GB2054945A - Electric storage batteries - Google Patents

Abstract

There is disclosed an electric storage battery having for each cell a vent plug presenting a flat upper surface 8 in which are formed two gas venting apertures 14. The apertures are covered by a strip of gas permeable adhesive material 16 which reduces the rate at which gas can flow through the venting apertures when the battery is in the 'drain-dry' condition. <IMAGE>

Description

SPECIFICATION Electric storage batteries This invention relates to electric storage batteries and in particular to venting arrangements for such batteries.

In service, conventional storage batteries incorporate vents that allow the flow of gas into or out of the battery if a pressure differential should occur. The amount of gas that is developed when, for instance, a lead acid battery is overcharged may be substantial and such vents must therefore be able to pass a substantial volume of gas.

However lead acid batteries are sometimes sold to the wholesale and retail trades in a condition known as "drained dry", that is to say the batteries are charged and the electrolyte is then poured out.

This increases the charged shelf life of the battery, and the batteries are only filled with electrolyte shortly before sale by the retailer. During drain-dry storage, which may last months or even years, pressure differentials may also occur, and vents are therefore required to reduce or eliminate these differentials to prevent damage to the batteries.

However drain-dry vents do not need to pass large gas flow rates and indeed it is desirable that they should not be able to admit large volumes of air since free ozygen in a drain-dry battery accelerates its rate of internal discharge.

Due to the different requirements of drain-dry and service venting it is customary to provide lead acid batteries with two vent plugs per cell, one of which, the drain-dry storage vent is thrown away before sale of the batteries by the retailer and replaced by the other, namely the service vent.

This practice is not only time consuming but is also uneconomical of materials and Tt Is therefore - an object of the present invention to provide an electric storage battery with a venting arrangement that may be used with a very simple modification for both drain-dry and service venting.

According to the present invention an electric storage battery has for each cell a surface in which is formed at least one gas venting aperture communicating with the interior of the cell, the surface carrying gas permeable material adhesively secured thereto overlying the aperture.

The surface is preferably the upper surface of a vent plug, and the material is preferably adhesive tape.

In a preferred construction two or more vent plugs are ganged together presenting a single flat upper surface carrying a single piece of adhesive tape overlying the or each gas venting aperture of each cell.

Further features and details of the invention will be apparent from the following description of one specific embodiment which is given by way of example with reference to the accompanying drawings in which:~ Figure 1 is a scrap sectional elevation through a ganged vent plug in position on a battery in accordance with the invention Figure 2 is a plan view of the vent plug; and Figure 3 is an enlarged scrap view of part of the adhesive tape.

Figure 1 shows part of the lid 2 of a 12 volt lead acid storage battery in which six venting and filling holes were formed, one for each cell, in each of which a vent plug is received. Each vent plug comprises a generally tubular body 4 of plastics material having an integral lid 6. The vent plugs are for convenience and simplicity ganged together in two three-gang vent plugs, each of which is an integral moulding having a flat external upper surface 8. Received within each tubular body 4 is an insert 10 (shown only in the right hand of the two vent plugs in Figure 1) which defines a circular section space immediately beneath the lid 6 within which is a flame retarding body 12, for instance of sintered polyethylene, which is retained in position by the insert 10.The plug 10 affords a venting path which connects the interior of the cell with the underside of the flame retarding body, so that gas vented from the cell passes through the flame retardant body and thence to the atmosphere through two space venting apertures 14 formed in the lid 6, each having a diameter of 1.5 mm.

The vent plugs as described are capable of passing the relatively large volume of gas that may be evolved when, for instance, the battery is overcharged, and are therefore service vents. Such vents are inherently unsuitable for use as drain-dry vents since they would permit a relatively large volume of air to enter the battery which would lead to a premature loss of charge.

According to the present invention these service vents are converted to function as draindry vents by the application to their upper surface 8 of a strip of gas permeable adhesive tape 1 6.

This tape substantially reduces the maximum possible flow rate of gas into and out of the battery through the venting apertures and thus renders the vents suitable for use as drain dry vents. Particularly suitable is the microporous polyvinyl chloride tape sold by Smith s Nephew Limited under the registered Trade Mark AIRSTRIP. Such tape has a porosity of about 44%, the mean pore diameter being about 3 Hm, and the maximum pore diameter being 50 ,um. The air permeability of such tape without any adhesive applied is preferably between 30 and 90 mls/cm2/minute and in the present example is 65 mls/cm2minute at a pressure of 10 cm. of mercury.

It will be appreciated that, depending on the type of adhesive used, the portions of tape bearing adhesive may be substantially non-permeable.

Thus in the preferred embodiment the tape bears a discontinuous pattern of adhesive so arranged in relation to the diameter of the venting apertures that each least one of the two venting apertures of each cell will communicate with a portion of the tape that does not bear any adhesive. It is preferred that gas entering or leaving the battery will do so through the tape rather than between the tape and the upper surface of the vent plugs, and for this reason in the illustrated embodiment the adhesive is arranged in a series of closed formations, in this case intersecting strips 18 defining squares whose sides are inclined to the length of the adhesive tape. However other formations such as diamonds, triangles and contacting circles could also be used.In the specific embodiment the width of the intersecting adhesive strips is 1 mm. and the diagonals of the squares defined are 2.5 mm. long. The width of the adhesive strips is less than the diameter of the venting apertures which can therefore not be blocked regardless of how the adhesive tape applied to the vents.

It will be appreciated that the function of the adhesive strip is to restrict the possible flow rate of gas into and out of the battery, and the permeability of the tape and the diameter of the venting holes are preferably so dimensioned relatively to each other that the gas flow rate into or out of an individual cell at a pressure differential of 10 cm. of mercury is between 3 and 20 mis/minute. In the preferred construction the flow rate is between 5 and 10 mis/minute. In practice the pressure differentials occurring in a drain-dry battery are generally only a few cm. of mercury and the gas flow rate will therefore be lower than that indicated above.

Thus, after a battery has been charged and drained of electrolyte in the factory, i.e. is in the drain-dry state, the service vents can be fitted immediately, and the adhesive tape applied so that the vents exhibit the required drain-dry venting characteristics. Shortly before sale, the battery is filled with electrolyte, and the adhesive tape is simply peeled off rendering the battery ready for immediate service. The use of the adhesive tape renders the conversion of the battery from the drain-dry to service state a simple operation, enables the services vents to be fitted in the factory and obviates the requirement of manufacturing a second set of drain-dry vents which are subsequently thrown away.

Although the use of adhesive tape has been described, it will be appreciated that the tape or material may be non-adhesive, with the adhesive applied to the surface in which the venting apertures are formed. It is not essential that the surface be flat, but merely that it be such that the material or tape is capable of adequate adhesive connection to it. In addition the invention is not only applicable to batteries with one or more flame retardant vents, so that in the embodiment described the flame retarding body 12 may be omitted leaving the insert 10 to serve a sprayarresting function and the insert 10 may also be omitted. The vent plugs may be omitted altogether, with the venting apertures formed in the battery lid.

It will be appreciated that the invention is also applicable to flooded batteries in which there is free electrolyte. Thus the application of the adhesive tape over the venting apertures substantially blocks the venting apertures and renders the battery spill-resistant if the battery should be tilted or inverted.

Claims (9)

1. An electric storage battery having for each cell a surface in which is formed at least one gas venting aperture communicating with the interior of the cell, the surface carrying gas permeable material adhesively secured thereto overlying the aperture.

2. A battery as claimed in Claim 1 in which each surface has two venting apertures formed in it.

3. A battery as claimed in Claim 1 or Claim 2 in which the surface is the upper surface of a vent plug.

4. A battery as claimed in any one of the preceding claims in which the material is adhesive tape.

5. A battery as claimed in any one of the preceding claims in which the total area of the venting aperture or apertures and the air permeability of the material is such that the flow rate of gas into or out of each cell at a pressure differential of 10 cm. of mercury is between 3 and 20 mI./minute.

6. A battery as claimed in claim 5 in which the flow rate is between 5 and 10 mis/minute.

7. A battery as claimed in any one of the preceding claims in which the air permeability of the material without adhesive is between 30 and 90 mls/cm2/minute.

8. A-battery as claimed in any one of the preceding claims in which two or more vent plugs are ganged together presenting a single flat upper surface carrying a single piece of adhesive tape overlying the or each gas venting aperture of each cell.

9. An electric storage battery substantially as herein described with reference to the accompanying drawings.