The present invention relates to a rechargeable electric cell. More particularly, the invention is concerned with providing a mechanically-rechargeable single-cell consumer electric battery for generating up to two volts, mechanical recharging being effected by exchanging the zinc anode.
Electric batteries intended for use in flashlights, portable radios, toys, telephones, calculators and simlar uses may be defined as consumer batteries, in contrast to the larger batteries in industrial use, as for example for powering electric vehicles such as fork-lift trucks, standby batteries for computer systems and batteries found in hospitals for the operation of medical appliances.
Batteries are further classified as primary or secondary. Primary batteries are not electrically rechargeable. Examples of primary batteries are the Leclanche dry cell and the Magnesium (Mg-MnO.sub.2) dry cell. Such batteries are discarded when exhausted and replaced with a new battery.
It is, however, to be noted that discarded batteries, particularly those containing mercury, have been found to cause substantial ecological damage, and in ecologically-aware countries such as Germany, disposal of batteries is allowed only into specially-provided containers.
In contradistinction to the primary type, batteries classified as secondary are electrically rechargeable, and justify their higher cost also by having higher power density, flat discharge curves and better low-temperature performance. Examples of secondary batteries are the lead-acid battery used in automobiles and the nickel-cadmium types.
Secondary batteries based on zinc typically use this metal as the negative electrode, have an alkaline electrolyte, a separator system, and a positive electrode either of manganese dioxide, nickel hydroxide or silver oxide. However, after 10 to 50 deep recharge cycles, even secondary batteries must be discarded due to a degradation in performance.
Zinc anodes are used in both primary and secondary batteries. Such batteries include the Leclanche, the alkaline manganese dioxide, in which the anode is zinc, the mercury cell, in which the anode is zinc and the cathode is mercuric oxide, the silver-zinc type, in which the cathode is silver oxide, the nickel-zinc type, in which the cathode is nickel hydroxide, among others.
With regard to zinc-anode secondary batteries, it has been found that the dominant cause of declining performance is the poor recyclability under electrical recharge of the zinc negative electrode, which results in a gradually lower output voltage and higher internal battery resistance after each recharge, until the unit is no longer able to power the appliance to which it is connected, whereafter the user discards the battery. Replacement costs are higher than for primary batteries.
In some cases, the cell will develop a short circuit as a result of zinc dendrites growing through to the positive electrode, and will consequently cease to function.
Examination and testing of the components of such failed cells have, however, yielded a significant result: the cell cathode is still in good condition and capable of further service.
As has been explained, even secondary batteries have a limited life. Whether or not this is of concern to the user depends on the amount of electrical power that is required. When a small battery is required for occasional use, as for example in electric test instruments, the cost of the battery to the consumer is of little significance. This is particularly so where the consumer is allowed to discard the used battery without being obliged to bear the cost of its environmentally-safe disposal, particularly for a battery containing toxic metals, such as cadmium and mercury.
The consumer will, however be more interested in lower total-life battery cost where the battery is larger, for use in applications such as in the operation of radios and larger tape recorders where more power is required.
It is known that for a specified current drain, a larger battery will achieve substantially longer life as measured in watt-hours per kilogram of battery weight, in comparison with a smaller battery of the same type. For example, Suppose a starting current drain of 15 milliamp is specified for a zinc-carbon cell. A small N size battery will have a service capacity of 24 hours only; the larger F size cell will operate for 520 hours, although weighing only about 10 times as much as the N cell.
Except in applications where there are severe weight or volume restrictions, it is, therefore, evident that a larger battery will serve the consumer in a more cost-effective manner than will a smaller battery. In order to offer the consumer a larger battery at a competitive cost, it is necessary either to use low-cost materials or to offer a battery with an outstandingly long service life.
In this regard, it is of interest to note that the more expensive systems, such as the nickel-metal hydride, and the lithium rechargeable types, are unavailable in the larger cell sizes such as the C size and above. Such cells would be economically unattractive to the user due to a combination of high material cost and limited rechargeability while the safety of such systems is still questionable, and consequently such batteries are not presently manufactured.
It is, therefore, one of the objects of the present invention to obviate the disadvantages of the prior-art battery cells and to provide an electric battery cell which provides an extended service life at a viable cost.
This the present invention achieves by providing a mechanically-rechargeable single-cell consumer electric battery for generating up to two volts, said battery being capable of electrical recharge and comprising a replaceable zinc anode; a housing containing said anode and provided with an aperture sealed by a removable closure, said aperture being sufficiently large to allow removal and replacement therethrough of at least said zinc anode; a cathode selected from the group consisting of manganese dioxide, nickel hydroxide, silver oxide, and air (oxygen) reduction types also contained in said housing; a non-spillable electrolyte in contact with both cathode and anode; and a separator system physically separating the anode from the cathode.
In a preferred embodiment of the invention, the aperture is sufficiently large to allow removal and replacement therethrough of said zinc anode and the cathode.
In another preferred embodiment of the present invention there is provided a rechargeable battery wherein the housing is at least as large as "C" cell, as defined by the American Standards Institute standard for cylindrical and flat zinc-carbon cells.
Preferably, the anode is composed of a slurry of zinc powder, pressed into a central current collector and optionally combined with an organic binder, such as carboxymethyl cellulose binder.
While most of the battery cells of the present invention require factory equipment for replacing the zinc anode, in a most preferred embodiment of the present invention there is provided a user-rechargeable electric battery cell wherein the removable closure and said housing are assembled by means of a matching screw thread, whereby the user can exchange the zinc anode without needing industrial tools.
For effective use of the battery cell of the present invention, means are also provided for differentiating between a battery which can still be electrically recharged and between a battery which has further deteriorated and should now undergo replacement of its zinc anode.
The battery cell of the present invention will also be of value in producing a cell of improved energy density.
Considering the energy density of the widely-used nickel-cadmium cell, it is known that such a cell will produce approximately 35 watt-hours per kilogram or 100 watt-hours per liter volume.
These figures would be approximately 75% higher were the cadmium replaced by zinc; however, this would only be economically justified for a battery which could be recharged sufficiently often to provide an extended service life. This requirement is provided for by the present invention.
The invention will be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood.
With reference to the figures, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.