Method and apparatus for diagnosing abnormal state of sodium (NA)--sulfur (S) cell

Disclosed is a method and apparatus which can detect progress of degradation of a sodium-sulfur cell and predict possibility of final breakdown of the cell in a very near future before such a final breakdown of the cell actually occurs. The method and apparatus for diagnosing the presence or absence of an abnormal state in the sodium-sulfur cell, containing a porous electrical conductive material impregnated with molten sulfur or sodium polysulfide as its positive electrode reactant and containing sodium as its negative electrode reactant, comprises observing the operating voltage of the cell during charging and discharging so as to detect occurrence of a minute ripple in the operating voltage, and, when the amplitude of the ripple and the frequency of occurrence of the ripple increases as a result of repeated cycles of charging and discharging, deciding that degradation of the cell has progressed to an extent that final breakdown of the cell will occur in a very near future.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method and apparatus for diagnosing a 
sodium-sulfur cell for the presence or absence of any abnormal state, and 
more particularly to a method and apparatus of the kind described above 
which can detect progress of degradation of a sodium-sulfur cell and 
predict possibility of catastrophic breakdown of the cell in a very near 
future before such a final breakdown of the cell actually occurs. 
2. Description of the Prior Art 
A sodium-sulfur cell (which will be often abbreviated hereinafter as an 
Na-S cell) is a secondary cell of a type operating at a high temperature. 
In the sodium-sulfur cell, molten sodium which is a reactant of a negative 
electrode is disposed on one side of an electrolyte tube permeable to 
sodium ions only, while molten sulfur which is a reactant of a positive 
electrode is disposed on the other side of the electrolyte tube. The 
sodium-sulfur cell is charged and discharged at a temperature as high as 
about 300.degree. to 350.degree. C. The following cell reaction occurs 
when the sodium-sulfur cell is charged and discharged: 
##STR1## 
That is, in the discharge mode, sodium turns into sodium ions by freeing 
electrons, and the sodium ions permeate the electrolyte tube, which is the 
separator, and react with sulfur to produce sodium polysulfide, Na.sub.2 
Sx, as a discharged product. On the other hand, in the charge mode, a 
voltage higher than the cell voltage is applied across the cell to cause a 
reaction reverse to that occurring in the discharge mode. 
FIG. 1 shows a practical structure of a prior art sodium-sulfur cell 9. 
Referring to FIG. 1, a bladder-like electrolyte tube 1 made of a material 
such as .beta.-alumina having an Na-ion conductivity is coaxially disposed 
in a positive electrode container (an outer electrode) 2 in the form of a 
cylindrical metal member having a closed bottom, in such a relation that a 
predetermined spacing a is maintained between the electrolyte tube 1 and 
the positive electrode container 2. The electrolyte tube 1 is joined or 
fixed at the outer periphery of its upper end opening 3 to a member 4 of 
an electrical insulator such as .alpha.-alumina ring in a cantilever 
fashion by a jointing member 5 such as glass solder. A negative electrode 
container 6 in the form of a cylindrical member having a closed top is 
disposed opposite to and fixed to the positive electrode container 2 
through the insulator member 4 to constitute the cell body. 
In the space b enclosed by the electrolyte tube 1 and negative electrode 
container 6, sodium acting as a negative reactant is filled together with 
porous metal fibers to constitute a negative electrode 7. 
On the other hand, in the space c enclosed by the electrolyte tube 1 and 
positive electrode container 2, porous carbon impregnated with sulfur 
acting as a positive reactant is filled to constitute a positive electrode 
8. The metal fibers filled in the space b to constitute the negative 
electrode 7 together with the sodium have a function of holding the sodium 
so as to prevent occurrence of a violent exothermic reaction between the 
sodium and the sulfur which direct reaction occurs when the electrolyte 
tube 1 is ruptured. Further, the carbon filled in the space c to 
constitute the positive electrode 8 acts to give an electron conductivity 
to the sulfur. That is, a graphite (carbon) mat is impregnated with 
sulfur. 
In order that the cell reaction can easily take place in the Na/S cell 
constructed in the manner described above, the wall thickness of the 
partitioning electrolyte tube 1 permeable to sodium ions is advantageously 
as small as possible. 
However, minute cracks tend to occur in the electrolyte tube 1 as the Na/S 
cell is repeatedly charged and discharged, and the cracks will develop to 
such an extent that the electrolyte tube 1 is finally ruptured. In such an 
event, the sodium directly reacts with the sulfur thereby destroying the 
function of the cell. 
Primarily, many sodium-sulfur cells are connected for the purpose of 
storage of electric power. Accordingly, impossibility of electric power 
supply or storage due to breakdown of any one of such sodium-sulfur cells 
greatly adversely affects the social life as a result of, for example, a 
sudden stoppage of electric power supply. 
JP-A-47-28431 (Japanese patent laid-open in 1972) is a newest publication 
relating to an Na/S cell. However, this known publication does not 
disclose any especial technique for diagnosing an abnormal state of the 
Na/S cell. That is, the known publication merely discloses the structure 
of an Na/S cell in which a conductive composite having a porosity of 50 to 
98% and a pore diameter of 10 to 1,000 .mu. is used to constitute its 
positive electrode. 
SUMMARY OF THE INVENTION 
In view of the circumstances described above, it is a main object of the 
present invention to provide a method of diagnosing the Na/S cell so that 
an abnormal state attributable to occurrence of minute cracks in an 
electrolyte tube due to progressive degradation of the cell as a result of 
repeated cycles of charging and discharging can be predicted and 
discovered before the cracks penetrate the electrolyte tube resulting 
finally in destruction of the Na/S cell. 
A further object of the present invention is to provide a cell diagnosing 
method which can simply predict the presence of an abnormal state in any 
one of Na/S cells constituting each of modules which are minimum 
controlled units of a power storage system, so that a module including a 
faulty Na/S cell can be bodily replaced by a sound one. 
A further another object of the present invention is to automatically stop 
the charge-discharge operation of the Na/S cell when the Na/S cell is 
detected abnormal. 
In accordance with the present invention which attains the objects 
described above, there are provided method and apparatus for diagnosing 
the presence or absence of an abnormal state in the sodium-sulfur cell 
containing a porous electrical conductive material impregnated with molten 
sulfur or sodium polysulfide as its positive electrode and containing 
sodium as its negative electrode, wherein the operating voltage of the 
sodium-sulfur cell during charging and discharging is observed to detect 
occurrence of a ripple in the operating voltage, and, when the amplitude 
of the ripple and the frequency of occurrence of the ripple increases as a 
result of cycles of repeated charging and discharging, the Na/S cell is 
decided that its degradation has progressed to the extent that 
catastrophic breakdown of the cell will occur in a very near future. 
By so observing and deciding, breakdown of the Na/S cell can be predicted 
before the Na/S cell is finally damaged (for example, at time earlier by 
several charge-discharge cycles than the time of actual breakdown). 
Therefore, an appropriate countermeasure of stopping the operation of the 
damaged Na/S cell and repairing or replacing the cell can be taken before 
actual breakdown occurs.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before describing preferred embodiments of the present invention, the basic 
principle of the present invention will be briefly described. 
The inventors made researches and studies on symptoms exhibited by an Na/S 
cell with progressive degradation of the cell toward the time of final 
breakdown, and measured and analyzed all of data which could be externally 
observed. As a result of the measurement and analysis, the inventors have 
discovered that, when the Na/S cell is charged and discharged, minute 
fluctuation is observed in the operating voltage of the cell. 
As a result of the analysis of vast data over a long period of time, the 
inventors have ascertained that the voltage fluctuation described above 
(that is, a voltage ripple) occurs immediately before the Na/S cell having 
been progressively degraded is finally damaged. 
Since this voltage fluctuation is considered to be related to deterioration 
and rupture of the electrolyte tube 1, the inventors have measured in 
detail the phenomena of voltage fluctuation under various operating 
conditions of the cell and investigated the features of the voltage 
fluctuation. 
FIG. 2 is a systematic diagram of a voltage ripple measuring apparatus 
which is a basic form of the present invention, FIG. 3 shows voltage 
ripples recorded by the x-t recorder shown in FIG. 2 to illustrate a 
tendency of occurrence of the voltage ripples, and FIG. 4 shows the 
voltage ripple waveforms displayed on an electromagnetic oscillograph. 
Referring to FIG. 2, an Na/S cell 9 is of a type operable at a high 
temperature of 300.degree. to 350.degree. C. and is placed in an electric 
furnace 10 where the cell 9 is heated by an electric heater 11 to be 
maintained at the operating temperature. 
The Na/S cell 9 is charged and discharged at an interval of a predetermined 
period of time by a charge-discharge apparatus (not shown) automatically 
repeating charging operation and discharging operation. The voltage, 
current and temperature of the Na/S cell 9 are detected by a voltage 
measuring terminal 13 of a cycler 12 (a charging and discharging device), 
an electric power supply cable 14 and a temperature sensor 15 respectively 
and recorded by a recorder 16. 
Besides this recorder 16, the operating voltage of the Na/S cell 9 is 
directly read by a voltmeter 17. Further, a d.c. amplifier 18 acts to 
drift a d.c. component of the operating voltage of the Na/S cell 9 and, at 
the same time, amplifies a ripple component of the operating voltage of 
the Na/S cell 9. The output of the d.c. amplifier 18 is applied to a 
low-pass filter 19 which removes a power-source noise component from the 
input, and the ripple component only of the operating voltage of the Na/S 
cell 9 is recorded by an electromagnetic oscillograph 20. 
At the same time, the output voltage of the Na/S cell 9 is passed through a 
capacitor C which acts to remove the d.c. component therefrom, and the 
output of the capacitor C is applied to an x-t recorder 21 which records a 
tendency of appearance of voltage ripples during charging and discharging 
the Na/S cell 9. 
FIG. 3 shows voltage fluctuations recorded by the x-t recorder 21, and FIG. 
4 shows the fluctuating voltage waveforms recorded by the electromagnetic 
oscillograph 20. 
From FIGS. 3 and 4, the following facts have been found: 
(1) A minute fluctuation of the operating voltage starts to be observed 
from the time earlier by several cycles than the time of final breakdown 
of the Na/S cell 9. The amplitude of the fluctuations and the frequency of 
occurrence of the fluctuations increase progressively towards the time of 
final breakdown of the Na/S cell 9. 
(2) The tendency of occurrence of the voltage fluctuations during charging 
the cell 9 is more marked than that during discharging the cell 9. 
Further, the voltage fluctuation is almost not observable in the 
open-circuit condition between the charging operation and the discharging 
operation immediately before the final breakdown of the cell 9 takes 
place. 
(3) The voltage fluctuation has a pulse waveform, and the pulse width of a 
typical waveform is about 50 msec. 
(4) After the final breakdown of the cell 9, a voltage variation considered 
to be attributable to cracking of the electrolyte tube 1 occurs even in 
the open-circuit condition. Further, although not shown, a greater voltage 
variation occurs at a temperature in the vicinity of the freezing point of 
the reactant as the temperature goes down after the final breakdown of the 
cell 9. 
Although the mechanism of occurrence of such a voltage variation is not 
clear at the present time, the inventors consider that this mechanism has 
some relation with the progress of formation of minute cracks in the 
electrolyte tube 1. 
Thus, by measuring these phenomena, the time of final breakdown of the 
sodium-sulfur cell can be easily and accurately predicted. 
An embodiment of the present invention will now be described with reference 
to FIG. 5 which shows an electric power storage system using Na/S cells. 
Referring to FIG. 5, a plurality of Na/S cells 9 are connected in parallel 
to each other to constitute a submodule 22. A plurality of such submodules 
22 are connected in series with each other, and a thermo-couple 23, a 
heater 24 and a heat insulating member 25 are combined with the submodules 
22 to constitute a module 26. A plurality of such modules 26 are connected 
in series with each other to constitute a unit bank 27 of an electric 
power storage system. 
In order that the temperature measured by the thermo-couple 23 installed in 
each module 26 can be maintained constant, the heater 24 is on-off 
controlled, and the rotation speed of a blower 28 is also controlled. 
According to circumstances, the current of a DC/AC inverter 29 is 
controlled. 
The Na/S cells 9 in the modules 26 constituting the unit bank 27 of the 
electric power storage system are charged with direct current and 
discharged with direct current. The d.c. output voltage of the unit 27 is 
converted by the Dc/Ac inverter 29 into an a.c. voltage, and this a.c. 
voltage is supplied to an electric power system after being boosted by a 
transformer 30. 
A volt-meter 31 is connected across each of the modules 26 which are 
smallest units to be controlled, and a minute fluctuation of the operating 
voltage during charging and discharging the module 26 is observed. By 
observing such a minute voltage fluctuation, breakdown of any one of the 
Na/S cells 9 constituting the module 26 and a symptom of the cell 
breakdown can be easily predicted. 
For the purpose of concretely detecting the symptom of the breakdown of any 
one of the Na/S cells 9 on the basis of the observed minute fluctuation of 
the cell operating voltage, a ripple component having a level exceeding a 
predetermined pulse peak value is detected by a pulse detector 32 from the 
output of the volt-meter 31, and the frequency of appearance of such 
ripple components or pulses within a predetermined period of time is 
counted. 
When the frequency of appearance of such pulses exceeds a limit, the 
presence of a damaged Na/S cell 9 is identified by an alarm generator 33 
connected to the pulse detector 32, and a switch 34 is opened to stop 
charging or discharging the Na/S cells 9. 
By taking an appropriate measure as described above, the module 26 
including the damaged Na/S cell 9 can be repaired or replaced before 
breakdown of many Na/S cells 9 occurs, so that the reliability of the 
entire power storage system using the Na/S cells can be improved. 
According to the method and apparatus of the present invention described in 
detail above, breakdown of an Na/S cell which has been degraded due to 
repeated charging and discharging can be predicted before its final 
breakdown occurs. Because of the practical effect described above, the 
present invention contributes greatly to improvements in the reliability 
of an electric power storage system using Na/S cells. 
Further, according to the method and apparatus of the present invention, 
the presence or absence of a damaged Na/S cell in each of modules which 
are smallest controlled units of an electric power storage system can be 
simply predicted, so that the module including the damaged Na/S cell can 
only be replaced without requiring expensive detecting means. 
Furthermore, according to the method and apparatus of the present 
invention, the operation for charging or discharging the Na/S cells can be 
automatically stopped when a damaged Na/S cell is detected.