How to estimate the lifespan of nickel-metal hydride batteries

Abstract

To estimate the life of a nickel hydrogen battery more accurately.SOLUTION: In a nickel hydrogen battery 1 of which the life is to be estimated, a positive electrode plate 3 and a negative electrode plate 4 which are superposed on each other through a separator 5 composed of a nonwoven fabric are contained in an outer can 2 together with a liquid electrolyte. The life of the battery is estimated from a liquid quantity of the liquid electrolyte held by the separator during a time when the battery is charged uninterruptedly, and a quantity of corrosion of the negative electrode plate.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for estimating the life of a nickel-metal hydride battery. [Background technology]

[0002] For example, nickel-metal hydride batteries, which constitute backup power sources for uninterruptible power supplies, require accurate lifespan determination to maintain the reliability of the power source. However, predicting lifespan is difficult because battery capacity and internal resistance remain stable until the end of their lifespan. Backup power sources also require periodic battery replacement to maintain functionality. Therefore, extending battery life is required to reduce the frequency of battery replacement, but the considerable time required to evaluate batteries to confirm lifespan has been a development challenge.

[0003] Since batteries utilize electrochemical reactions of chemical substances, corrosion of the electrode plates progresses as the batteries deteriorate. For example, as disclosed in Patent Document 1, batteries are evaluated under various environmental temperatures and charge / discharge conditions to measure the amount of corrosion of the negative electrode until the battery reaches the end of its life, and the battery life is estimated based on the amount of corrosion. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2006 / 013881 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when batteries are used as backup power sources in low-temperature environments, the actual lifespan may be shorter than the initial estimated lifespan if the estimation method is based solely on the amount of corrosion of the negative electrode. Therefore, to ensure the battery's reliability as a backup power source, the battery is replaced earlier than the predicted period of actual use, resulting in battery waste.

[0006] An object of the present invention is to provide a method for estimating the life of a nickel-metal hydride battery that can accurately estimate the battery life. [Means for solving the problem]

[0007] In order to achieve the above object, the method for estimating the life of a nickel-metal hydride battery of the present invention is a method for estimating the life of a nickel-metal hydride battery in which a positive electrode plate and a negative electrode plate are stacked together with a separator made of nonwoven fabric sandwiched between them and housed in an outer can together with an electrolyte, and the life is estimated from the amount of electrolyte retained by the separator and the amount of corrosion of the negative electrode plate. [Effects of the Invention]

[0008] According to the method for estimating the life of a nickel-metal hydride battery of the present invention, the life of the battery can be estimated more accurately even when the nickel-metal hydride battery is continuously charged in a low-temperature environment. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a partially cutaway view of a nickel-metal hydride battery according to an embodiment of the present invention; [Figure 2] FIG. 10 is a diagram showing the correlation between the estimated value of the separator retention capacity estimated according to the present embodiment and the actual measured value of the separator retention capacity. DETAILED DESCRIPTION OF THE INVENTION

[0010] A method for estimating the life of a nickel-metal hydride battery according to this embodiment will now be described.

[0011] A nickel-metal hydride battery (hereinafter referred to as "battery") 1 is an AA-size cylindrical battery, as shown in Fig. 1. Battery 1 contains an electrode group 6, which is made up of a positive electrode plate 3 and a negative electrode plate 4 stacked with a separator 5 interposed therebetween and wound into a spiral shape, housed in an outer can 2 having a cylindrical shape with an open top and a closed bottom, together with an alkaline electrolyte, and the top end is sealed with a sealing body 7.

[0012] The bottom wall 8 of the outer can 2 is conductive and functions as a negative electrode terminal. The sealing body 7 includes a lid plate 9 and a positive electrode terminal 10. The lid plate 9 is conductive and has a gas vent hole 11 in the center, and a rubber valve body 12 that closes the gas vent hole 11 is disposed on the outer surface of the lid plate 9. The lid plate 9 is disposed on the open end of the outer can 2 via a ring-shaped gasket 13, and the opening edge of the outer can 2 is crimped to close the opening. A positive electrode terminal 10 is attached to the lid plate 9.

[0013] The electrode group 6 is composed of strip-shaped positive electrode plates 3, negative electrode plates 4, and separators 5, which are wound in a spiral shape with the separator 5 sandwiched between the positive electrode plates 3 and negative electrode plates 4, forming a substantially cylindrical shape. That is, the positive electrode plates 3 and negative electrode plates 4 face each other with the separator 5 interposed between them, and are overlapped in the radial direction of the outer can 2.

[0014] In the exterior can 2, a positive electrode lead 14 is disposed between one end of the electrode group 6 and the cover plate 9, and each end of the positive electrode lead 14 is electrically connected to the positive electrode plate 3 and the cover plate 9, respectively.

[0015] The positive electrode plate 3 comprises a conductive positive electrode substrate having a porous structure and a positive electrode mixture held in the pores of the positive electrode substrate and on the surface of the positive electrode substrate. The positive electrode substrate may be, for example, a nickel-plated mesh-like, sponge-like, or fibrous metal body or foamed nickel.

[0016] The positive electrode mixture contains positive electrode active material particles, a conductive material, a positive electrode additive, and a binder. The positive electrode active material particles are nickel hydroxide (Ni(OH)2) particles or higher-order nickel hydroxide particles. It is preferable that at least one of zinc, magnesium, and cobalt is solid-dissolved in these nickel hydroxide particles.

[0017] The negative electrode plate 4 has a strip-shaped conductive negative electrode core, and a negative electrode mixture is supported on this negative electrode core. The negative electrode core is made of a sheet-shaped metal material with distributed through holes, such as a punched iron sheet with a nickel-plated surface. When held on the negative electrode core, the negative electrode mixture forms a negative electrode mixture layer.

[0018] The negative electrode mixture contains particles of a hydrogen storage alloy, a negative electrode additive, a conductive material, and a binder.

[0019] The hydrogen storage alloy is an alloy capable of absorbing and releasing hydrogen, which is a negative electrode active material. A typical hydrogen storage alloy can be used as the hydrogen storage alloy. In the present disclosure, it is preferable to use a rare earth-Mg-Ni-based hydrogen storage alloy containing rare earth elements, Mg, and Ni.

[0020] The separator 5 is made of, for example, a nonwoven fabric made of polypropylene fibers that have been subjected to a fluorine treatment or sulfonation treatment.

[0021] The electrode group 6 is housed in the outer can 2 so that the negative electrode side is in contact with the bottom wall 8 of the outer can 2 .

[0022] After a predetermined amount of alkaline electrolyte is poured into the outer can 2, the opening of the outer can 2 is sealed. The alkaline electrolyte is impregnated into the positive electrode plate 3, the negative electrode plate 4, and the separator 5, and is involved in the electrochemical reaction, i.e., the charge / discharge reaction, between the positive electrode plate 3 and the negative electrode plate 4. An alkaline electrolyte containing NaOH as the main solute is used as the alkaline electrolyte.

[0023] The battery 1 produced as described above is subjected to an initial activation process to make the battery 1 usable.

[0024] Next, a method for estimating the battery life will be described below. When used as a backup power supply, the battery 1 is electrically connected between a load, such as electrical equipment, and a commercial power source. The backup power supply discharges and supplies power to the load when the commercial power source is interrupted or a power outage occurs. For this reason, the battery 1 is constantly charged, and after being fully charged, a small amount of current flows to compensate for self-discharge, thereby continuing charging, a state known as continuous charging (overcharging). In other words, "continuous charging" refers to a state in which, after the battery 1 is fully charged, a small amount of current flows to compensate for self-discharge, thereby continuing charging.

[0025] When the battery 1 is continuously charged in a low-temperature environment of, for example, 0° C., the positive electrode plate 3 swells (positive electrode swelling) and the electrolyte is absorbed into the positive electrode plate 3 .

[0026] [ka]

[0027] Due to the positive electrode swelling, the positive electrode plate 3 increases in thickness in the radial direction of the electrode group 6. However, because the inner diameter of the outer can 2 remains constant, when the positive electrode swelling occurs, the separator 5 located between the positive electrode plates 3, each of which expands radially, is compressed in the thickness direction, i.e., radially, by the positive electrode plate 3 from both the center and the outside of the electrode group 6. This compression causes the electrolyte held in the separator 5 to be pushed out and absorbed into the swollen positive electrode plate 3. Therefore, the amount of electrolyte held in the separator 5 decreases. In this disclosure, the capacity of the electrolyte held in the separator 5 is referred to as the "separator electrolyte retention volume." A decrease in the separator electrolyte retention volume increases the overall internal resistance of the battery 1, and therefore, the progression of positive electrode swelling leads to a shortened battery life. Positive electrode swelling tends to progress more rapidly in a low-temperature environment, such as 0°C, where the battery 1 is installed, compared to a high-temperature environment.

[0028] This is because oxygen is generated on the positive electrode plate 3 as a side reaction after full charge during continuous charging. Nickel-metal hydride secondary batteries form sealed cells by converting oxygen generated on the positive electrode back into water on the surface of the hydrogen storage alloy of the negative electrode. However, side reactions are more likely to occur on the positive electrode when the ambient temperature is high, and less likely to occur when the ambient temperature is low. Therefore, continuous charging of the battery 1 in a low-temperature environment where side reactions are less likely to occur makes the positive electrode more likely to swell.

[0029] Furthermore, it is known that corrosion of the hydrogen storage alloy (negative electrode corrosion) progresses in the negative electrode plate 4. The corrosion of the hydrogen storage alloy consumes the electrolyte in the negative electrode plate 4, and similar to positive electrode swelling, it sucks the electrolyte out of the separator 5, leading to a reduction in the amount of electrolyte held by the separator 5.

[0030] As described above, when the liquid retention amount of the separator 5 decreases due to the swelling of the positive electrode and the corrosion of the negative electrode, the internal resistance of the battery 1 increases, shortening the battery life.

[0031] Therefore, the inventor of the present invention has found an equation that can more realistically estimate the life of the battery 1 in consideration of the amount of electrolyte taken into the bipolar plates 3 and 4 due to not only the corrosion of the negative electrode but also the swelling of the positive electrode, as follows.

[0032] The battery 1 is evaluated for its characteristics under each environmental temperature and each charge-discharge condition, and based on these evaluations, the life of the battery 1 is estimated as follows.

[0033] The liquid retention amount of the separator 5 can be obtained by the following formula (1).

[0034]

Equation

[0035] Each parameter in the above formula (1) will be described. Regarding the initial liquid amount, C: Liquid retention amount of the separator during continuous charging [g] L: Amount of electrolyte injected during battery manufacturing [g] Sr: Distribution ratio of the injected electrolyte to the separator, 0 < Sr < 1 Ce: Amount of electrolyte at which the infiltration of the electrolyte into the separator starts during injection [g] The initial liquid retention amount of the separator is the amount of liquid retained by the separator at a ratio Sr with respect to the amount of electrolyte exceeding Ce from the injection amount.

[0036] Regarding the amount of positive electrode uptake during continuous charging, Wγ: Amount of electrolyte consumption per amount of positive electrode swelling progress [g] Dγ: Positive electrode swelling progress rate [0 < Dγ < 1]. Dγ = 0 indicates that 100% of the positive electrode active material is in the β phase, and Dγ = 1 indicates that 100% of the positive electrode active material is in the γ phase.

[0037] Regarding the negative electrode corrosion, W M : Electrolyte consumption per amount of corrosion of the negative electrode [g^2 / emu] M: Amount of corrosion progress of negative electrode [emu / g] M is an index of the amount of magnetization per weight of the negative electrode, the amount of corrosion progress.

[0038] When using formula (1), the unknown coefficient can be determined by preliminarily evaluating the battery in advance at various ambient temperatures and under various charge / discharge conditions, and measuring the amount of electrolyte held in the separator each time.

[0039] Next, the formula for calculating the positive electrode expansion progress rate Dγ will be explained below. More specifically, the amount of positive electrode uptake is calculated from the following formulas (2) to (4).

[0040]

number

[0041] In the formulas (2) to (4), the parameters are as follows: i rate : Current rate of battery during continuous charging [C] i side : Current rate used for side reaction at the positive electrode [C] i γ : Current rate used for the positive electrode swelling reaction [C] i side ,i γ The value of the interfacial overvoltage on the positive electrode surface: Δφ pos It is calculated by solving simultaneous equations (2) and (3) with unknowns. I: Reference temperature T for each of the side reactions and the cathode swelling reaction in the Butler-Volmer equation ref Exchange current density I[C] α: Transfer coefficient for each of the side reactions and the cathode swelling reaction in the Butler-Volmer equation Δφ: Interfacial overvoltage [V] for the side reaction and the positive electrode swelling reaction in the Butler-Volmer equation Q: Activation energy of each of the side reactions and the positive electrode swelling reaction in the Butler-Volmer equation [J / mol] T: Temperature during continuous charging [K] T ref :Reference temperature [K] R: Gas constant [J / mol k] F: Faraday constant [A sec / mol] The reference temperature T ref is the reference temperature when determining each coefficient. For example, if the temperature T when charging is T ref When it is equal to, the exponential part of equations (2) and (3) becomes 1, so it can be calculated without any correction due to temperature. That is, I side , Iγ is the charge temperature T ref This means that the exchange current density coefficient is the coefficient when T ref can be set arbitrarily, and the set T ref In response to I side However, when calculating the life expectancy and comparing and analyzing each coefficient, T ref It is preferable to set the temperature at 25°C.

[0042] As described above, the formula for the amount of electrolyte taken up into the positive electrode is expressed by two equations (2) that represent the rate of side reactions during continuous charging of the positive electrode and the rate of swelling of the positive electrode (current rate): (3). The side reaction and the positive electrode swelling reaction occur based on the charging current flowing through the battery during continuous charging. As shown in the following equation (4), the sum of the two reaction currents is the charging current i rate Since it is equal to the unknown interfacial overvoltage Δφ of the positive electrode surface, pos can be derived, and each reaction current i side , i γ can be calculated.

[0043] Next, the swelling progress rate Dγ of the positive electrode is calculated from iγ by the following procedure.

[0044] (1) At predetermined intervals, for example, every hour, equations (2) and (3) are solved to calculate iγ.

[0045] (2) If the time interval is Δt = 1 hour, the reaction progress rate at each calculation interval is iγ × Δt [A × (hours)], and the total amount of positive electrode swelling during continuous charging is Σ(iγ × Δt).

[0046] The amount of oxygen generated as a result of the side reaction at the positive electrode is measured, and the reaction amount of the side reaction Σ(i side × Δt) is calculated. Note that Σ means integrating the current during the period when continuous charging continues. The calculated reaction amount of the side reaction Σ(i side × Δt) is the total charge amount Σ(i rate The charge capacity Cγ of the battery at which swelling ends is calculated in advance by subtracting this from the positive electrode swelling rate Dγ. The positive electrode swelling rate Dγ can then be calculated by Dγ = Σ(iγ × Δt) / Cγ.

[0047] As shown in equation (4), the sum of the two reaction currents is equal to the charging current of continuous charging, so the unknown interfacial overvoltage Δφ on the positive electrode surface pos By deriving the reaction current i side , i γ The reaction current i due to swelling of the positive electrode active material can be calculated. γ By sequentially calculating and integrating the above, the amount of positive electrode swelling Dγ of the positive electrode plate 3 in an overdischarged state can be calculated. It can be seen that such a calculation formula and calculation method can explain the increase in the rate of positive electrode swelling in a low-temperature environment.

[0048] Furthermore, the negative electrode corrosion can be calculated from the following formula (5).

[0049]

number

[0050] In addition, the parameters in equation (5) are as follows: Mo: Initial magnetic susceptibility [emu / g] The starting value of magnetic susceptibility at the start of use (t=0). t is the time elapsed since the battery was used (the total of the period during which normal charging and discharging was performed, the period during which continuous charging was performed, and the unused period during which charging and discharging was not performed), but when predicting the lifespan under continuous charging, it is the period during which continuous charging continues, and is expressed in months. At: Corrosion progression coefficient per month of exposure [emu / g month] Ac: Corrosion coefficient per charge / discharge cycle [emu / g cyc] Ncyc: Number of charge / discharge cycles [cyc] Qt, Qc: Activation energy of corrosion progression over time and corrosion progression due to charging and discharging [J / mol] T ref : Reference temperature [K] The reference temperature for At and Ac

[0051] The amount of magnetization progress M of the negative electrode is determined by two factors: the total elapsed period of the battery (the total of the period during which normal charging and discharging is performed, the period during which continuous charging is performed, and the period during which no charging and discharging is performed) and the number of charge and discharge cycles. The amount of corrosion progress for each is calculated and added together. Since it is known that the amount of electrolyte absorbed is proportional to the progress of positive electrode swelling and negative electrode corrosion, it is evaluated in advance and the coefficients (Wγ, W M ) can be calculated, it is possible to estimate the amount of electrolyte in the separator. Therefore, the battery life can be more accurately estimated from the internal resistance at that time.

[0052] From the above, the lifespan can be estimated by using the formulas (1) to (5) as follows. (I) Preliminary experiments are carried out in advance by varying the storage condition parameters such as the battery storage temperature, charging current value, and storage period in the estimation formulas (2) to (5), and the amount of electrolyte retained by the separator at each time is measured. The storage period may be determined using an accelerated test.

[0053] (II) The coefficient parameters in the estimation formulas (2) to (5) are determined using the least squares method or the generalized reduced gradient method (GRG) so that the liquid retention capacity of the separator can be calculated correctly under each storage condition.

[0054] (III) After the estimation formulas (2) to (5) with determined coefficient parameters are obtained, the storage conditions such as temperature and current value are input into the estimation formulas (2) to (5), and the liquid retention capacity of the separator is calculated from formula (1).

[0055] (IV) The internal resistance of the battery can be determined from the amount of electrolyte held in the separator, and the battery life can be determined. In this way, the battery life can be estimated from the history of the amount of electrolyte held in the separator 5 and the amount of swelling of the positive electrode plate, which are measured under various ambient temperatures and charge / discharge conditions of the battery 1.

[0056] (Example) A continuous charge test was conducted using an AA-size, 1000 mAh battery. The continuous charge test conditions were: (1) ambient temperature: -20 to 60°C; (2) charge rate: 0.05 It to 0.2 It; and (3) discharge frequency: none, once per month, once every two months, or once per three months. After a certain period of time, the battery was removed from the test, and the positive electrode electrolyte uptake, negative electrode electrolyte consumption, and separator electrolyte retention were measured after continuous charging. These measurement results were used to formulate the equations. Figure 2 shows the correlation between the actual separator electrolyte retention volume and the predicted separator electrolyte retention volume calculated using the above equations (1) to (5). Figure 2 confirms that there is a correlation between the actual separator electrolyte retention volume and the predicted separator electrolyte retention volume.

[0057] Specifically, the following table shows the relationship between the actual battery life and the life calculated using the life estimation formula when an AA-size, 1000 mAh battery is continuously charged (without discharging) in a low-temperature environment, for example, at an ambient temperature of 0°C and a current rate of 0.05 C. In the table, the comparative example is a battery of the same type and continuously charged under the same conditions as the example in which the life was calculated based only on conventional anode corrosion.

[0058] [Table 1]

[0059] As can be seen from Table 1, the battery life was 6.2 years when continuously charged in a low-temperature environment of 0°C at a current rate of 0.05C. On the other hand, the battery of the comparative example, whose life was estimated from the amount of corrosion of the negative electrode, was estimated to have a life of 12.8 years, and the battery of the example was estimated to have a life of 5.8 years. As such, it can be seen that the battery life estimation according to the example can calculate a number of years closer to the actual measured value than the comparative example. Therefore, it is clear that the accuracy of the life estimation according to this embodiment can be improved.

[0060] The estimated lifespan of the comparative example is thought to be longer than the actual measured value due to the influence of the environmental temperature during continuous charging. When continuously charging in a low-temperature environment, such as 0°C, a conventional estimation method based on the amount of anode corrosion would estimate a longer lifespan than the actual measured value because anode corrosion does not progress as much as when continuously charging in a temperature environment, such as 25°C. When estimating lifespan based solely on anode corrosion, a battery continuously charged in a low-temperature environment would be estimated to have a longer lifespan than the actual measured value. Therefore, a battery incorporated in a backup power supply would deteriorate and become unusable sooner than the number of years originally estimated based on anode corrosion, and would therefore be unable to perform its backup function in the event of a voltage drop or power outage in the commercial power supply.

[0061] Therefore, as in this embodiment, when the battery life is estimated from the amount of liquid retained in the separator, a backup power supply incorporating such a battery can function appropriately in unexpected situations such as a voltage drop or power outage in the commercial power supply, and the battery life can be utilized effectively without waste. [Explanation of symbols]

[0062] 1 battery 2 outer can 3 positive electrode plate 4 negative plates 5 Separator

Claims

1. A method for estimating the service life of a nickel-metal hydride battery in which a positive electrode plate and a negative electrode plate, which are stacked with a separator made of nonwoven fabric sandwiched between them, are housed in an outer can together with an electrolyte, comprising: A method for estimating the life of a nickel-metal hydride battery, comprising estimating the life from the amount of electrolyte held by the separator and the amount of corrosion of the negative electrode plate.

2. 2. The method according to claim 1, wherein the retained electrolyte volume is calculated from an initial amount of electrolyte held in the separator before the nickel-metal hydride battery is first used and an amount of electrolyte absorbed into the swollen positive electrode plate when the nickel-metal hydride battery is maintained at full charge.

3. 2. The method according to claim 1, wherein the life is estimated by calculating the amount of retained electrolyte in the separator and the amount of swelling of the positive electrode plate from a history of the amount of retained electrolyte held in the separator and the amount of swelling of the positive electrode plate, the history being measured in a storage environment in which the nickel-metal hydride battery is stored and under predetermined charge and discharge conditions.

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