JPH03158781A - Method for estimating residual capacity of sodium-sulfur battery - Google Patents

Abstract

PURPOSE:To enhance estimation accuracy by obtaining the number of defectless series units from a difference between the series-parallel units of discharge depths, adding the correction by the total number of the series unit to the energization quantity of a set battery with the discharge depth as a beginning point and calculating the respective discharge depths. CONSTITUTION:The set battery 8 is housed into a heat insulating box 10 where the series-parallel units 6 are connected in series. A voltmeter 14 measures the voltage across the units 6 and an ammeter 16 measures the current flowing in external terminals 12a, 12b. The electromotive force E of the units 6 is estimated from a change with time, of the voltage of the units 6 right after the stop of the charge and discharge and the discharge depth D of the units 6 is obtd. by collating the electromotive force E and the theoretical value. The number K of the defectless series units of the units 6 is obtd. by making comparison with depths D each other between the units 6. The charging and discharging current quantity Ah of the battery 8 is obtd. by integrating the current value measured by the ammeter 16 with time and can be referenced at all time. The residual capacity R of the battery 8 is calculated from the max. of the depth D of the units 6, the number K of the units and the number K of the units of the battery 8.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to sodium-sulfur batteries, and in particular, to accurately estimate the remaining capacity of the battery and to inspect the number of damaged single cells in the battery. The present invention relates to a voltage measurement method and data processing method suitable for

[Conventional technology]

Conventional technology generally involves calculating the depth of discharge starting from the time of initial loading by integrating the current flowing through the battery, and then obtaining the remaining capacity from the difference from the theoretical discharge capacity.
The accuracy of the remaining capacity was increased by increasing the measurement accuracy. In the case of lead-acid batteries, there is also a method of estimating the depth of discharge by measuring the specific gravity of the electrolyte. In addition, as a special method, as described in JP-A-60-166875, an AC voltage is applied superimposed on a DC voltage and the harmonics of the flowing current are measured, and the remaining capacity state is determined in advance. A method has also been proposed for estimating the remaining capacity by comparing it with the value of . However, an effective means for accurately estimating remaining capacity has not been established.

[Problem to be solved by the invention]

However, the above conventional technology has the following problems.

In the first example, errors in current value and integration accumulate while repeating a large number of charge/discharge cycles, resulting in a decrease in accuracy;
In the case of an assembled battery in which a large number of single cells, which are the basic unit, are connected in series and parallel, damage to the above-mentioned single cells will greatly change the distribution of current within the assembled battery, and the depth of discharge obtained from the integration of external currents will decrease. teeth.

There is a problem in that it no longer represents the depth of discharge of a single cell. On the other hand, if one were to try to measure the current for each cell, it would require a huge amount of quantitative determination, and would not be an effective method in terms of errors.

The second example of specific gravity measurement is possible for batteries that use electrolytes, such as lead batteries, but cannot be applied to batteries that use solid electrolytes, such as sodium-sulfur batteries.

In the third example, it is necessary to obtain the relationship between the remaining capacity and harmonic components in advance.

An object of the present invention is to accurately estimate the remaining capacity of a sodium-sulfur battery in which a large number of single cells are connected in series and parallel, without the need for prior measurement.

[Means to solve the problem]

The method of the present invention for achieving the above object consists of the minimum unit of cells connected in series and parallel in a sodium-sulfur battery, that is,
After measuring the electromotive force and the corresponding theoretical depth of discharge for each unit that has a closed circuit in which a circulating current can flow even when the external circuit is opened, it is possible to determine whether the battery is damaged based on the difference in electromotive force. Estimate the number of cells, find a correction coefficient for the current value flowing through the single cells in the assembled battery unit, and use this correction coefficient and the depth of discharge that indicates the maximum depth among the above depths of discharge, This method calculates the remaining capacity of the entire battery.

In addition, when measuring the electromotive force mentioned above, by approximating the voltage change immediately after the charging/discharging current stops to a functional form including an exponential function, the true electromotive force value, which approaches asymptotically after an extremely long period of time, can be measured for a short time. It can be estimated that

[Effect]

According to the above configuration, the remaining capacity of the sodium-sulfur battery when charging and discharging is stopped is calculated within a short time, and using this as a starting point, the battery current is integrated as before, and multiplied by the above correction coefficient. This makes it possible to constantly obtain a highly accurate remaining capacity even after repeated charge/discharge cycles over a long period of time, and because it is possible to estimate the number of damaged cells, it is also possible to estimate the battery repair time. .

The operation principle of the present invention will be explained in detail below using appropriate drawings.

A single cell, which is the smallest structural unit of a sodium-sulfur battery, uses molten sodium as the negative electrode active material, sulfur or sodium polysulfide as the positive electrode active material, and a solid electrolyte such as β''-alumina that has sodium ion conductivity as the electrolyte. During discharge, sodium liberates electrons and becomes ions, which enter the positive electrode through the solid electrolyte and react with sulfur to form sodium polysulfide.At this time, it combines electrons flowing in from the outside. The discharge reaction is completed by neutralization.In the positive electrode, as the discharge progresses, sodium increases and becomes Na2S, Na, S, and so on.

The sulfidity of sodium polysulfide decreases as Na, SJ, and so on. Usually, the melting point of Na□S is about 285℃
When it comes to Na, S, the melting point rises to over 1000°C, so the operating temperature is set to about 300 to 350°C, and the discharge limit is set to Na, S. 6 In other words, Na,
The theoretical discharge capacity can be obtained by converting the amount of Na required to reach S□ into the amount of charge. In practical terms,
Even if sodium is supplied in excess of Na and S, sodium polysulfide will begin to partially solidify within the positive electrode, increasing internal resistance and impairing battery functionality.

It is appropriate to set the discharge limit to Na2S.

In addition, if the battery is constructed in such a way that sodium is not supplied until Na, S,
The unit cell is configured so that a high energy density can be obtained by setting the molar ratio of sodium and sulfur to about 2=3. During charging, by applying a voltage higher than the electromotive force from the outside in the opposite direction, electrons are taken from sodium polysulfide and it is ionized.The ionized sodium passes through the solid electrolyte in the opposite direction to that during discharging, and enters the negative electrode. It returns, where it picks up electrons and becomes neutral, completing the charging reaction. At this time, as charging progresses and the proportion of sulfur in the sodium polysulfide in the positive electrode increases, elemental sulfur begins to precipitate, but since sulfur does not have electronic conductivity, it has a high internal resistance at the end of charging. come to have.

Therefore, the discharge capacity is usually 10 to 1, assuming the above discharge capacity as 100%.
The practical charging limit is about 5%, and charging ends with some sodium remaining in the positive electrode. Therefore, the normal operating range ranges from 10 to 15% to a maximum of 100% of the discharge capacity.

Figure 2 shows the electromotive force of a sodium-sulfur battery. The electromotive force of a sodium-sulfur battery shows different changes depending on the depth of discharge, and until the molar ratio of sodium and sulfur in the positive electrode reaches 2:5, the positive electrode contains elemental sulfur, Na,
In the mixture of S, the electromotive force is constant at about 2.07V.

In the state where the discharge has progressed from the state of Na, S, Na,
S,,Na.

It becomes a mixture of S, Na, S, etc., and as a whole,
It decreases almost linearly with the depth of discharge, reaching about 1.76 V for Na2S5. Therefore, Na.

S, at more advanced depths of discharge, the depth of discharge can be identified by knowing the electromotive force.

However, this theoretical value is the value when the composition of sodium and sulfur is uniform; in reality, different compositions of electromotive force may be distributed within the positive electrode, which has a finite volume, and the true It is often difficult to measure the electromotive force; in other words, as shown in Figure 3, the voltage after discharge stops slowly approaches the true electromotive force; It takes several hours for it to reach an asymptotic state.

In the present invention, this voltage change is defined as V (t)=E
By approximating the function to @-(Eo-vo)e""-(1), the true electromotive force can be estimated within a short time after charging and discharging are stopped. Here, V (t) is the measured voltage value, and E is the true electromotive force that V (t) approaches asymptotically.

respectively represent voltage values at time 1=0. Although the matters described so far are aimed at single cells, this method has also been found to be effective for assembled batteries in which many single cells are connected in series and parallel, as explained in the following example. .

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings.

First, a specific example of the configuration of a sodium-sulfur battery and its operation will be described. In general, a single sodium-sulfur battery has an output of several tens of watts due to manufacturing constraints, but in practical terms, for example, for use in power storage or electric vehicles, an output of several kilowatts to several megawatts is required. It becomes necessary. Therefore, a large number of single cells are connected in series and parallel to obtain a predetermined voltage and current.

FIG. 1 shows an example of the configuration of a sodium-sulfur battery assembly constructed in this way. In FIG. 1, one or more cells 2 are connected in series to form a series unit 4.

A series-parallel unit 6 is constructed by connecting a plurality of these in parallel. Furthermore, one or more of these are connected in series to form an assembled battery 8.

Note that since the sodium-sulfur battery operates at high temperatures, it is normally housed in a heat-insulating box 10 that is a heat-insulating structure. In the assembled battery configured in this way, the rated current of the single cell 2 is calculated. , if the average discharge voltage is v0, the number of series in series-parallel units 6 is n, the number of parallel units is m, and the number of series-parallel units 6 is N, then
Approximately mI at both ends of external terminals 12a and 12b. At a rated current of -, an average voltage of nNV is obtained. The voltmeter 14 is for measuring the voltage across the series-parallel unit 6, and the ammeter 16 is for measuring the current flowing through the assembled battery 8.

This assembled battery 8 is a unit for operation, temperature control, and maintenance.
Usually configured with outputs ranging from a few kilowatts to several tens of kilowatts. Furthermore, if a large amount of power needs to be stored, these assembled batteries are connected in series or in parallel.

The operation of this assembled battery 8 is nothing but a superposition of the operation of the single cells 2. Therefore, if all the single cells 2 are working properly, the electromotive force shown in FIG. Inside the series-parallel unit 6, which has an electromotive force in the form of nN times the change, the current distribution is not uniform due to the difference in the resistance value of the series unit 4, and therefore the depth of discharge differs for each series unit. Charging and discharging progress in this state, and the electromotive force is a value corresponding to the average value of these different depths of discharge. Immediately after stopping charging and discharging, the voltage of the series-parallel unit 6 measured by the voltmeter 14 shows a change as shown in FIG. 3. During this pause, charge is exchanged between the series units 4 having different depths of discharge, that is, different electromotive forces. In other words, since the series unit with a shallow discharge depth has a high electromotive force, it discharges while charging other series units. Therefore, the change in the measured voltage is
This is the change in the average value of the electromotive force, which is the change in the electromotive force due to the change inside the cell 2 multiplied by five.

Therefore, by approximating the voltage change of the series-parallel unit 6 immediately after charging and discharging is stopped as a function as in the case of the single cell,
The electromotive force of the series-parallel unit 6, that is, the depth of discharge can be known in a short time.

When all the cells 2 in the assembled battery 8 are healthy, the depth of discharge of each series-parallel unit 6 obtained as described above is equal. This is because the current supplied is the same.

However, the situation is different if the cell is damaged.

Damage to sodium-sulfur batteries is basically due to deterioration of the solid electrolyte. As a result, what definitely occurs is a decrease in the electromotive force. Even a short circuit outside the cell is equivalent to a decrease in electromotive force when viewed from the outside. Changes in internal resistance are not constant; they may become larger or smaller than in a healthy state, and changes over time vary.

Now, when the cell 2 is damaged and the electromotive force decreases, a current corresponding to the difference in electromotive force flows from another healthy series unit 4 of the series-parallel unit 6. As a result, the series unit including the damaged cell is charged, and the healthy cell in the series unit reaches the high resistance region at the end of charging as described above. Then, the series unit including the damaged assembled battery becomes substantially open circuited. This process progresses at different speeds depending on the number n in series, the number m in parallel, and the internal resistance value of the damaged cells, but in the end the series unit including the damaged cells is virtually disconnected, so the initial The number of serial units 4, which were m, becomes (m-1).

In addition, while the above process is progressing, the amount of discharge of a healthy series unit is actually greater than the amount of charge, and the amount of charge corresponding to the depth of discharge of the series unit including the damaged cell at the time of damage is increased. bear. Therefore, at maximum 1/m − of discharge capacity
The depth of discharge shifts by 1 to the discharge side. Also, a healthy series unit in a series-parallel unit containing a damaged cell carries an average of m/m-1 times more current than a series unit in a healthy series-parallel unit. Therefore, if we compare the depth of discharge of a healthy series-parallel unit and a series-parallel unit containing a damaged cell, the latter will necessarily have a deeper depth of discharge.

As a result, the voltage of each series-parallel unit in the assembled battery was measured,
By comparing the obtained depths of discharge, it is possible to check whether a particular series-parallel unit is healthy or not. If the depth of discharge deviates, it can be determined that damage has occurred in the cell, and this can be checked. Can be recorded.

Now, the remaining capacity of an assembled battery is the amount of current that passes through the battery in the series-parallel unit with the most advanced discharge depth, as seen from the outside of the assembled battery, until the single cell in the series-parallel unit with the most advanced discharge depth reaches its discharge capacity. If the number of units is k, then

As described above, by measuring the voltage change immediately after charging and discharging is stopped for each series-parallel unit in the assembled battery 8, calculating the electromotive force to obtain the depth of discharge, and comparing the mutual values, the maximum discharge depth can be determined. Then, we can know the healthy number of series units in each series-parallel unit. Using this as a starting point, if you update the depth by converting the accumulated current obtained by the current meter 16 to 1/1 to the discharge (charge) amount, the remaining capacity can be maintained at all times even during operation. You will be able to know.

An embodiment of the present invention will be described below with reference to FIG.

In the assembled battery 8 where the series-parallel units 6 are connected in series and stored in the heat insulation box 10, the voltmeter 14 measures the voltage across the series-parallel units 6, and the ammeter 16 measures the current flowing through the external terminals 12a and 12b. Measure. Electromotive force E of series-parallel unit 6 (symbol 1
8) is estimated from the time change in the voltage of the series-parallel unit 6 immediately after charging and discharging is stopped.

The depth of discharge D (symbol 20) of the series-parallel unit 6 is obtained by comparing the electromotive force 18 with a theoretical value.

The number of healthy series units K (symbol 22) of the series-parallel units 6 is obtained by mutually comparing the depth of discharge 20 between the series-parallel units. The charge/discharge amount Ah (reference numeral 24) of the assembled battery 8 is obtained by time-integrating the current value measured by the ammeter 16, and can be referenced at all times.

The remaining capacity R (symbol 26) of the assembled battery 8 is determined by the maximum depth of discharge 20 of the series-parallel unit 6 and its series-parallel unit 6.
The number of healthy series units is 22, and the amount of charging and discharging of one set of batteries 8 is 24.
It is calculated from

When calculating the depth of discharge, a theoretical formula based on the above-mentioned Fig. 2 is used.

E=A (D-B)+C (D>60%)...(3)(
A, B, and C are constants), but in order to identify D with an error of about 1%,
It is necessary to obtain E with an accuracy of at least 4 digits.

Therefore, as the voltmeter 14, a measuring instrument having a resolution of at least 5 digits, preferably about 7 digits is used. This voltmeter 14
The change in voltage obtained by
1) is the formula.

-α t V(t)=E (E'/'+o+)6
, , E is found by functional approximation to (1)/.
In this example, the following steps were taken. In other words, when formula (1)' is differentiated, the differential coefficient at time tz+tz (t□<C2) is approximated as follows, and further, at time 1. , 1. (12<1□), divide the sides and get...(7) From this equation (7), 1=1. Obtain the α value at ~t3.

Several points are sampled to find the average α, and this α is used to find E. In this example, the voltage was measured at 6-second intervals, and after sampling (30 points) for 3 minutes, the above calculation was performed to obtain E. As a result of determining the depth of discharge D (%) using E thus obtained, it was found that the depth of discharge was correct with an error of approximately 1%.

Next, the depths of discharge 20 of the series-parallel units 6 obtained in this way are compared with each other, and it is checked whether there is any one whose depth of discharge has advanced. As a result, if there is one that is progressing, the counter for the number of healthy series units 22 of that series-parallel unit 6 is decremented by one. By this operation, the depth of discharge 2o and the number of healthy series units 22 for each series-parallel unit 6 are as follows. Updated when charging/discharging stops.

However, if the electromotive force is n times about 2.07V, the depth of discharge cannot be identified and therefore cannot be updated.

Now, the remaining capacity of an assembled battery can be calculated by focusing on the series-parallel unit with the most advanced depth of discharge.In other words, assuming the discharge capacity of a single cell is 80, the discharge depth of that series-parallel unit at the time of renewal is D0, and from that point on, the cumulative amount of charge and discharge (discharge is positive,
If charging is negative) is Ah, and the number of healthy series units is 0, then the remaining capacity R is.

R=, xA, X (1-D,) - AhX - (8) , and if the integral of Ah is updated, then always.

R is updated.

In addition to the above method, if the average temperature of each series-parallel unit is measured and used when calculating the depth of discharge from the electromotive force, the accuracy will be further improved. The electromotive force of a sodium-sulfur battery is
It is known that the constant A in the above-mentioned formula (3) decreases as the temperature increases.

For B and C, several points are given in the literature with respect to temperature.

Furthermore, since the number of damaged single cells can be roughly grasped during the process of this method, it is possible to determine when to repair the assembled battery.

〔Effect of the invention〕

According to the present invention, since the remaining capacity of the assembled sodium-sulfur battery can be constantly monitored with high accuracy, there is an effect that the subsequent operation plan can be easily planned even during operation.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing an example of the configuration of a sodium-sulfur battery;
Figure 2 is a graph showing changes in electromotive force with respect to depth of discharge of a sodium-sulfur battery, Figure 3 is a graph showing changes in battery voltage immediately after discharging is stopped, and Figure 4 is a concept of an embodiment of the present invention. FIG. 2... Single battery, 4... Series unit, 8... Assembled battery, 14... Voltmeter. 18...Emotive force E in series and parallel units. 20... Depth of discharge D in series/parallel units, 22... Number K of healthy series units in series/parallel units, 24...
Charge/discharge amount of the assembled battery Ah, 26...Remaining capacity R0 of the assembled battery 6...Series/parallel unit,

Claims (1)

[Claims] 1. A series unit in which one or more single cells of a sodium-sulfur battery are connected in series is connected in parallel to form a series-parallel unit,
In a method for estimating the remaining capacity of a sodium-sulfur battery assembly formed by connecting a plurality of these batteries in series, the electromotive force and depth of discharge are estimated from the voltage change immediately after charging and discharging are stopped for each series-parallel unit, and the The number of healthy series units in the series-parallel unit is obtained from the mutual difference in depth between the series-parallel units, and using the depth of discharge as a starting point, the amount of current flowing through the assembled battery is corrected by the number of healthy series units. A method for estimating remaining capacity of a sodium-sulfur battery, comprising calculating the depth of discharge for each series-parallel unit, and calculating the remaining capacity of the assembled battery based on the maximum depth of discharge. 2. A series unit in which one or more sodium-sulfur battery cells are connected in series is connected in parallel to form a series-parallel unit;
In a method for estimating the remaining capacity of a sodium-sulfur battery assembly formed by connecting multiple batteries in series, the voltage change immediately after charging and discharging is stopped for each series-parallel unit is approximated to a functional form including an exponential function. The electromotive force of the series-parallel unit is estimated, the depth of discharge of the series-parallel unit is estimated from this electromotive force, and the healthy series in the series-parallel unit is estimated from the mutual difference in the depth of discharge between the series-parallel units. get the number of units,
Using this depth of discharge as a starting point, calculate the depth of discharge for each series-parallel unit by correcting the current flow amount of the assembled battery by the number of healthy series units, and calculate the depth of discharge for each series-parallel unit from the maximum depth of discharge. A method for estimating the remaining capacity of a sodium-sulfur battery, the method comprising calculating the remaining capacity. 3. A series unit in which one or more sodium-sulfur battery cells are connected in series is connected in parallel to form a series-parallel unit;
In a method of estimating the remaining capacity of a sodium-sulfur battery assembly formed by connecting multiple batteries in series, the voltage change immediately after charging and discharging of each series-parallel unit is approximated to a functional form including an exponential function. The electromotive force of the series-parallel unit is calculated, the depth of discharge of the series-parallel unit is estimated from this electromotive force, and the healthy series in the series-parallel unit is estimated from the difference in the depth of discharge between the series-parallel units. get the number of units,
Using this depth of discharge as a starting point, calculate the depth of discharge for each series-parallel unit by correcting the current flow amount of the assembled battery by the number of healthy series units, and calculate the depth of discharge for each series-parallel unit from the maximum depth of discharge. A method for calculating remaining capacity, in which the functional form calculates the measured voltage for each series-parallel unit at time t by V(t
), the true electromotive force asymptotic to this voltage V(t) is E_
0, the voltage at time t=0 is V_0, the base of the natural logarithm is e, and the exponent is α, V(t)=E_0-(E_0-V_0)e^-^α^t
A method for estimating the remaining capacity of a sodium-sulfur battery, characterized in that the functional form is: 4. A series unit in which one or more sodium-sulfur battery cells are connected in series is connected in parallel to form a series-parallel unit;
In a method of estimating the remaining capacity of a sodium-sulfur battery assembly formed by connecting multiple batteries in series, the voltage change immediately after charging and discharging of each series-parallel unit is approximated to a functional form including an exponential function. The electromotive force of the series-parallel unit is calculated, the depth of discharge of the series-parallel unit is estimated from this electromotive force, and the healthy series in the series-parallel unit is estimated from the difference in the depth of discharge between the series-parallel units. get the number of units,
Using this depth of discharge as a starting point, calculate the depth of discharge for each series-parallel unit by correcting the current flow amount of the assembled battery by the number of healthy series units, and calculate the depth of discharge for each series-parallel unit from the maximum depth of discharge. A method for calculating the remaining capacity, wherein the electromotive force is determined by measuring the voltage of each series-parallel unit at several different times, and based on the voltage measurement value and the measurement time,
There are mathematical formulas, chemical formulas, tables, etc. where time is t, a natural number is n, the measured voltage is V, the base of the natural logarithm is e, and the exponent is α.
1. A method for estimating remaining capacity of a sodium-sulfur battery, characterized in that the power exponent α value in 2 is calculated and the remaining capacity of a sodium-sulfur battery is determined from the average value of the α values.