WO1987007982A1

WO1987007982A1 - A method of proving a battery of alkali metal cells

Info

Publication number: WO1987007982A1
Application number: PCT/GB1987/000426
Authority: WO
Inventors: Peter John Bindin
Original assignee: Chloride Silent Power Limited
Priority date: 1986-06-23
Filing date: 1987-06-18
Publication date: 1987-12-30
Also published as: GB2193370A; ZA874477B; GB8714314D0; GB8615256D0

Abstract

A battery of sodium sulphur cells has series connected banks of parallel connected cells. Each cell has a series connected fuse. The battery is proved by first fully recharing a bank of cells and then applying a higher proving voltage sufficient to blow a fuse in series with a faulty cell.

Description

A METHOD OF PROVING A BATTERY OF ALKALI METAL CELLS

The present invention relates to a method of proving batteries of alkali metal cells.

Known alkali metal cells include sodium sulphur cells employing liquid sodium metal as the anode and sulphur/sodium polysulphides as the cathode, the two being separated by a solid electrolyte material, typically of beta alumina.

Problems arise when these cells are connected into batteries. Alkali metal cells and particularly sodium sulphur cells become high resistance when over- discharged, when completely charged or on failure involving fracture of the solid electrolyte. Thus, in order to ensure that most of the cells in a battery receive a full charge during a charging cycle, it is an established practice to connect the cells in the battery as several banks of cells in a parallel or series/parallel configuration. Strings of series connected cells in the battery are kept to a minimum number of cells to reduce the effect of a single cell becoming high resistance. It will be appreciated that one cell of a series connected string becoming high resistance effectively prevents any further charging or discharging current flowing through the other cells in the particular string.

However there are disadvantages in taking this parallel connecting of cells to the extreme, i.e. building a battery in which there are no cells which are connected only in series with other cells. Such a battery would comprise banks of individual cells all connected in parallel with one another, the various banks being then connected in series to provide the necessary output voltage for the complete battery.

With such an arrangement, failure of one cell in a bank, resulting in that cell becoming relatively low resistance, would provide a discharge path for all the other parallel connected cells in the bank, resulting in the entire bank becoming discharged. The present invention is concerned with providing an arrangement which will permit batteries to be made which include banks of single cells all connected in parallel, but reducing the attendant disadvantages. The invention is also applicable to batteries comprising one or more banks of parallel connected strings of series connected cells

The present invention provides a method of proving a battery of alkali metal cells including at least one bank of parallel connected single cells or parallel connected strings of series connected cells, each single cell or string of the bank having a series connected fuse arranged to fail in response to a current through the fuse which is at or above a predetermined value for a predetermined minimum time, said predetermined value being greater than the maximum operating current of the cell during normal discharging or charging cycles, wherein the or each said bank of parallel connected single cells or strings is charged to bring all cells in the bank to full charge by applying to the bank a charge voltage selected to produce through each healthy single cell or string of the bank a charging current which is less than said predetermined value of current; and, after charging, a proving voltage in excess of the charge voltage is then applied to the bank for at least said predetermined minimum time, said proving voltage being selected to produce through any failed single cell or string of failed cells of the bank a charge current which is at or above said predetermined value of current, so as to cause failure of the fuse in series with the failed cell or string. For the purposes of this document, the process of proving a battery involves testing the battery to determine that all cells are healthy or alternatively to isolate failed cells within the battery. It will be appreciated that if occasional cells within a large battery are isolated, this may have a relatively small effect on the overall capacity and operational value of the battery as a whole.

Proving of the battery as described causes any failed cells or strings of failed cells in each bank of the battery to be isolated from the other cells or strings of the battery, without any deleterious effect on healthy cells. This is because healthy cells become high resistance when they are fully charged, so that application of the proving voltage to a healthy cell or string results in negligible current through the cells and no damage to the cells.

The invention is especially suitable when applied to batteries containing banks of parallel connected single cells.

It will be appreciated that the wires interconnecting the parallel connected cells of a bank must be capable of carrying at least the typical charge current produced through a failed cell during application of the proving voltage. In fact, in case there may be more than one failed cell in a particular bank, the interconnection wires should be capable of handling at least twice and preferably about five times the typical charging current through a failed cell under the proving voltage. Where there are a large number of cells in a battery, such as for a load levelling application, a greater handling capacity would be necessary to allow for greater expectancy of cell failure.

In a typical example of the battery, the fuse connected in series with each cell of the parallel connected bank of cells may be arranged to fail in response to a current in excess of 30 amps for at least 20 seconds. Then, application of the proving voltage of 10 volts to the bank of cells will result in a charge current causing failure of the fuse in series with a failed cell which has a leakage resistance less than about one quarter ohm.

The proving voltage applied to each bank of cells may be 20 volts or more. It has been established that failed cells which still show a relatively high resistance at low charge voltages will develop a low resistance when a proving voltage as high as 20 volts is applied across the cell, resulting in failure of the series connected fuse.

Examples of the present invention will now be described in more detail and with reference to the accompanying drawings in which:

Figure 1 is a schematic representation of a battery of alkali metal cells embodying the present invention; and

Figures 2 and 3 are views partially in cross- section of a design of fuse which may be incorporated in the battery of Figure 1. Referring firstly to Figure 1, a battery is illustrated comprising fully parallel interconnected banks of cells. Thus, the battery comprises a series of banks 10, 11... of cells 12 with the cells in each bank connected to each other in parallel. The cells in bank 10 are connected in parallel between busbars

13 and 14, whereas the cells in bank 11 are connected in parallel by means of busbars 14 and 15. The various banks of cells are themselves connected in series so that the complete battery provides the desired output voltage between the output terminals 16 and 17. A fuse 18 is connected in series with each cell 12.

In normal operation of the battery, discharge current drawn from terminals 16 and 17 results in discharge current flowing from each cell 12 through its series connected fuse 18. The battery will have a maximum discharge current limit so that a corresponding maximum discharge current from each cell 12 can be determined. The fuse 18 is designed so as not to blow in response to such a normal maximum discharge current.

Similarly, the cells of the battery can be recharged and in the present case the battery is designed to enable each parallel connected bank 10, 11... of cells to be topped -up independently by supply of a charging current directly to the busbars between which the cells of the bank are connected. Thus, bank 10 of cells can be topped-up by connected a charging current between terminal 16 of busbar 13 and a terminal 19 of busbar 14. Bank 11 of cells is topped- up by applying the current between terminal 19 of busbar 14 and a terminal 20 of busbar 15. When recharging, the whole battery is first given its main charge by connecting a suitable source of charging current to terminals 16 and 17. Then, the banks are topped-up one by one.

In accordance with the normal requirements of alkali metal cells, there is a maximum rate at which the cells should be recharged to avoid damage to the cell, and therefore a maximum charging current to each cell. The fuses 18 in series with the cells are arranged so as not to blow in response to this maximum normal charging current applied to its associated cell, so long as that cell is healthy.

In summary therefore the fuses 18 are selected so as to fail only in response to a current through the fuse which is at or above a predetermined value and for a predetermined minimum time, where this predetermined value is greater than the maximum discharge or charging current of a healthy cell during normal operation. As mentioned previously, a battery made in this way can be "proved" to isolate failed cells within the battery. In order to prove the battery, each bank 10, 11 etc. of cells is first brought to full charge. It is a feature of alkali metal cells, and particularly sodium sulphur cells, that at the top of charge each cell becomes substantially high resistance. By topping -up each bank of cells in the battery separately, it can be insured that all healthy cells in the battery are brought to top of charge at which they become high resistance.

Then, a charging voltage substantially in excess of that normally applied during a charging cycle is applied to each bank of cells for a period of time longer than the above mentioned predetermined minimum time for a fuse to blow. This "proving voltage" is set high enough so that any faulty or failed cell in the bank will conduct a current in excess of the aforementioned predetermined value, so that the series connected fuse associated with the failed cell will in due course itself fail, isolating the cell.

By isolating faulty or failed cells within the battery by this technique, the useful life of the battery can be substantially extended between periods when the battery must be dismantled for replacement of failed cells. It should be appreciated that with the complete parallel interconnection of cells in the battery, as illustrated in Figure 1, the isolation of a small number of cells in a battery which may comprise many tens or even hundred of cells, has a relatively small effect on the overall capacity and performance of the battery.

On the other hand, failed cells in a battery eventually become relatively low resistance, so that the presence of a failed cell which is not isolated provides a discharge route for all the cells which are in parallel with the failed cell in the same bank of cells. It can be seen therefore that failure of one cell can effectively discharge an entire bank of cells, reducing the output voltage of the battery and the remaining capacity of the battery. Indeed, sodium sulphur cells when fully discharged also become high resistance, so that discharge of a bank of cells in the battery of Figure 1 would result in the entire battery becoming inoperative, unless a short circuit is provided around the bank containing the failed cell.

In practice, with typical sodium sulphur cells forming the battery, the cells may be fitted with a fuse which is designed to blow in response to a current of between 30 to 60 amps applied for a period of 20 seconds. Then a proving voltage of 10 volts applied to the bank for at least 20 seconds would normally result in a current flowing through a failed cell sufficient to blow its associated fuse.

It should be appreciated that the busbars and interconnection wires of the cell must be themselves capable of carrying currents likely to flow during the battery proving operation. It may therefore be necessary to provide interconnection wires and busbars capable of carrying 100 to 200 amps for 20 seconds, to allow for the possibility of more than one failed cell in a single bank being proved.

In the case of sodium sulphur cells, higher proving voltages can be desirable of as much as 20 volts or more. It has been established that the application of a charging voltage of 20 volts to a fully charged sodium sulphur cell has no damaging effect on the cell. However, the application of a proving voltage of this magnitude can be desirable in isolating damaged cells which nevertheless still show a relatively high resistance under lower charge voltages. Then a proving voltage of 20 volts or more causes the damaged cell to develop a low resistance resulting in blowing of its associated fuse to isolate the cell. A proving voltage of 20 volts may be applied to each bank of cells in turn for a period of typically 1 minute to ensure that any damaged or failed cells are duly isolated.

It may be appreciated that a number of designs of fuses can be contemplated for use as fuses 18 in the battery of Figure 1. A particular example of fuse is illustrated in Figures 2 and 3 and comprises a glass tube 21 having a cap 22 at one end of insulating material, typically alumina, and a tie pin 23 located transversely across the width of the tube at the other end. A length of fuse wire 24 extends through a first hole in the cap 22 into the tube 21, forms a loop through a connecting plate 25 and extends out of the tube again through a second hole in the cap. The ends 26 and 27 of the fuse wire 24 are connected respectively to a busbar of the battery (e.g. busbars 13, 14 in Figure 1), and to one terminal of the associated cell.

The connecting plate 25, which may be made of clay, is pulled towards the tie pin 23 by means of a tension spring 28, typically of stainless steel, so as to hold the loop of fuse wire 24 within the glass tube

21 under tension. When sufficient current flows through the fuse wire 24 for long enough to blow the fuse, the fuse wire 24 breaks in the region between the cap 22 and the connecting plate 25 allowing the spring 28 to draw the connecting plate 25 towards the tie pin 23. Other structures and forms of fuse may also be used in embodiments of the present invention providing that they have the desired characteristics as described above. For example, the cells may themselves be formed with internal fuse devices which are effective to isolate the terminals of the cell from each other on passage through the cell, at least in the direction of charging of the cell of a current in excess of that experienced by a healthy cell during a normal recharging cycle.

In a further embodiment, the battery to be proved is formed of banks of parallel connected strings of series connected cells. A fuse is provided connected in series with each string and busbars enable charging voltage to be applied independently to each bank. If a single cell in a string should fail low resistance, the remaining strings of the bank will supply charging current through the string with the failed cell until a healthy cell in that string reaches top of charge and goes open circuit isolating the string. Thus the series connected cells of each string protect the rest of the bank. However, in the event that all cells of the string fail, then applying the proving voltage ensures blowing of the fuse to isolate the string.

Claims

1. A method of proving a battery of alkali metal cells including at least one bank of parallel connected single cells or parallel connected strings of series connected cells, each single cell or string of the bank having a series connected fuse arranged to fail in response to a current through the fuse which is at or above a predetermined value for a predetermined minimum time, said predetermined value being greater than the maximum operating current of the cell during normal discharging or charging cycles, wherein the or each said bank of parallel connected single cells or strings is charged to bring all cells in the bank to full charge by applying to the bank a charge voltage selected to produce through each healthy single cell or string of the bank a charging current which is less than said predetermined value of current; and, after charging, a proving voltage in excess of the charge voltage is then applied to the bank for at least said predetermined minimum time, said proving voltage being selected to produce through any failed single cell or string of failed cells of tthe bank a charge current which is at or above said predetermined value of current, so as to cause failure of the fuse in series with the failed cell or string.

2. A method as claimed in claim 1 wherein the cells are sodium sulphur cells and the proving voltage is at least 10 volts.

3. A method as claimed in claim 2 wherein the proving voltage is at least 20 volts.

4. A method as claimed in any preceding claim wherein the proving voltage is applied for at least 30 seconds.

5. A method as claimed in claim 4 wherein the proving voltage is applied for at least 1 minute.