GB1599644A - Electrical batteries for use as capacitors - Google Patents

Description

(54) ELECTRICAL BATTERIES FOR USE AS CAPACITORS (71) We, NATIONAL RESEARCH DEVELOPMENT CORPORATION, a British Corporation established by Statute, of Kingsgate House, 66-74 Victoria Street, London, S.W.1, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to the construction of electrical cells or batteries for use as capacitors for example as filtering capacitors for removing alternating voltages in d.c. power supplies or as memory elements in stores for computers.

Secondary cells are similar to polarised capacitors in that if a unidirectional voltage is applied to a battery, electrical energy is stored and the battery can be discharged later. Thus although the mechanism of charge and discharge is different as between battery and capacitor the effect is similar.

According to the present invention there is provided an electrical battery for use as a capacitor comprising an electrolyte system in contact with and between first and second electrode means, wherein the ratio of short circuit current as hereinafter defined from the battery divided by the storage capacity as hereinafter defined of the battery is not less than one amp per joule.

In this specification and claims "short circuit current" means the ratio of opencircuit voltage of the battery divided by the internal resistance measured at 50 Hz, and storage capacity is the maximum energy extractable from the battery over a ten hour period. The term battery includes a single cell.

Advantageously, the product of internal resistance of the battery measured at 50 Hz times the maximum energy which can be stored by the battery is at most 6 ohmjoules.

If a battery is to be used as a capacitor it is important, in most cases, that the battery can discharge successfully at a high rate. The battery will then behave in a way similar to a capacitor. Also internal resistance should be low if the battery is to make a useful capacitor and the limit for the above mentioned product helps to ensure that this condition is met.

The ability to supply power for an extended period is no longer required where a battery is to be used only as a capacitor and the battery can therefore be made considerably smaller than a conventional battery. Furthermore, in relation to a capacitor having a similar specification, such a battery may be considerably smaller, weigh less and cost less.

Where the battery or cell employs one or more secondary cells containing an active material which is changed chemically during charging, the limits for the above mentioned ratio and product imply that only thin layers of active material are used compared with an ordinary battery. As applied frequency increases, charging and discharging times decrease and, for a given depth of discharge, currents increase. It is thought that batteries according to the invention will be useable as filtering elements up to fairly high frequencies such as 10 kHz. The thick layers of active material necessary in an ordinary battery to allow power to be supplied over an extended period, though porous, each form a high resistance layer if charge and discharge at high rates is attempted. Such high charge and discharge rates are usually required of a battery acting as a capacitor and thus a conventional battery is not suitable for this application. In secondary cells or batteries of such cells according to the present invention it is therefore preferred that the, or each, layer of active material is less than 100 microns thick.

Advantageously, nickel-iron cells are used for a battery according to the invention in which the first electrode means comprises a plurality of nickel plates each with a layer of nickel hydroxide (Ni(OH)2) on one side, the second electrode means comprises a plurality of iron plates, and the electrolyte is an aqueous solution of potassium hydroxide (KOH). The potassium hydroxide solution occupies spaces between the plates and is, advantageously, in gel form. The battery is formed by a plurality of cells each comprising a nickel plate (with hydroxide layer) and an iron plate preferably with a porous, perhaps corrugated, separator between the plates. Typical materials for the separator include cellophane, non-woven polypropylene, Pellon 2506-K4 and plastic fibre. In a preferred embodiment of such a nickel-iron battery each nickel plate carries a "thick" film of nickel hydroxide about 30 microns thick and each nickel plate is about 0.1 millimetres thick. The spacing between the nickel and iron plates is then about 0.2 millimetres and each iron electrode is also about 0.1 millimetres in thickness.

The operation of the cell is improved if the iion plates are covered on one side by an oxide layer of about the same thickness as the nickel hydroxide layer on the nickel plates.

Nickel hydroxide may be formed by depositing a nickel oxide layer which when it comes into contact with the electrolyte becomes nickel hydroxide. The nickel oxide is preferably deposited on the nickel plates by first roughening the plates with emery paper or grit blasting them and then immersing them in a bleach such as sodium hypochiorite and having 12% by weight per volume of available chlorine for two minutes at 75 'C. The oxide layer on the iron plates may be formed by an equivalent method. Alternatively the nickel plates can be sprayed with nickel oxide powder in an arc plasma or they can simply be heated in an oven in the presence of oxygen.

The upper limits for plate separation and electrode thickness (except for outer electrodes in a battery) are expected to be 1.5 millimetres and 1.0 millimetres, respectively, for batteries which can be used successfully as capacitors. Again the limits of the said ratio and product imply small separations and thicknesses such as are given above.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawing which shows a partially sectioned battery of cells according to the invention.

In the Figure a plastics casing 10 is tubular in shape and contains a plurality of nickel plates 11 each having a nickel hydroxide film 12 made in one of the ways described above. The tubular casing also contains a number of iron plates 13 each in contact with one of the nickel plates 11 on one side, and separated on the other side, from a further nickel plate and its hydroxide film by means of a cellulosic separator 17 and a gel 14 which contains a 21% by weight aqueous solution of potassium hydroxide. The gel is prepared using an appropriate known gellifying agent. As can be seen from the diagram the battery is formed from a plurality of cells connected in series by means of a contact between the nickel and iron plates. A positive terminal 15 is indicated at one end of the battery and a negative terminal 16 is indicated at the other end. The thicknesses of the plates and the hydroxide film, and the separation between the plates, is as given above in connection with a preferred nickel-iron battery.

The chemical reactions which are believed to take place at the plates are: Positive plate: NiOOH + H20 = Ni(OH)2 + OH Negative plate: Fe + 20H = Fe(OH)2 + 2e where e is the electronic charge.

The overaII reaction which takes place during charge and discharge of the battery is believed to be as follows:- 2 NiOOH + Fe + 2HX0 = 2Ni(OH), + Fe (OH)2 During discharge the reaction progresses from left to right of the equation and vice- verso Thus the Ni(OH)2 layer and the surface layers of the iron electrode provide the active materials and the KOH remains chemically unchanged during the reaction.

Where the battery shown in the drawing is to be used as a filtering element, the number of series cells required as determined by the working voltage of the element, each cell having a voltage of 1.37 volts.

Expected parameters of a single nickel-iron cell in a battery constructed as described above are compared with those of various batteries and a capacitor in the following table (where the word Varta is a trade mark):

Single Cell Varta Varta Varta Varta Siemens For use as battery battery battery battery capacitor capacitor 20DK 100 RS RS1 RS6 4200 F, 10v Amp Hour Rating AH 0.577.10-3 20.10-3 0.1 1. 6. Internal Resist. R (m#) 24 5000 190 19 9 280 Volume V (cm ) 0.253 0.521 2.95 14.96 82.9 56.8 Energy stored E (J) 2.58 89.3 446 4464 26800 0.21 Short circuit current ISC(A) = VS/R 54.2 0.26 6.84 68.4 144 35.7 F = R x E (#J) 0.062 446.5 84.8 84.8 241.1 0.059 The product internal resistance of the battery times maximum energy which can be stored by the battery is given by F in the table. A typical value of capacitance for a single cell for use as a capacitor in which each plate has an area of seven cm is 18,000 F. The volume of a 4200 F capacitor and a cell according to the invention can be compared using the above table. The Varta batteries types 20 DK, 100 RS, RS1 and RS6 mentioned in the table are a small button type for example for a selfcontained hearing aid or a watch, a larger button type, a battery for a portable radio, and a small accumulator, respectively.

It will be realised that the invention can be put into practice in many other ways, for example using different types of cell and using different geometrical constructions for the various plates and separators. For example the "swiss-roll" construction may be used, when a single cell only is required, in which the nickel and iron plates are formed from strips with the nickel oxide film and separator containing the electrolyte between the strips, the strips being rolled up to form a swiss roll. Other electrolytes and active materials suitable for the chosen electrodes may also be used. Other common types of battery may be used such as the nickel cadmium battery, especially since it has a good cycle life, low standing loss, high charge and discharge rates and low maintenance requirements. Less common type of battery may also be used such as the silver oxide battery normally not considered useful as a conventional battery in view of the cost of materials but suitable where the battery is to be used as a capacitor since only small amounts of expensive materials are then required.

Cells or batteries according to the invention may be used as capacitors in several applications which include for example as a filter capacitor at the output of a power pack producing d.c. from a.c. and employing rectifiers. Another use for cells or batteries according to the invention is as a non-volatile storage element in an electrical memory such as may for example be used in a computer. For example it has been calculated that theoretically a million cells used as 1 microfarad capacitors can be deposited on a four centimetre square chip. The batteries or cells then used as capacitors are either charged or discharged according to binary information to be stored. When the power supply for the computer or store is switched off the battery/capacitor elements retain their charged or discharged states with the advantage that information is not lost.

Claims (14)

WHAT WE CLAIM IS:

1. An electrical battery for use as a capacitor comprising an electrolyte system in contact with and between first and second electrode means, wherein the ratio of short circuit current as hereinbefore defined from the battery divided by the storage capacity as hereinbefore defined of the battery is not less than one amp per joule.

2. A battery according to Claim 1 comprising one or more secondary cells, each including one or more layers of active material which is or are changed chemically during charging, wherein the, or each layer, is less than 100 microns thick.

3. A battery according to Claim 1 or 2 wherein the first and second electrode means comprise a plurality of first and second plates made from first and second metals, respectively, each first plate being separated on one side by the electrolyte and active material which is changed during charging from an adjacent second plate, and each first plate, except one, being in contact with a second plate on the other side.

4. A battery according to aaim I or 2 wherein the first electrode means comprises a plurality of nickel plates, the second electrode means comprises a plurality of iron plates, and the electrolyte is an aqueous solution of potassium hydroxide (KOH), each nickel plate having a nickel hydroxide (Ni(OH)2) layer which, together with the electrolyte, separates the nickel plate on one side from an adjacent iron plate, and each nickel plate, except one, being in contact with an iron plate on the other side.

5. A battery according to Claim 4 between the potassium hydroxide solution is in gel form.

6. A battery according to Claim 4 or 5 including a plurality of porous separators, one separator in the electrolyte between each nickel plate and the adjacent iron plate.

7. A battery according to Claim 4, 5 or 6 wherein each iron plate has an oxide layer on that side which would otherwise be in contact with the electrolyte.

8. A battery according to any of Claims 3 to 7 wherein the separation between plates which are not in contact is not greater than 1.5 millimetres.

9. A battery according to any of claims 3 to 8 wherein each plate which is in contact with another plate is not greater than 1.0 millimetres in thickness.

10. A battery according to Claim 1 or 2 which is a nickel cadmium battery or a silver oxide battery.

11. A battery according to any preceding claim wherein the product of internal resistance of the battery measured at 50 Hz times the maximum energy which can be stored by the battery is at most 6 ohm-joules.

12. An electrical direct-current power supply including a battery according to any preceding claim connected as filter capacitor for smoothing out variations in the output voltage of the power supply.

13. An electrical memory circuit including a plurality of batteries, each according to any preceding claim and used as a storage element

14. A battery for use as a capacitor substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.