JPH0850930A - Judging method for remaining capacity and remaining electric energy of sodium-sulfur cell, and charging/ discharging control method - Google Patents

Abstract

PURPOSE:To prevent a part of sodium-sulfur cells from being subjected to excessive burden by properly judging the remaining capacity and remaining electric energy of each sodium-sulfur cell, thereby preventing over-charge and over- discharge, and concurrently recognizing deterioration and the like surely. CONSTITUTION:A control means 6 is provided, in which each module 2 composed of a plurality of NaS cells is made into one unit, and the voltage of each module 2 is measured through insulating amplifiers 4 and 5. The depth of discharge is computed based on release voltage measured by the control means 6, and the remaining capacity and remaining electric energy of each NaS cell is judged based on the aforesaid depth of discharge.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for determining the remaining capacity and remaining power of a sodium-sulfur battery, and a charge / discharge control method.

[0002]

2. Description of the Related Art A sodium-sulfur battery is operated by repeating charging and discharging within a set operable range. Operating outside of this operable range leads to over-discharge or over-charge, leading to deterioration in battery performance. Therefore, it is necessary to accurately grasp the remaining capacity of the battery.

Conventionally, as a method for determining the remaining capacity of a sodium-sulfur battery, for example, the charging current or discharging current of the sodium-sulfur battery is integrated, and the integrated value is compared with the rated capacity to determine the remaining capacity. There was a way to do it. That is, when discharging, the integrated value of the discharge current is subtracted from the rated capacity to calculate the remaining capacity, and when charging, the integrated value of the charging current is added to the remaining capacity.

[0004]

However, in the above-mentioned conventional technique, when an error occurs in the measurement of the charge current and the discharge current, even if the error is very small,
The error increased due to the integration, and the remaining capacity could not be accurately determined. Further, when the rated capacity of the sodium-sulfur battery decreases due to deterioration of the sodium-sulfur battery or internal failure due to long-term operation of the battery, etc., even if the determination of the remaining capacity value by integration is accurate, An error may occur between the determined remaining capacity and the actual capacity, and the remaining capacity may not be accurately determined.

The fact that the remaining capacity cannot be accurately determined due to deterioration of the sodium-sulfur battery means that the sodium-sulfur battery is operated outside its operable range, and This will result in discharge or overcharge. When this over-discharging or over-charging is executed, deterioration of the sodium-sulfur battery, internal failure, etc. are further promoted, leading to a vicious circle.

In addition, when one or a plurality of batteries are deteriorated or an internal failure occurs, the internal resistance value of the battery becomes larger than the internal resistance values of other normal batteries. As a result, deterioration or internal failure of the battery is overloaded even if the normal battery is not energized. This also causes the vicious circle as described above.

Particularly, the determination of the remaining capacity of a sodium-sulfur battery is performed by using a plurality of mutually connected sodium-sulfur batteries as one unit, and a plurality of units are totalized. Even if a small amount of the sodium-sulfur battery is deteriorated in performance due to deterioration or internal failure of the sulfur battery, the state is not surfaced. Therefore, sodium-
The sulfur battery is often operated with a load exceeding its capacity, resulting in the above-mentioned vicious circle.

In addition to this, a method of monitoring the time spent for charging / discharging so that charging / discharging is not performed for a certain time or longer can be considered. However, in this case, since the remaining capacity and the charge / discharge amount are ignored, there is a possibility that the result may be worse than that in the above case.

The present invention has been made in view of the problems existing in the above-mentioned prior art, and its purpose is to accurately determine the remaining capacity and prevent overcharge and overdischarge. Especially.

Further, an object of the present invention is to surely recognize deterioration and the like, to prevent an excessive load from being applied to a part of the sodium-sulfur battery, and to prevent the vicious cycle as described above. It is in.

[0011]

Means for Solving the Problems In claim 1,
The voltage of the sodium-sulfur battery is measured, the depth of discharge is obtained from the measured value, and the remaining capacity of the sodium-sulfur battery is determined from the depth of discharge.

In claim 2, in claim 1,
The voltage measurement is performed with a plurality of sodium-sulfur batteries as one unit. In a third aspect of the present invention, the sodium-sulfur battery according to the first or second aspect is discharged to a predetermined depth of discharge region, and a voltage is measured after a certain time has passed from the end of the discharge.

In claim 4, in claim 2,
The remaining capacity of a plurality of sodium-sulfur batteries is determined, and the remaining capacity value of the sodium-sulfur battery showing the minimum remaining capacity among them is defined as the remaining capacity of other sodium-sulfur batteries,
Based on that, the total remaining capacity of the plurality of sodium-sulfur batteries is calculated.

In claim 5, in claim 4,
A plurality of sodium-sulfur batteries are used as one unit to determine the respective remaining capacities of the plurality of units, and the remaining capacity value of the unit showing the lowest remaining capacity among the respective units is set as the remaining capacity of the other unit, and the plurality of units are calculated based on the remaining capacity. Calculate the total remaining capacity in units of.

In a sixth aspect of the present invention, in any one of the first to fifth aspects, the charging / discharging is stopped when the battery voltage reaches a specified value. In Claim 7, the remaining capacity of the sodium-sulfur battery is obtained from the depth of discharge in Claim 1, and the remaining power amount is calculated based on this remaining capacity and the discharge voltage.

In claim 8, in claim 1,
The current discharge depth is obtained from the measured voltage, and the discharge voltage and discharge current after the discharge depth are calculated based on the current discharge depth to determine the remaining power of the sodium-sulfur battery.

[0017]

According to the first aspect of the present invention, the remaining capacity is determined irrelevantly without using the integrated value of the current.
There is no error in the judgment. Further, it is possible to determine the degree of deterioration or failure of the sodium-sulfur battery based on the remaining capacity.

According to the second aspect of the present invention, the determination of the remaining capacity and the recognition of deterioration etc. are performed with a plurality of sodium-sulfur batteries as one unit. According to the third aspect, the voltage is measured in the stable region, and the remaining capacity and the like are accurately determined.

In claim 4, a plurality of sodium-
Since the total remaining capacity of the sulfur battery is calculated on the basis of the minimum remaining capacity value, the sodium-sulfur battery can be operated with a margin and overcharge and overdischarge can be prevented.

In claim 5, a plurality of sodium-
The total remaining capacity value of a plurality of units with the sulfur battery as one unit is based on the minimum remaining capacity value. According to the present invention, the sodium-sulfur battery can be prevented from being deteriorated or damaged without being operated in a state where the battery voltage exceeds the specified value.

In the invention of claim 7, the remaining capacity of the battery is obtained according to the depth of discharge, and the remaining power amount is calculated based on the remaining capacity and the discharge voltage. In the invention of claim 8, the discharge voltage and the discharge current after that are obtained based on the current depth of discharge, and the remaining power amount of the battery is determined. Therefore, the remaining power amount of the battery can be accurately determined in accordance with the discharge voltage and the discharge current that change from time to time.

[0022]

EXAMPLES First Example Hereinafter, a first example embodying a remaining capacity determination method and a charge / discharge control method of a sodium-sulfur battery (hereinafter referred to as NaS battery) of the present invention will be described with reference to FIGS. 1 and 2. Follow the instructions below.

The unit 1 shown in FIG. 1 is composed of a plurality of modules 2 connected in series. Each module 2
Is composed of a plurality of blocks (not shown) connected in series or in parallel. Each block is configured by connecting a plurality of strings (not shown) in parallel. Each string is composed of a plurality of unit cells, that is, NaS batteries (not shown) connected in series. Therefore, in this embodiment, each module 2 is composed of many NaS batteries, and each module 2 constitutes one unit.

The shunt resistor 3 is connected to the unit 1. The isolation amplifier 4 is connected between both terminals of the shunt resistor 3. The plurality of isolation amplifiers 5 are connected in parallel to each module 2. Each of the isolation amplifiers 4 and 5 is composed of a transformer or the like, and outputs a voltage that is insulation-converted according to the voltage of the unit 1 or the module 2 from the secondary side.

The control device 6 having various functions such as a control function, a data storage function, a voltage measurement function, a multiplexer function, etc., inputs the outputs of the respective isolation amplifiers 4 and 5 in a sequential scanning manner after a fixed time, In response to this, various operations such as determination of remaining capacity and charge / discharge control, which will be described later, are performed. The display device 7 displays the operation result of the control device 6.

In the system configured as described above, charging and discharging are alternately performed at regular time intervals (for example, 12 hours). And when the discharge ends,
The control device 6 calculates the remaining capacity of each module 2 based on the output of each isolation amplifier 4, 5.

That is, the charge / discharge characteristics of the NaS battery are as follows. In FIG. 2, the horizontal axis represents the depth of discharge (A
h), and the vertical axis represents the battery voltage (V) corresponding to the depth of discharge
Is shown. The solid line in FIG. 2 represents the open-circuit voltage (voltage when the charging / discharging operation of the NaS battery is stopped), the upper dotted line represents the battery voltage during charging, and the lower dotted line represents the battery voltage during discharging. There is. Both the solid line and the dotted line are the two-phase region (two-phase state in which sodium and sulfur are polarized) showing a constant voltage, and the solid and dotted line is the one-phase region (the sodium and sulfur Is a single-phase state in which sodium polysulfide is formed). It should be noted that the voltage rises in a sharp curve due to a sharp rise in the internal resistance at the end of charging (to the left of the upper dotted line in FIG. 2), but this portion is also in the two-phase region. The depth of discharge corresponds to the capacity of the NaS battery.

Now, for example, when the discharge is completed at the position of point A in FIG. 2, the battery voltage reaches the position of point B for a certain time (5 to 5).
It takes about 30 minutes) to rise naturally to the open circuit voltage and stabilize. Therefore, the voltage of each module 2 is measured in this stable region. On the other hand, the open circuit voltage data corresponding to the limit discharge depth of each module 2 is stored in the control device 6.
The open circuit voltage corresponding to this limit depth of discharge is point C in FIG. The control device 6 compares the measured voltage with the open circuit voltage indicating the limit discharge depth, takes the difference, and determines the discharge depth for each module 2 by the difference. The value of the determined depth of discharge is displayed on the display device 7.

The limit discharge depth is set in advance as a calculated value from the amount of the active material of the NaS battery, the safety design of the battery system, the reliability design, and the like. Therefore, in this embodiment, unlike the above-mentioned conventional example, since the depth of discharge is not calculated based on the integrated value of the discharge current,
An accurate value of the remaining capacity can be obtained without causing an error in the calculated remaining capacity or accumulating the error. For this reason, always grasp the proper remaining capacity,
Over discharge can be prevented in advance.

Further, in this embodiment, since the remaining capacity is calculated for each module 2, the module 2
By determining the full discharge capacity (actual discharge capacity + remaining capacity) for each module, the degree of deterioration of each module 2 can be grasped, which can be used as a guideline for maintenance and the like. That is, the voltage of each module 2 and the unit 1 detected by the isolation amplifier 4
The control device 6 recognizes the remaining capacity and the actual discharge amount based on the behavior of the discharge voltage. And the deteriorated NaS
Since the battery has a low remaining capacity, it is possible to accurately grasp the degree of deterioration.

Further, the value of the full discharge capacity (actual discharge capacity + remaining capacity) of each module 2 goes up several cycles (one charging / discharging cycle is one cycle) from the present time to obtain a plurality or a single piece of historical data. Each is stored in the control device 6. Then, the new full discharge capacity data and the historical data are compared, and if a sharp change is found in the new data, it is determined that an internal failure has occurred in the module 2, and the signal is displayed on the display device 7. It is sent and a message to that effect is displayed.

Further, in this embodiment, in one phase region,
The voltage of each module 2 is measured a plurality of times.
Therefore, the slope of the voltage curve in the one-phase region and the inflection point between the one-phase region and the two-phase region can be known, and the deterioration or failure of the NaS battery in the module 2 can be recognized based on the inflection point. That is, the voltage curve of a normal NaS battery has a slope of a predetermined angle, and the inflection point is located at a position corresponding to the predetermined depth of discharge, but when the NaS battery deteriorates or malfunctions, the slope angle changes ( Normally, the inclination angle becomes steep) and the inflection point moves (usually the inflection point moves to the right in FIG. 2). Therefore, it is possible to determine whether or not the NaS battery in each module 2 is normal, that is, whether or not the NaS battery is deteriorated or has an internal failure, by measuring the voltage at a plurality of positions in the one-phase region. .

In addition, if the voltage / discharge depth slope rate in the one-phase region is used as a correction parameter and a change in the slope rate due to deterioration or failure is corrected, more accurate remaining capacity calculation and deterioration / internal failure determination can be performed. It will be possible.

In addition, the remaining capacity value of the module 2 having the lowest remaining capacity is used as a reference, and the remaining capacity value is multiplied by the number of modules to be converted into a total remaining capacity value for one unit. It is possible to anticipate a safe remaining capacity without imposing an overload on. Therefore, even if there is a variation in the remaining capacity of each module 2, that is, even if the degree of deterioration is different, it is possible to operate the module smoothly without causing deterioration of the module. The isolation amplifier 4 outputs a secondary voltage according to the current value of the entire unit 1, and the control device 6 determines the remaining capacity of the entire unit based on the output secondary voltage.

Further, when the discharge end voltage or the charge end voltage of any of the modules 2 reaches the specified voltage as the discharge or charge progresses (when the discharge end voltage or the charge end voltage becomes less than the specified voltage at the time of discharge, at the time of charge (When the voltage exceeds the specified voltage)
At the same time, the control device 6 stops the operation of discharging or charging, and at the same time, sends a signal to the display device 7 to display that effect.

Incidentally, the specified voltage is set as follows. The specified voltage VL at the time of discharge is VL = Vocv × n−Im × rd Vocv: an open circuit voltage (electromotive voltage) of one NaS battery corresponding to a predetermined discharge depth, which is a set value.

N: The number of NaS batteries in series in one unit. Im is a discharge current of 1 unit (unit is ampere) and is measured by the control device 6 through the isolation amplifier 4.

Rd: Internal resistance (set value) of the unit 1 during discharging. The specified voltage VH at the time of charging is: VH = (Vp + α) × n + Im × rc Vp: Open voltage in the two-phase region, which is a set value.

Α: Voltage rising limit constant during charging (converted by the amount of increase in internal resistance, which limits the voltage applied to the NaS battery). n: The number of NaS batteries in series in the unit 1.

Im: The charging current of the unit 1 (unit is ampere), which is measured by the control device 6 via the isolation amplifier 4. rc: Internal resistance (set value) of the unit 1 during charging. As can be seen from the above formula, all values other than the current Im are constants known at the stage of configuring the battery system.

Therefore, the optimum specified voltages VL and VH according to the state of the battery are set based on the change of the current Im. As a result, even with a single NaS battery, performance deteriorates due to deterioration and the like, and even if the value of the current Im becomes low, the specified voltage VL is set high during discharging and set low during charging. Therefore, it is possible to prevent over-discharging or overcharging caused by the decrease in battery capacity due to the deterioration of the battery, and to reliably protect the NaS battery. (Second Embodiment) Next, a second embodiment of the present invention will be described with reference to FIGS.

In this embodiment, the amount of remaining power is calculated as follows based on the remaining capacity (Ah) obtained in the first embodiment and can be displayed. That is, the remaining power amount W ₁ (Wh) is calculated by the following equation (1).

W ₁ = remaining capacity (Ah) × V _D (1) However, V _D represents an initial setting value of the rated discharge voltage. This V
_{Since D} considers Joule heat loss due to the internal resistance of the battery, it is desirable to correct D based on the internal resistance R _m by the following method. That is, the average internal resistance R _m of each cycle is calculated by the following equation (2).

R _m = (W _c −W _D ) / (∫I _C (t) ² dt + ∫I _d (t) ² dt) (2) where W _c is the charging power amount of one cycle, W _D discharged electric power amount of one cycle, I _C (t) is the charge current (measured value), I
_d (t) represents discharge current (measured value).

Using R _m obtained by this evaluation, V _D can be corrected by the following equation (3). _{V D = V 0 -R m ×} I D ··· (3) where, V ₀ is the average discharge open voltage (set point), I _D represents the rated discharge current (set value).

Alternatively, the average internal resistance R _m can be calculated by the following equation (4).
It can also be calculated by R _m ≈ (V _0e −V _de ) / I _de (4) where V _0e is the open circuit voltage after discharge, V _de is the battery voltage at the end of discharge, and I _de is the current value at the end of discharge. .

This internal resistance R _m is obtained based on the previous charge / discharge cycle, and is used to calculate the discharge voltage V _D of the next charge / discharge cycle based on the value. Further, when the discharge conditions of each cycle are the same, it is also possible to directly obtain an appropriate discharge voltage V _D by the following equation (5).

V _D = W _D / ∫I _d (t) dt (5) Further, V _D varies depending on the discharge power, and the residual power amount also varies accordingly. Therefore, as shown in FIG. 3A, the relationship between the discharge power (W) and the discharge voltage (V _D ) can be graphed, or as shown in FIG. Then, it is desirable that the discharge voltage can be easily read from the discharge power and used for internal calculation.

Further, as shown in FIG. 4 (a), the relationship between the discharge power (W) and the remaining power amount (Wh) is graphed, and as shown in FIG. It is desirable for the user to see at a glance the amount of remaining power corresponding to the discharge power by using the table below.

In addition, the discharge voltage (V _D ) is the depth of discharge (A
Since it also differs depending on h), more accurate evaluation can be made possible by expressing the discharge voltage as a function of the depth of discharge. That is, FIG. 5 shows the relationship between the depth of discharge of the battery and the open circuit voltage. Then, V _D = V _D (q ₀ )
Then, evaluation is performed in the depth region from the current discharge depth (q ₀ ) to the end of discharge, and the residual power amount is evaluated using this V _D.

V _D (q) is preset as an initial setting function, and there are a method of estimating it from test data and a method of giving it universally by a theoretical formula. When the latter theoretical formula is used, the following procedure is performed, for example. That is, in the first embodiment, the relationship between the depth of discharge and the open circuit voltage has been theoretically shown, and the open circuit voltage at that depth is already obtained when the discharge is once completed.
Therefore, the average open circuit voltage V from the current depth to the end of discharge
_D (q ₀ ) can be easily calculated.

That is, the average open circuit voltage V _D (q ₀ ) from the current depth to the end of discharge can be evaluated by the following equation (6). _{V D (q 0) = V} 0D (q 0) -R m × I de ··· (6) average internal resistance R _m is also possible to use a correction value by several methods described above. I _de is the current value at the end of discharge as described above. The rated discharge current I _d may be used instead for the evaluation in a shallow depth region, but I suppose that the evaluation is performed at a relatively deep depth, the higher evaluation accuracy is obtained for I _de .

The current discharge depth is the total battery capacity (A
It is a value obtained by subtracting the remaining capacity (Ah) from h). When higher accuracy is required, it is effective to use the following equation (7) with the rated discharge power as P _D.

P _D = [V _0D (q) −R _m × I _d (q)] × I _d (q) (7) where q (function of time t) represents the depth of discharge, and It is obtained by performing arithmetic processing on the equation (7) and the following equations (8) and (9).

[0055]

[Equation 1]

[0056]

[Equation 2]

However, the Ah remaining capacity is a known value as described above, and te represents the dischargeable time from the current depth q ₀ . Therefore, the remaining electric energy (Wh) is expressed by the following equation (10).

[0058]

(Equation 3)

As described above, the remaining electric energy can be evaluated with high accuracy by performing a simulation arithmetic operation from the time point q = q ₀ (t = 0) to the end of the discharge.

As described above, in this embodiment, the remaining electric energy (Wh) can be measured easily and accurately.
Moreover, since the remaining power amount can be displayed, the remaining capacity (Ah) cannot easily grasp the dischargeable time when discharging with constant power, whereas
The dischargeable time can be immediately grasped.

In addition, by evaluating the remaining power amount (Wh), not only the deterioration of the battery based on the remaining capacity (Ah) but also the deterioration of the battery can be determined by considering the change in the internal resistance R _m . The deterioration of the battery can be determined more accurately.

Further, based on the current discharge depth, the discharge voltage and the discharge current after that are obtained, and the remaining power amount of the battery is determined, so that the battery remaining amount is made to correspond to the discharge voltage and the discharge current which change from time to time. The amount of power can be accurately determined.

The present invention may be embodied by changing the configuration as follows, for example. (1) The integration of the discharge current and the charging current is used in combination with each of the methods of the above-mentioned embodiments. In this way, the remaining capacity can be recognized in real time based on the integrated value during battery operation. (2) Perform voltage measurement with one block as one unit.
Therefore, the voltage measurement unit can be subdivided, and the remaining capacity determination and deterioration determination can be performed with higher accuracy. (3) The voltage is measured with a string in which batteries are connected in series as one unit. As a result, the remaining capacity, deterioration, etc. can be accurately determined.

Further, the technical idea understood from the above embodiment will be described below. (A) The residual power amount determination method for a sodium-sulfur battery according to claim 7, wherein the residual power amount can be displayed. With this configuration, the dischargeable time with constant power can be immediately grasped visually. (B) The method for determining the amount of remaining power of a sodium-sulfur battery according to claim 7, wherein the amount of remaining power is determined by also considering the internal resistance of the battery. According to this configuration, it is possible to more accurately determine the deterioration of the battery.

[0065]

As described in detail above, the following effects are exhibited in the present invention. In claims 1 and 2, the remaining capacity is determined irrespective of the use of the integrated value of the current, so that there is no error in the determination. Further, it is possible to determine the degree of deterioration or failure of the sodium-sulfur battery based on the remaining capacity.

According to the third aspect, the voltage is measured in the stable region, and the remaining capacity and the like are accurately determined. In claims 4 and 5, the total remaining capacity of the plurality of sodium-sulfur batteries is calculated on the basis of the minimum remaining capacity value, so that the sodium-sulfur battery can be operated with a margin, and Discharge can be prevented.

According to the sixth aspect, the sodium-sulfur battery can be prevented from being deteriorated or damaged without being operated in a state where the battery voltage exceeds the specified value. According to the invention described in claim 7, it is possible to easily and accurately obtain the remaining power amount, it is possible to easily know the dischargeable time by constant power from the remaining power amount, and the deterioration of the battery can be accurately performed. Can be determined.

According to the eighth aspect of the present invention, the remaining electric energy of the battery can be determined more accurately in correspondence with the discharge voltage and the discharge current which change from time to time.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a system of a sodium-sulfur battery of a first embodiment.

FIG. 2 is a graph showing the relationship between the depth of discharge and voltage.

FIG. 3A is a graph showing the relationship between discharge power and discharge voltage in the second embodiment, and FIG. 3B is a tabular diagram showing the relationship between discharge power and discharge voltage.

FIG. 4A is a graph showing a relationship between discharge power and residual power, and FIG. 4B is a tabular diagram showing a relationship between discharge power and residual power.

FIG. 5 is a graph showing the relationship between the depth of discharge and the open circuit voltage.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Unit which comprises a sodium-sulfur battery, 2 ... Module which comprises a unit, 3 ... Shunt resistance used for measuring a voltage, 6 ... Control device which performs residual capacity determination and charge / discharge control of a sodium-sulfur battery .

Front page continuation (72) Inventor Hiroyuki Abe No. 2-56 Sudamachi, Mizuho-ku, Nagoya City Insulator Japan Inc.

Claims (8)

[Claims] 1. Measuring the voltage of a sodium-sulfur battery,
A method for determining the remaining capacity of a sodium-sulfur battery, in which the depth of discharge is obtained from the measured value and the remaining capacity of the sodium-sulfur battery is determined from the depth of discharge. 2. The method for determining the remaining capacity of a sodium-sulfur battery according to claim 1, wherein the voltage is measured by using a plurality of sodium-sulfur batteries as one unit. 3. The method according to claim 1 or 2, wherein sodium-
A sulfur battery is discharged to a specified depth of discharge area, and the voltage is measured after a certain time has passed from the end of the discharge.
Method for determining remaining capacity of sulfur battery. 4. The plurality of sodium compounds according to claim 2,
The remaining capacity of the sulfur battery is determined, and the remaining capacity value of the sodium-sulfur battery showing the lowest remaining capacity among them is taken as the remaining capacity of the other sodium-sulfur battery, and based on that, the remaining capacity of the plurality of sodium-sulfur batteries A method for determining the remaining capacity of a sodium-sulfur battery for calculating the capacity. 5. The plurality of sodium compounds according to claim 4,
With the sulfur battery as one unit, the remaining capacity of each of the plurality of units is determined, and the remaining capacity value of the unit showing the lowest remaining capacity among the respective units is set as the remaining capacity of the other unit, and based on this, the remaining capacity of the plurality of units is determined. Remaining capacity determination method for calculating the remaining capacity. 6. The charge / discharge control method for a sodium-sulfur battery according to claim 1, wherein charge / discharge is stopped when the battery voltage reaches a specified value. 7. The method for determining the remaining power of a sodium-sulfur battery according to claim 1, wherein the remaining capacity of the sodium-sulfur battery is determined from the depth of discharge, and the remaining power is determined based on this remaining capacity and the discharge voltage. 8. The present invention according to claim 1, wherein the current discharge depth is obtained from the measured voltage, the discharge voltage and the discharge current after the current discharge depth are calculated based on the current discharge depth, and the residual electric energy of the sodium-sulfur battery is calculated. Method of determining remaining capacity of sodium-sulfur battery for determination.