GB2295718A

GB2295718A - Arrangements of batteries comprising an array of cells interconnected to give the required energy storage/operational voltage

Info

Publication number: GB2295718A
Application number: GB9424441A
Authority: GB
Inventors: John Molyneux
Original assignee: SILENT POWER GmbH
Current assignee: SILENT POWER GmbH
Priority date: 1994-12-02
Filing date: 1994-12-02
Publication date: 1996-06-05
Also published as: GB9424441D0

Abstract

In one arrangement, a layer of electrochemical cells e.g. sodium-sulphur cells is arranged as at least one two-dimensional array of rows and columns of cells, each cell having one end at which both cell terminals are provided and the array being one cell thick with all of the cells having the same relative orientation whereby all the cell terminals are accessible from one surface of the array, the cells in the array being electrically connected to provide a plurality of strings of series-connected cells, the strings being connected in parallel with each string having the same number of cells to provide a desired operational voltage with intermediate cross-connections being provided between points intended to be of equal voltage in the strings at at least one selected intermediate voltage. The series connections between adjacent cells in each string have series current carrying capacity at least sufficient for the maximum rated charge and discharge currents for the string. At least some of the intermediate cross-connections between points intended to be of equal voltage have an intermediate cross-connection current carrying capacity less than said series current carrying capacity. In a further arrangement, cells having a defined orientation with a top and a base and a height no greater than three times the width of the cell are arranged in layers each comprising at least one two-dimensional array of rows and columns of cells. Each array and each layer have a height of one cell and at least one layer is stacked on top of another layer. A panel effective as a heat sink or as a heat source may be arranged interadjacent two layers of electrochemical cells. A matrix e.g. silica hydrogel may be provided for holding the cells in the array during assembly. <IMAGE>

Description

EATIERIES The present invention relates to batteries, and in particular, to composite batteries comprising at least one array of electrochemical cells interconnected as necessary to give a required energy storage capacity and desired operational voltage. The present invention is particularly applicable to secondary batteries, such as batteries made of sodium sulphur cells, which may be used for starting automobiles, powering forklift trucks and electric vehicles and providing standby power for building and telephone exchange equipment.

W089/00344 (Chloride Silent Power) discloses a battery construction formed of sodium sulphur cells. Three cells (practical arrangements may use four cells) are stacked on top of one another and connected in series to form a "string" of cells. A number of such strings are arranged between a pair of electrically conductive bus plates. Each bus plate serves to interconnect corresponding terminals at the ends of the strings, thereby electrically connecting the strings in parallel. The strings are arranged in mutually staggered rows with corrugated insulating separators arranged between the rows.

The separators serve to electrically- insulate the cells of the strings in adjacent rows from each other while maintaining the strings in their positions in the complete array. The number of cells in each string determines the operational voltage of the array while the number of cells in the array (and their interconnection) determines the storage capacity of the array. If required, two or more arrays may be arranged in side-by-side relationship and connected in series to provide a battery having an operational voltage which is a multiple of the operational voltage of an array. A heat sink may be provided in thermal connection with one of the bus plates of the array (or each array in a multi-array battery).

It is an object of the present invention to provide an improved battery construction.

According to a first aspect of the present invention, there is provided a battery comprising a layer of electrochemical cells arranged as at least one two-dimensional array of rows and columns of cells, each cell having one end at which both cell terminals are provided, the array being one cell thick with all the cells having the same relative orientation whereby all the cell terminals are accessible from one surface of the array, cells in the array being electrically connected to provide a plurality of strings of series-connected cells, the strings being connected in parallel with each string having the same number of cells to provide a desired operational voltage with intermediate cross-connections being provided between points intended to be of equal voltage in the strips at at least one selected intermediate voltage.

In contrast to the prior art, the first aspect of the present invention provides an arrangement of cells in which all the cell terminals are easily connected to one another, enabling intermediate cross-connections to be provided between points in the strings at at least one selected intermediate voltage at which equalisation of voltage is required. The intermediate cross-connections also enable a failed cell to be bypassed so that failure of one cell in a string does not mean that the whole string is taken out of the battery.

The cells may be cylindrical and arranged in a hexagonal arrangement in which the majority of cells have six adjacent cells.

The layer may comprise a plurality of arrays electrically connected in series. This allows a number of arrays of a structurally convenient size to be connected together to form a layer of larger cross-sectional area (which might otherwise be difficult to handle) of the desired operational voltage. In particular, the layer may be formed of a plurality of arrays of standard size and voltage while, if necessary, non-standard arrays may be used in the layer if the required battery operational voltage is not an integer multiple of the voltage of the standard arrays. Conveniently, the battery may comprise a plurality of layers physically arranged parallel to one another and electrically connected in series.

- As the intermediate cross-connections are provided between points intended to be of equal voltage in the strings at at least one selected intermediate voltage, the intermediate cross-connections are required only to carry current due to imbalance in voltages of different strings.

Thus, - although the series connections between adjacent cells in each string must have a series current carrying capacity at least sufficient for the maximum rated charge and discharge currents for the string, at least some of the intermediate cross-connections between points intended to be of equal voltage may have an intermediate cross-connection current carrying capacity less than said series current carrying capacity and, indeed, less than that required for said maximum rated charge and discharge currents for the storing. In addition, the resistance of said intermediate cross-connections may be optimised without compromising battery power as the intermediate cross-connections are not in the main current paths in the battery.

Advantageously, at least some of said intermediate cross-connections-are adapted to limit the current flowing therethrough. If a cell or group of cells is damaged and caused to short-circuit, this current limiting feature of the intermediate cross-connections means that the current flowing from the damaged cell or group of cells is limited, alleviating the problem of excess current causing other cells in the battery to fail. Preferably, the intermediate cross-connections are fusible at a current limit and so prevent current flow above said current limit.

A second aspect of the present invention therefore provides a battery comprising at least one array of electrochemical cells, the array comprising a plurality of strings of series-connected cells, the strings being connected in parallel with each string having the same number of cells to provide a desired operational voltage and intermediate cross-connections being provided between points intended to be of equal voltage in the strings at at least one selected intermediate voltage, the series connections between adjacent cells in each string having a series current carrying capacity at least sufficient for the maximum rated charge and discharge currents for the string and at least some of the intermediate cross-connections between points intended to be of equal voltage having an intermediate cross-connection current carrying capacity less than said series current carrying capacity.

A third aspect of the present invention provides a battery comprising a plurality of layers of electrochemical cells, each layer being arranged as at least one two-dimensional array of rows and columns of cells, each cell having a defined orientation with a top and a base and a height no more than three times the width of the cell each array and each layer having a height of one cell, at least one layer being stacked on top of another layer.

One particular advantage of the first aspect of the present invention is that the arrangement of the cells in layers, each layer having a height of one cell, allows a panel effective at a heat sink or as a heat source to be arranged interjacent two layers of cells. This arrangement provides for greater control and greater uniformity of temperature across the battery.

Thus, a fourth aspect of the present invention provides a battery comprising two layers of electrochemical cells, each layer being arranged as at least one two-dimensional array of rows and columns of cells, each cell having a defined orientation with a top and a base, each array and each layer having a height of one cell, said layers being stacked one on top of the other and the battery further comprising a panel effective as a heat sink or as a heat source, said panel being arranged interjacent said two layers.

Advantageously, for a battery comprising at least three layers of electrochemical cells stacked one on top of the other, a sufficient number of such panels are provided such that each layer of cells is separated from a panel by a distance less than the height of one cell.

The fourth aspect of the present invention is particularly applicable to panels effective as heat sinks to avoid excessive temperature rise during operation of the battery. Thermal insulation may be provided interjacent a heat sink and a layer of electrochemical cells to provide a region in which the thermal gradient required for cooling can be established.- The heat sink may comprise a cooling duct having a serpentine pattern and means-for introducing a coolant into the cooling duct.

For batteries of high temperature cells, such as sodium sulphur cells operational at 3500C, the panel may comprise a source of heat by which the battery may be brought from room temperature to its operating temperature.

The battery therefore advantageously includes a temperature sensorfor sensing the temperature of the cells.

A fifth aspect of the present invention provides a battery comprising a plurality of electrochemical cells arranged as at least one array of cells and a matrix for holding the cells in the array in a position fixed during assembly to form a solid array of cells, the matrix comprising a silicate hydrogel.

Silicate hydrogels provide a relatively rigid material capable of cementing components, such as cells, together but which offers little resistance to liquid diffusion and so is able to absorb any liquid which might escape from a cell if damaged. This advantage is particularly applicable to batteries comprising sodium sulphur cells as the silicate hydrogel is absorbent of sulphur vapour and sulphur liquid.

Advantageously, the silicate hydrogel includes sodium silicate.

The silicate hydrogel may be chemically set, advantageously by addition of an organic ester. The array of cells may therefore be conveniently handled during manufacture before it is inserted into the battery.

The silicate hydrogel advantageously includes an alumina silicate, preferably an alumina silicate derived from pulverized fly-ash. Such an alumina silicate can be found in the form of microspheres filled with carbon dioxide or nitrogen gas and therefore has a relatively low density with the weight advantage this provides in the finished battery. Other types of coarse-grained filler may be used.

To ensure that the electrochemical cells are electrically insulated from one another, a sheet of electrical insulation, such as mica, may be provided around each cell. This feature also provides the advantage that the matrix adheres to the electrically insulating sheet and so individual cells may be extracted from the matrix for repair or replacement as required.

The matrix is preferably derived from a mixture of alumina silicate powder, sodium silicate solution and an organic ester.

Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 shows a schematic plan view of at least part of a layer of electrochemical cells for an embodiment of a battery in accordance with the present invention; Figure 2 shows schematically two of the layers of Figure 1; Figure 3 shows a layer comprising a plurality of arrays for an embodiment of a battery in accordance with the present invention.

Figure 4 shows schematically electrical connections in a prior art battery construction; Figure 5 shows schematically electrical connections in an embodiment of a battery provided in accordance with the present invention; Figure 6 shows a plan view of a layer of electrochemical cells for an embodiment of a battery in accordance with the present invention; Figure 7 shows a side view of part of a battery having two layers provided in accordance with an embodiment of the present invention and Figure 7a shows, in greater detail, the interface of the two layers of Figure 7, the interface including a heat sink; and Figure 8 shows a plan view of the heat sink of Figure 7.

Figure 1 shows a plurality of electrochemical cells 2, each cell 2 being provided with both the positive and negative terminals 4a, 4b at one end of the cell. The cells 2 are arranged to have the same relative orientation so that all the cell terminals are accessible from one side or surface of the layer. Each layer comprises at least one two-dimensional array of rows and columns of cells. As shown, the cells have a cylindrical cross-section and are conveniently arranged in a hexagonal arrangement, in which the cells are arranged in mutually staggered rows (with the columns having a zig-zag configuration), and the majority of cells have six adjacent (or nearest neighbour) cells, although other arrangements such as orthogonal rectilinear rows and columns are alse possible.A sufficient number of cells are electrically connected in series to form a string 6a of cells having a desired operational voltage. Figure 1 shows, for example, 14 cells connected in series in each string 6a, 6b, 6c..., providing an operational voltage of about 29 volts if the cells are sodium sulphur cells each giving a voltage of 2.076 volts. The ends of the strings 6a, 6b, 6c... are electrically connected in parallel, as indicated at 8a and 8b to provide the positive and negative terminals of the array.

As shown in Figure 2, two or more layers 10, 12, each layer being one cell thick, may be stacked on top of one another to form a battery. The layers 10, 12 may be connected in series, to provide a battery having an operational voltage which is the sum of the operational voltages of the layers 10, 12. This feature is particularly advantageous with short cells having a height no more than three times the width of the cell such as sodium sulphur cells having a height no more than twice the width of the cell or, indeed, equal to the width of the cell.

Figure 3 shows the layer 10 as being comprised of six arrays 24, 25, 26, 27, 28, 29 electrically connected in series and having positive and negative terminals. Four of the arrays 24, 25, 26, 27, are standard arrays having a standard size and shape and a standard voltage of e.g. 50 volts. During the design of the battery, it may be established that the required operational voltage of the battery is not an integer multiple of the standard voltage. However, as the arrays are designed so that cell terminals may be easily connected to one another, it is straightforward to include, in the design of the battery, two arrays 28, 29 having a voltage such as to make up the required operational voltage of the layer. The layer 10 may then be connected in series with an identical layer 12 to form a battery having the required operational voltage.

In a hypothetical example, the required operational voltage of the battery is 240 volts. If the battery is made of two layers electrically connected in series, then each layer must have an operational voltage of 120 volts.

Each such layer may therefore be made of two standard arrays having standard voltages of 50 volts and a non-standard array having a voltage of 20 volts. Thus, the present invention allows for flexibility in the design of batteries to meet the voltage requirements of different uses.

Figure 1 shows, in addition to the parallel connections 8a, 8b at the ends of the strings, intermediate parallel cross-connections 14, 16 connecting in parallel points intended to be of equal voltage in the strings. For ease of reference in the accompanying description, sections of strings between parallel connections of the array will be termed sub-strings 18a, 20a, 22a for the string 6a etc.

Current flow through a prior art battery construction and through the construction of Figure 1 will be described with respect to Figures 4 and 5 of the accompanying drawings.

Figure 4 shows a prior art arrangement of first and second arrays 30, 32 of cells connected in series. First array 30 comprises a plurality of strings of cells A, C, D, E, Fj G, H..., the ends of which are connected in parallel by electrically conductive bus plates. Each string of cells comprises a number of cells, typically four for a sodium sulphur battery, connected in series. Each string A therefore connects directly (as shown in solid line) to typically six nearest neighbour parallel strings and indirectly (dotted lines) to all the other strings within the array via several paths in the bus plate. Current flow from string A to its next series neighbour B in second array 32 is via the bus plate and the array inter-connector plate.This type of electrical connection has the advantage, if a string fails open circuit, that the series current load - of the failed string is distributed approximately uniformly over all other strings within the array i.e. if A fails open circuit then the current from B would be shared between all the other strings C, D...H....

However, this ability -to current share makes parallel electrical flow within the array particularly difficult to restrain. For example, if a cell in the string B fails short circuit, then the voltage across this string is decreased and becomes less than the voltage across its nearest neighbour strings. The consequence of this voltage imbalance is that additional current will flow from all the other strings C. H in the adjacent array through the string B as well as current from the next series neighbour A. Should a number of cells fail short circuit, e.g. due to structural damage of the cells as a result of impact, this could lead to excess current flowing through the damaged strings leading to overheating and a potential fire risk.

In view of the problems of restraining current flow through the bus plates, prior art attempts to deal with this problem have concentrated on the provision of a fuse or current limiter within the series connections of the strings Such fuses" must be capable of operating at some 20% above nominal string discharge currents to cope with the variations in current flow during the lifetime of the cells. Conventional electrical fuses are not capable of discriminating between such expected variation in current flow and the circulating currents seen during battery failure. Also, the fuse is in series with the main current path in the battery and so must be of a low resistance.

Figure 5 shows schematically electrical connections between what have been termed sub-strings in the battery of Figure 1. Each sub-string A in the array has two nearest parallel neighbours C, D which are singly connected i.e. if the connection 34a between sub-string A and sub-string C is broken, there is no other current path by which current may flow from sub-string A to sub-string C. In addition, the parallel connections 34a, 34b between sub-string A and its nearest parallel neighbours C, D does not form part of the series connection 36b to the series neighbour, sub-string B. Thus, the electrical connection of Figure 5 provides the possibility of separating the series and parallel current paths.A theoretical disadvantage is that uniform current sharing is less likely and so, if sub-string A failed open circuit, the series current from B would flow predominantly through C and D such that the series current through these sub-strings could rise by up to 50%. In practice, it has been found that the current rise due to sub-string failure open circuit is not so severe.

One advantage of the electrical connections shown in Figure 5 is that current limitation or fusing can be provided in at least some (or all) of the parallel current paths without interfering dramatically with the resistance of the battery. The series connections, 36a, 36b must have a series current carrying capacity at least sufficient for the maximum rated charge and discharge currents for the string. The parallel connections at the ends of the strings through which the strings are connected in series to the terminals of the array or battery must have a current carrying capacity at least sufficient for the maximum rated charge and discharge currents for the array or battery.However, the intermediate cross-connection current carrying capacity of the intermediate cross-connections 34a, 34b need -only be sufficient to cope with-imbalance flow, not power flow, and can therefore be less than the series current carrying capacity of the series connections 36a, 36b and indeed less than the maximum rated charge- and discharge currents for the string. The discrimination of any fusing provided in such intermediate cross-connections having an intermediate cross-connection carrying capacity less than the series current carrying capacity is thereby increased. Typically, the series current carrying capacity needs to be three times the expected maximum rated charge and discharge currents for the string.The required intermediate cross-connection current carrying capacity is more difficult to formulate as it will depend on the configuration of the battery and the position of the intermediate cross-connection within the battery.

Typically, a battery . requiring series-connections with series current carrying~capacity of 110 to 150 A may be able to use intermediate cross-connections having an intermediate cross-connection current carrying capacity of only 34 to 40 A. In this way, the possibility of excess current circulating through the battery due to short circuit of cells in an impact is limited to the current which can flow through the intermediate cross-connections while the current flowing through the series-connections, during normal operation of the battery, is not affected.

Figure 6 shows, in plan view, a layer 40 of a sodium sulphur battery. The layer 40 comprises a plurality of strings 42, 44 ..., the ends of the strings being connected in parallel by a positive bus bar 46 and a negative bus bar 48. Additional intermediate parallel connections 50 are provided across the strings of cells at selected points of the strings which are intended to be at equal voltages.

Thus, the layer 40 may be considered to comprise a plurality of sub-arrays 40a, 40b, 40c, 40d, 40e, each sub-array being provided between adjacent parallel connections 50. Conveniently, the layer 40 is assembled by positioning the sub-arrays 40a, 40b, 40c, 40d, 40e in a battery box and then adding the intermediate parallel connections 50 at the required positions. Each sub-arrays is made up of a plurality of series connected cells, termed "sub-strings" as before, which are held together in a matrix so that the cells can be conveniently handled as a block. If required, additional electrical insulation may be provided between the sub-arrays under the intermediate parallel connections 50.

Figure 7 shows the interface region 52 between first and second layers 40 while Figure 7a shows the interface region 52 in greater detail. The interface region 52 includes a heat sink 54 which is used to remove excess heat from the cells during operation of the battery. For high temperature batteries, such as the sodium sulphur battery which operates at about 350 0C, the interface region 52 also includes a heater board assembly 56 so that the temperature of the cells in the battery can be raised from ambient temperature to the required operating temperature when the battery is used. A thermocouple board assembly 58 is also provided to monitor the temperature at the interface region 52 and hence control operation of the heat sink 54 and heater board assembly 56.A thermal shield 60, made of graphite foil, is provided which acts as a barrier against polysulphides and hence reduces the risk of fire in the battery, particularly the risk of fire in one layer spreading to the next. Mica boards 62, 64 are provided for electrical insulation between the layers of cells and also between the layers 40 and the various components of the interface region 52. To ensure that the cooling effect of the heat sink 54 on each layer of cells 40 is the same, the interface region 52 may additionally comprise additional thermal insulation 66, the position of the board of additional thermal insulation 66 being dependent on the relative positions of the other boards within the interface region 52.

Figure 8 shows a heat sink 54. Cooling fluid, such as air or oil, flows through a serpentine cooling duct 68 which extends over the area of the layers 40. The presence of thermal insulation, as shown-in Figure 7a, between the heat- sink 54 and each layer of cells permits the cooling fluid to be at a relatively low temperature, comparable to room temperature, without excess cooling effect.

As discussed above, in a battery of two layers of cells, an interface region 52 may be provided between the two layers. In a four layer battery, it is envisaged that only two such interface regions 52, between the first and second layers and between the third and fourth layers, need be provided. Such an arrangement would allow each layer of cells to be in close proximity with a heat sink and a heater board assembly, without unduly increasing the thickness of the battery. The interface region between the second and third layers may comprise a thermal shield, a thermocouple assembly board electrical insulation and additional thermalinsulation as required.

A matrix material is used to hold the cells in an individual array or suh-array together in their correct orientation and relative position. The matrix is based on an alumina silicate powder which is extracted from pulverised fly-ash (from coal fired power stations). A dry inert free flowing powder consisting of hollow alumina silicate microspheres is sold under the trade name Fillite. The Fillite microspheres are filled with a mixture of carbon dioxide and nitrogen gas and range in size from 5 to 500 microns. A typical chemical composition for the Fillite microspheres is set out below: SiO2 55% - 65% A1203 27% - 33% Fe203 4% maximum K2 /Na2 0.5% - 4% CaO 0.2% - 1% MgO 1% - 2% The Fillite microspheres are mixed with an aqueous solution of sodium silicate and an organic ester.The sodium silicate solution is an aqueous solution of silica and sodium oxide having a pH of 13 and a density of 1.62 g/cc. The organic ester is typically a blend of water miscible organic esters such as glycerol diacetate and glycerol triacetate having a density of 1.17 g/cc and sold under the trade name Carset 533. This mixture produces a material which, when mixed together, forms a slurry that is initially pourable and that will "set" in ambient air over a nominal overnight standing period. The rigid set material has good mechanical properties and is capable of bonding the individual cells together in the array or sub array. The time taken for setting to occur depends on the pH, the concentration of solutions and the temperature of the mixture.Different grades of Carset are available which allow a range of setting times from a few minutes to beyond an hour (Carset 533 allows a working time of about 50 minutes).

The relative proportions of the materials in the mixture can be derived experimentally. The slurry must have sufficient fluidity to completely fill the small voids between cells in the required configuration. The slurry must also have an adequate working time, setting in air at ambient temperatures within a convenient time period. The set material must then be able to hold the cells together in their correct configuration. Variation in the relative amount of Fillite powder will affect the viscosity of the slurry. Sufficient sodium silicate solution must be used to provide adequate bonding between cells, insufficient silicate solution resulting in poor cell-to-cell adhesion.

An increase in the amount of sodium silicate solution generally results in a greater set strength and greater bonding capability but has the disadvantage of increasing the density of the matrix material (and hence the weight of the battery). The fluidity of the slurry may be improved by increasing the proportion of sodium silicate solution although, alternatively, water may be added. There is also a minimum proportion of Carset solution that is required to produce a rigid handleable material after an overnight setting period which is approximiately 7.5% by weight.

(Quantities below this were found not to produce a uniform set.)- The optimum proportion of Carset depends on the sodium silicate solution and also the need to wet the Fillite powder. Increased amounts of Carset, up to about 15% by weight, also aid in giving the slurry an oily characteristic with good mould release properties.

Each cell may be surrounded by a layer of mica to ensure that the cells are electrically insulated from each other in the array or sub array. However, it is envisaged that the electrical resistivity of the matrix may be sufficient to provide the necessary electrical insulation without the need for the mica layers. For the matrix, it was found that the resistance varied with temperature and time as found - for a simulated battery warm-up. At room temperature, the resistance of the matrix was relatively low (probably due to thewater content of the matrix) but increased suddenly at a temperature of between 100 150 0C. The resistance suddenly decreased at a 0 temperature of just above 250 C and then recovered to a more-or-less stable value after a period of a few hours.

However, it is possible that this variation may not be seen during subsequent-battery warm-ups as the water content of the matrix will have been reduced by the first warm-up period.

Release of entrapped water at elevated temperatures may cause distortion of the material and so a restraining fixture may be provided to clamp the array to hold the array to a defined overall size during its first exposure to elevated temperatures. Subsequent reheating of the array does not affect the dimensional stability.

The matrix material also has sufficient compressional strength and resistance to vibration. The material was also found to be capable of absorbing up to 10% by weight of sulphur vapour and 50% by weight of sulphur liquid.

It is envisaged that the properties of the matrix material are due to the production of silica hydrogel bonds produced when the Carset ester lowers the pH of the silicate to a point where gelling and eventually setting occurs. The "set" matrix material contains chemically bonded silica and water molecules. Some of this water may be released as the matrix material is left to stand, the release being accelerated by application of temperature.

It may be significant that, although the Fillite microspheres have a typical particle size of the order of microns, some of the microspheres may be broken resulting in particles of smaller sizes which may assist in the setting of the gel.

Setting of the silicate may also be effected by heating or by introducing carbon dioxide gas into the slurry (in place of using an organic ester).

Claims

1. A battery comprising a layer of electrochemical cells arranged as at least one two-dimensional array of rows and columns of cells, each cell having one end at which both cell terminals are provided, the array being one cell thick with all the cells having the same relative orientation whereby all the cell terminals are accessible from one surface of the array, cells in the array being electrically connected to provide a plurality of strings of series-connected cells, the strings being connected in parallel with each string having the same number of cells to provide a desired operational voltage with intermediate cross-connections being provided between points intended to be of equal voltage in the strips at at least one selected intermediate voltage.

2. A battery according to claim 1, wherein each cell has a cylindrical cross-section in the plane of the array and the cells are arranged in a hexagonal arrangement in which the majority of cells have six adjacent cells.

3. A battery according to claims 1 or 2, wherein the layer comprises a plurality of arrays electrically connected in series.

4. A battery according to any one of the preceding claims comprising a plurality of layers physically arranged parallel to one another and electrically connected in series.

5. A battery according to any one of the preceding claims, wherein each cell has a defined orientation with a top and a base and a height no greater than three times the width of the cell, each array and each layer having a height of one cell, at least one layer being stacked on top of another layer.

6. A battery according to any one of the preceding claims, comprising two layers of electrochemical cells, each cell having a defined orientation with a top and a base, each layer having a height of one cell, said layers being stacked one on top of the other and the battery further comprising a panel effective as a heat sink or as a heat source, said panel being arranged interjacent said two layers.

7. A battery according to Claim 6, comprising at least three said layers of electrochemical cells, said layers being stacked one on top of the other, the battery also comprising a sufficient number of said panels such that each layer of cells is separated from a panel by a distance less than the height of one cell.

8. A battery according to Claim 6, wherein said panel comprises a heat sink.

9. A battery according to Claim 8 further comprising a sheet of thermal insulation interjacent said heat sink and a said layer of electrochemical cells.

10. A battery according to Claims 8 or 9, wherein said heat sink comprising a cooling duct having a serpentine pattern and means for introducing a coolant into the cooling duct.

11. A battery according to any one of Claims 8 to 11, wherein said panel further comprises a source of heat.

12. A battery according to any one of Claims 6 to 11, wherein said panel further comprises a temperature sensor for sensing the temperature of the cells.

13. A battery according to any one of the preceding claims further comprising a matrix for holding the cells in the array in a position fixed during assembly to form a solid array of cells, the matrix comprising a silicate hydrogel.

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14. A battery according to Claim 13, wherein the silicate hydrogel includes sodium silicate.

15. A battery according to Claims 13 or 14, wherein the silicate hydrogel is chemically set.

16. A battery according to Claim 15, wherein the silicate hydrogel is set by addition of an organic ester.

17. A battery according to any one of Claims 13 to 16, wherein the matrix includes a coarse-grained filler.

18. A battery according to any of the Claims 13 to 17, wherein the silicate hydrogel includes an alumina silicate.

19. A battery according to Claim 18, wherein the alumina silicate is derived from pulverized fly-ash.

20. A battery according to any one of Claims 13 to 19, further comprising a sheet of. electrical insulation around each cell.

21. A battery according to any one of Claims 13 to 20, comprising a matrix for holding the cells in the array in a position fixed during assembly to form a solid array of cells, the matrix being derived from a mixture of alumina silicate powder, sodium silicate solution and an organic ester.

22. A battery according to any one of the preceding claims, wherein the series connections between adjacent cells in each string have a series current carrying capacity at least sufficient for the maximum rated charge and discharge currents for the string and at least some of the intermediate cross-connections between points intended to be of equal voltage have an intermediate cross-connection current carrying capacity less than said series current carrying capacity.

23. A battery according to Claim 22, wherein said intermediate cross-connection current carrying capacity is less than that required for said maximum rated charge and discharge currents for the string.

24. A battery according to any one of the preceding claims, wherein at least some of said intermediate cross-connections are adapted to limit the current flowing therethrough.

25. A battery according to Claim 24, wherein said at least some of said cross-connections are fusible at a current limit.

26. A battery comprising at least one array of electrochemical cells, the array comprising a plurality of strings of series-connected cells, the strings being connected in parallel with each string having the same number of cells to provide a desired operational voltage and intermediate cross-connections being provided between points intended to be of equal voltage in the strings at at least one selected intermediate voltage, the series connections between adjacent cells in each string having a series current carrying capacity at least sufficient for the maximum rated charge and discharge currents for the string and at least some of the intermediate cross-connections between points intended to be of equal voltage having an intermediate cross-connection current carrying capacity less than said series current carrying capacity.

27. A battery according to Claim 26, wherein said intermediate cross-connection current carrying capacity is less than that required for said maximum rated charge and discharge currents for the string.

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28. A battery according to Claims 26 or 27, wherein at least some of said intermediate cross-connections are adapted to limit the current flowing therethrough.

29. A battery according to Claim 28, wherein said at least - some of said intermediate cross-connections are fusible at a current limit.

30. A battery comprising a plurality of layers of electrochemical cells, each layer being arranged as at least one two-dimensional array of rows and columns of cells, each cell having a defined orientation with a top and a base and a height no greater that three times the width of the cell, each array and each layer having a height of one cell, at least one layer being stacked on top of another layer.

31. A battery comprising two layers of electrochemical cells, each layer being arranged as at least one two-dimensional array of rows and columns of cells, each cell having a defined orientation with a top and a base, each array and each layer having a height of one cell, said layers being stacked one on top of the other and the battery further comprising a panel effective as a heat sink or as a heat source, said panel being arranged interjacent said two layers.

32. A battery according to Claim 31, comprising at least three said layers of electrochemical cells, said layers being stacked one on top of the other, the battery also comprising a sufficient number of said panels such that each layer of cells is separated from a panel by a distance less than the height of one cell.

33. A battery according to Claim 32, wherein said panel comprises a heat sink.

34. A battery according to Claim 33, further comprising a sheet of thermal insulation interjacent said heat sink and a said layer of electrochemical cells.

35. A battery according to Claims 32 or 33, wherein said heat sink comprises a cooling duct having a serpentine pattern and means for introducing a coolant into the cooling duct.

36. A battery according to any one of Claims 33 to 35, wherein said panel further comprises a source of heat.

37. A battery according to any one of Claims 32 to 36, wherein said panel further comprises a temperature sensor for sensing the temperature of the cells.

38. A battery comprising a plurality of electrochemical cells arranged as at least one array of cells and a matrix for holding the cells in the array in a position fixed during assembly to form a solid array of cells, the matrix comprising a silicate hydrogel.

39. A battery according to Claim 38, wherein the silicate hydrogel includes sodium silicate.

40. A battery according to Claims 38 or 39, wherein the silicate hydrogel is chemically set.

41. A battery according to Claim 40, wherein the silicate hydrogel is set by addition of an organic ester.

42. A battery according to any one of Claims 38 to 41, wherein the matrix includes a coarse-grained filler.

43. A battery according to any one of Claims 38 to 42, wherein the silicate hydrogel includes an alumina silicate.

44. A battery according to Claim 43, wherein the alumina silicate is derived from pulverized fly-ash.

45. A battery according to any one of Claims 38 to 44, further comprising a sheet of electrical insulation around each cell.

46. A battery comprising a plurality of electrochemical cells arranged as at least one array of cells and a matrix for holding the cells in the array in a position fixed during assembly to form a solid array of cells, the matrix being derived from a mixture of alumina silicate powder, sodium silicate solution and a organic ester.

47. A battery according to any one of the preceding claims, wherein the cells are sodium sulphur cells.