltage 1 hour after charging is stopped. This method can accurately detect the remaining capacity (SOC) without being affected by the accumulation of errors or the degree of degradation, but it requires stopping charging for 1 hour to detect the remaining capacity.

[0027] The following method for detecting the remaining battery capacity involves a pre-charge step, a voltage detection step, a function setting step, a calculation step, and a remaining capacity prediction step to detect the remaining battery capacity in a short time.

[0028] (Pre-charging step) The method for determining the remaining capacity (SOC) from the open-circuit voltage involves pre-charging the battery in a step prior to detecting the open-circuit voltage. This increases the open-circuit voltage of the battery, thereby increasing the change in the open-circuit voltage during the voltage detection step and allowing for more accurate identification of the prediction function. The time-enlarged diagram in Figure 3 shows the pre-charging step set to 10 seconds, but the pre-charging step can be 5 seconds or more and 30 seconds or less. However, the method for detecting the remaining capacity of a battery according to the present invention does not necessarily require a pre-charging step; the open-circuit voltage of the battery can also be detected in the following voltage detection step without pre-charging the battery.

[0029] (Voltage detection step) This step involves detecting the open-circuit voltage multiple times at predetermined time intervals after stopping the charging and discharging of the battery. Figure 3 shows the state in this step where the open-circuit voltage of the battery is detected a total of seven times, one second after stopping charging and six times at 10-second intervals after stopping charging, for a total time period of one minute. The voltage detection step can improve the accuracy of remaining capacity measurement by making the time period for detecting the open-circuit voltage longer than one minute, but this increases the stopping time of charging and discharging, so it should be set to, for example, three minutes or less, preferably two minutes or less, and less than 1 / 20 of the convergence timing. Furthermore, the accuracy of remaining capacity can also be improved by increasing the number of open-circuit voltage detections, so it is preferably three times or more, more preferably five times or more.

[0030] (Function setting step) This step calculates a prediction function for the convergence of the open-circuit voltage from multiple open-circuit voltages detected in the voltage detection step. The prediction function is a power function that shows how the open-circuit voltage converges to a constant voltage over time, and it calculates the predicted open-circuit voltage of the battery after the convergence timing. By identifying coefficients from multiple open-circuit voltages and time, the power function can calculate the open-circuit voltage after a predetermined time has elapsed with high accuracy.

[0031] The exponential function, using the following equation with time (x) as the variable, can accurately predict the open-circuit voltage after a predetermined time has elapsed. Open-circuit voltage (OCV [mV]) = ax b However, x is time [s], and a and b are the coefficients of the curve of approximation.

[0032] In the voltage detection step, the power function that changes with the voltage values ​​shown in Figures 2 and 3 calculates the approximate curve coefficients a and b from the following open-circuit voltages detected 1 second after charging is stopped and six times at 10-second intervals from the time charging is stopped. The open-circuit voltage decreases as shown in the graphs in Figures 2 and 3, and the open-circuit voltage after 1 to 60 seconds is as follows: 1 second later (x=1)...3887mV After 10 seconds (x=10)…3883mV After 20 seconds (x=20)…3879mV After 30 seconds (x=30)…3876mV After 40 seconds (x=40)…3874mV After 50 seconds (x=50)…3873mV After 60 seconds (x=60)…3871mV

[0033] By calculating the power functions a and b from the above open-circuit voltages, we can identify the approximate curve for the open-circuit voltage. a=3888.8 b = -0.00099.

[0034] The power function that decreases with open-circuit voltage in Figures 2 and 3 becomes an approximate curve of the open-circuit voltage with the above coefficients a and b. However, the decrease in the open-circuit voltage of the battery in the voltage detection step varies depending on the detected open-circuit voltage, so a and b are calculated from the open-circuit voltage detected in the voltage detection step to determine the approximate curve coefficients. The calculation to determine a and b of the power function can be performed quickly by using a high-speed calculation unit built into the load that is powered by the battery. For example, in the case of a battery pack built into a laptop computer as a power source, the approximate curve coefficients a and b can be quickly determined by performing the calculation on the laptop computer side without having to perform the calculation in the calculation unit inside the battery pack.

[0035] (Calculation step) This step calculates the predicted open-circuit voltage of the battery at the convergence timing based on the power function for which the approximation curve coefficients have been identified. If the convergence timing is set to 1 hour later, i.e., after 3600 seconds (x=3600), then, The predicted open-circuit voltage is 3857.34mV.

[0036] When the open-circuit voltage of the above battery was measured one hour after charging was stopped, the open-circuit voltage was 3857mV. The error from the predicted open-circuit voltage was extremely small, at 0.34mV (0.01%), and is practically negligible.

[0037] The above steps calculate the predicted open-circuit voltage assuming a convergence timing of 1 hour (3600 seconds) has elapsed. However, the present invention does not specify a convergence timing of 1 hour. For example, the predicted open-circuit voltage can be calculated within a range of 1 hour ± 30 minutes, or even further within a range of 1 hour ± 1 hour.

[0038] (Remaining capacity prediction step) This step identifies the predicted remaining capacity (SOC) [%] from the predicted open-circuit voltage calculated in the calculation step. Since the remaining capacity (SOC) for a given open-circuit voltage is determined by the battery itself, the remaining capacity (SOC) for a given open-circuit voltage is stored in memory or a lookup table beforehand, and the remaining capacity (SOC) is identified from the open-circuit voltage.

[0039] (Full charge capacity correction step) Batteries can accurately calculate the time they can supply power to a load by correcting for both their state of charge (SOC) and full charge capacity (FCC). This is because even with the same SOC, a battery that has degraded and whose FCC has decreased from 10Ah to 8Ah will only be able to supply power to the same load for 80% less time. Batteries that have been corrected for their FCC in this step have the advantage of being able to accurately calculate the time they can supply power to a load, i.e., the total power (Ah), at the moment the SOC is detected.

[0040] This step involves detecting the open-circuit voltage of the battery in the voltage detection step, then charging or discharging the battery to correct its full charge capacity (FCC). After stopping this charging and discharging, the open-circuit voltage of the battery is measured in the voltage detection step, the battery is charged until it reaches the charging stop voltage, the charge capacity (Ah) from the start of charging to the stop of charging is accumulated, and the full charge capacity (FCC) of the battery is corrected from the accumulated charge capacity (Ah) and the predicted remaining capacity (SOC).

[0041] Figure 4 shows the charging capacity (Ah) required to charge a battery with a remaining capacity (SOC) of 30%, as determined by its open-circuit voltage, up to the full charge voltage, which is the charging stop voltage. A battery with a remaining capacity (SOC) of 30% is fully charged by increasing its SOC by 70%. Therefore, a battery with a charging capacity (Ah) of 7Ah to increase its SOC by 70% will have a full charge capacity (FCC) of 10Ah.

[0042] The above method can be corrected by calculating the full charge capacity (FCC) using the following formula. Full charge capacity (FCC) = Charge capacity (Ah) × 100 / [100 - Remaining capacity (SOC)]

[0043] Furthermore, this step can also involve stopping the charging and discharging of the battery, measuring the open-circuit voltage of the battery in the voltage detection step, discharging the battery until it reaches a discharge stop voltage that reduces the remaining capacity (SOC) to 0%, accumulating the discharge capacity (Ah) from the start to the stop of discharge, and correcting the battery's full charge capacity (FCC) from the accumulated discharge capacity (Ah) and the predicted remaining capacity (SOC).

[0044] In Figure 4, when a battery with a state of charge (SOC) of 30%, determined from its open-circuit voltage, is discharged to a charging stop voltage that reduces the SOC to 0%, its discharge capacity (Ah) becomes 3Ah. In other words, a battery with a SOC of 30% is discharged to a SOC of 0% by 30%, so the charging capacity (Ah) required to reduce the SOC by 30% is 3Ah, and the full charge capacity (FCC) is 10Ah.

[0045] The above method can be corrected by calculating the full charge capacity (FCC) using the following formula. Full charge capacity (FCC) = Discharge capacity (Ah) × 100 / State of Charge (SOC)

[0046] The above method for detecting the remaining capacity of a battery corrects the full charge capacity (FCC) by setting the charging stop voltage of the battery being charged to the full charge voltage at which the remaining capacity (SOC) reaches 100%, and the minimum voltage at which the remaining capacity (SOC) reaches 0% and discharge stops for the battery being discharged. However, the method of calculating the full charge capacity (FCC) of a battery by charging can correct the full charge capacity (FCC) by setting the charging stop voltage to a voltage at which the remaining capacity (SOC) is between 80% and 100%, and the method of calculating the full charge capacity (FCC) by discharging the battery can correct the full charge capacity (FCC) by setting the discharge stop voltage at which the discharge of the battery is stopped to a voltage at which the remaining capacity (SOC) is between 0% and 20%.

[0047] Figure 5 shows a method for correcting the full charge capacity (FCC) by charging a battery whose state of charge (SOC) detected from the open-circuit voltage is 40%, stopping the charge at a voltage equivalent to 80% of the SOC, and then discharging the battery and stopping the discharge at a voltage equivalent to 20% of the SOC to correct the full charge capacity (FCC). This battery, when charged, has an integrated charge capacity (Ah) of 4Ah, which increases the SOC by 40%. Therefore, the full charge capacity (FCC), which increases the SOC by 100%, is 10Ah. Similarly, this battery, when discharged, has an integrated discharge capacity (Ah) of 2Ah, which decreases the SOC by 20%. Therefore, the full charge capacity (FCC), which decreases the SOC by 100%, is 10Ah. [Industrial applicability]

[0048] The present invention relates to a battery used in various electronic devices such as laptop computers, and is preferably employed as a method for detecting the remaining capacity of a battery to display its usable time.

Claims

1. A method for detecting remaining battery capacity, which involves detecting the open-circuit voltage of a battery and determining the remaining capacity from the detected open-circuit voltage, A voltage detection step in which the open-circuit voltage is detected multiple times at predetermined time intervals after the charging and discharging of the battery has been stopped, A function setting step which calculates a prediction function for the decrease in open-circuit voltage from multiple open-circuit voltages detected in the voltage detection step, A calculation step that calculates the predicted open-circuit voltage at the convergence timing when the open-circuit voltage of the battery decreases and converges, based on the prediction function identified in the function setting step, A remaining capacity prediction step that identifies the predicted remaining capacity (SOC) from the predicted open-circuit voltage calculated in the calculation step, A method for detecting the remaining capacity of a battery, comprising a pre-charging step in which the battery is pre-charged as a prior step to increase the open-circuit voltage of the battery before the first detection of the open-circuit voltage in the voltage detection step of detecting the open-circuit voltage multiple times.

2. A method for detecting the remaining capacity of a battery according to claim 1, A method for detecting the remaining capacity of a battery, wherein the time period for detecting the open-circuit voltage in the voltage detection step is 1 / 20 or less of the convergence timing.

3. A method for detecting the remaining capacity of a battery according to claim 1 or 2, A method for detecting the remaining battery capacity such that the convergence timing is within the range of 1 hour ± 30 minutes.

4. A method for detecting the remaining capacity of a battery according to any one of claims 1 to 3, A method for detecting the remaining capacity of a battery by measuring the open-circuit voltage three or more times at predetermined time intervals in the voltage detection step.

5. A method for detecting the remaining capacity of a battery according to any one of claims 1 to 4, A method for detecting the remaining capacity of a battery, wherein the prediction function is the following power function with time (x) as the variable. Open-circuit voltage (OCV) = ax ｂ However, x is time, and a and b are the coefficients of the curve of approximation.

6. A method for detecting the remaining capacity of a battery according to any one of claims 1 to 5, A method for detecting the remaining capacity of a battery, which is a non-aqueous electrolyte secondary battery.

7. A method for detecting the remaining capacity of a battery according to any one of claims 1 to 6, In the voltage detection step, after measuring the open-circuit voltage of the battery, the battery is charged to the charging stop voltage, and the charging capacity (Ah) from the start of charging to the charging stop voltage is detected. A method for detecting the remaining capacity of a battery, including a full charge capacity correction step of correcting the full charge capacity (FCC) of the battery from the charging capacity (Ah) and the predicted remaining capacity (SOC).

8. A method for detecting the remaining capacity of a battery according to claim 7, A method for detecting the remaining capacity of a battery, wherein the charging stop voltage in the full charge capacity correction step is a voltage that is 80% to 100% of the full charge voltage, which represents a remaining capacity (SOC) of 100%.

9. A method for detecting the remaining capacity of a battery according to any one of claims 1 to 6, In the voltage detection step, after measuring the open-circuit voltage of the battery, the battery is discharged to the discharge stop voltage, and the discharge capacity (Ah) from the start of discharge to the discharge stop voltage is detected. A method for detecting the remaining capacity of a battery, including a full charge capacity correction step of correcting the full charge capacity (FCC) of the battery from the discharge capacity (Ah) and the predicted remaining capacity (SOC).

10. A method for detecting the remaining capacity of a battery according to claim 9, A method for detecting the remaining capacity of a battery, wherein the discharge stop voltage in the full charge capacity correction step is a voltage that is 0% to 20% of the minimum voltage at which the remaining capacity (SOC) becomes 0.

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