Separator for starter batteries
The invention relates to a separator made of elastic thermoplastic material, which is inserted between the oppositely charged electrode plates of lead/sulphuric acid starter batteries. The microporous separator prevents short-circuits between the plates and the fall-out of plate material, and yet due to its porous structure it allows the ionic current flow in the electrolyte. 
     A normal starter battery consists of several cells, each of which contains a number of electrode plates and separators; there can be up to 20 plates and separators per cell. Due to the manufacturing tolerances the variations occurring in the thicknesses of the individual components, i.e. both of the electrode plates and of the battery separators, can add up, with the consequence that considerable variations in the pack thickness can occur. Previously, if the thickness of the pack was too small, this was compensated by inserting it with spacers into the cell divisions of the battery containers. This operation can only be carried out manually however, which is not possible with modern, fully automated lines, in which the electrode plates of one polarity, enclosed in pockets of the separator material, are stacked into packs with electrode plates of the other polarity and these are inserted mechanically into the battery containers. 
     Battery separators are known from DE-PS 12 69 212 with ribs of elastic-foam material, which can be pressed together slightly by making use of the compressibility of the ribs. However, because of the high resistance to deformation by pressure using such separators it is only possible to compensate for very small variations in thickness. In addition, because the manufacture of such ribs is only possible in an additional process stage by applying them subsequently to the separator sheet and foaming up the ribs, it is not possible for this to be integrated into modern manufacturing processes in which the separators are manufactured in roll form from thermoplastic material by extrusion and subsequent calendering. 
     In the DE-PS 17 71 227 separators for alkaline batteries are described which have ribs (spacers), which are punched out of a foil and then applied to the surface of a flexible separator foil and attached thereto. According to one embodiment the ribs are arranged so that they are staggered on both sides of the separator foil. Under pressure this type of separator between the electrode plates takes on a form similar to that of corrugated paper. The separator foil thus also comes into contact with the positive electrode plate and is therefore exposed to the danger of destructive oxidation. Due to the flexibility of the separator foil there is practically no restoring force. Moreover, this previously known battery separator also has the disadvantage that it can only be manufactured in a multi-stage process. 
     Also on the market are battery separators made of a synthetic-fibre fleece, with impress beads for reinforcement, which can be pressed out alternately from both surfaces. These beads act at the same time as spacers for the electrode plates in a similar way to the solid ribs in normal synthetic separators. When these are installed in batteries the usual compression pressure is only sufficient to press the beads together slightly; when on the other hand very high pressure is used the beads are pressed completely flat and the separator sheet lies directly on both electrode plates and consequently one cannot ensure an adequate acid supply to the electrodes. It is consequently also not possible to solve the problem with these separators made of fibre-fleece. 
     Another suggestion for the solution of this problem is known from DE-OS 31 17 917. According to this proposal flexible ribs are provided on the walls of the battery container, which act as elastic spacers when the plate pack is inserted. Manufacturing such battery containers is however extremely involved and complicated. 
     The battery separators made from thermoplastic synthetic material that are used to a great extent today for starter batteries to encase the electrodes have a sheet thickness of about 0.15 to 0.35 mm and have solid ribs integrally formed from the synthetic material. On the front side of the separator which faces the positive electrode plate a number of ribs is arranged, the height of which is greater than the thickness of the separator sheet. Should ribs also be arranged on the rear side of the separator which faces the negative electrode plate, they are less high than the ribs on the front side and at least in part lie opposite these back to back. With the compression pressure that exists under the conditions of insertion, the compressibility of these separators is very low, it amounts to only about 1 to 2%, which is not sufficient to compensate for the above mentioned variations in thickness of the electrode plates and battery separators. 
     The basis object of the invention is thus to improve the aforementioned previously known starter-battery separator made from thermoplastic synthetic material as regards their compressibility and recovery force, in such a way that they are sufficient to compensate for the variations in thickness that occur, and yet to maintain constantly the necessary distance for the supply of acid without at the same time complicating manufacture and without impairing their life expectancy under normal conditions of use. 
     The subject of the invention is a starter-battery separator of the previously specified type of elastic synthetic material in which unlike in the previously known rib arrangement, the ribs on the rear side are each staggered as regards the ribs on the front side of the separator, so that a front-side rib runs between two rear-side ribs arranged as a pair or a rear-side rib runs between two front-side ribs arranged as a pair, such that the distance between two ribs arranged as a pair is considerably less than the distance between the neighbouring groups each formed of three associated ribs. 
     Preferably, each of the single ribs is situated in the middle between the ribs arranged in pairs on the other side. The single ribs on the front- or rear-side of the separator are preferably arranged at the same distances from each other so that an uneven distribution only results on the other side of the separator due to the paired arrangement of the ribs provided there, which previously was not known. 
     The extent of compressibility and recovery force of the separators according to the invention can be influenced in various ways. Firstly, it is possible to arrange an opposing pair for only some of the ribs on one side of the separator. The fewer opposing rib pairs there are, the less the pressure that has to be exerted to compress and deform the separator. Further, the separator sheet thickness may be increased between the pairs of ribs, i.e. opposite to the single ribs, in order to raise the recovery force. 
     Moreover, one can vary the distance apart of the ribs arranged in pairs. The compressibility increases and the recovery force decreases when the distance between the ribs of a pair is increased. With synthetic separators for starter batteries of the usual separator sheet thickness of 0.15 to 0.35 mm the distance apart of two ribs arranged as a pair, between which On the other side of the separator an opposing rib runs, amounts to about 1.2 to 4 mm (measured from rib centre to rib centre). It is particularly preferred for this distance to be about 2.5 mm and the distance of two neighbouring groups each of three associated ribs is approx. 13 mm (measured from rib centre to rib centre of the centre rib) when the separator sheet thickness is about 0.25 mm. The ratio between the distance apart of two ribs arranged in a pair and the distance apart of two neighbouring groups each of three associated ribs amounts advantageously to 1:2 to 1:11, preferably 1:4 to 1:6, and particularly 1:5. 
     The ribs on the front side are preferably high enough to give a thickness of about 1 mm with the separator sheet thickness, i.e. about 0.65 to 0.85 mm. The ribs on the rear side are lower with a height of 0.1 to 0.6 mm and preferably about 0.3 mm. 
     Battery separators of the present type are manufactured from polyethylene, polypropylene or polyvinylchloride with the addition of inorganic fillers such as amorphous silicic acid. Particularly preferred are battery separators made from polyethylene of high molecular weight, as produced on a large scale in practice. 
    
    
     The accompanying drawings are intended to act as a more detailed explanation of the invention; they show 
     FIG. 1 a separator according to the invention in cross-section, 
     FIG. 2 a part of the cross-section of FIG. 1 enlarged in the ratio 5:1, 
     FIG. 3 a different embodiment of the separator according to the invention, in cross-section, 
     FIG. 4 a part of the cross-section of FIG. 3 enlarged in the ratio 5:1, 
     FIG. 5 a further embodiment of the separator of the invention, in cross-section. 
    
    
     The separator according to the invention represented in cross-section in FIG. 1 consists of a separator sheet 1, of which the edge areas 4 are for bonding when heat or ultrasonics is applied, in the manufacture of pockets. On the front side of the separator ribs 2 are provided running at equal spacings. To each of these ribs 2 a pair of opposing ribs 3,3&#39; is arranged on the rear side. As FIG. 2 shows, the ribs 2 each run in the centre of the opposing ribs 3,3&#39; which are arranged in pairs. The ribs 2 on the front side of the separator, which are turned towards the positive electrode plate on assembly, are clearly higher than the opposed ribs 3,3&#39;. It is consequently ensured that even when there is a certain deformation of the separator in the rib areas, such as occurs when the electrode pack is squeezed together, the separator sheet does not come into contact with the positive electrode plate and thus be destructively oxidized. FIG. 2 furthermore shows the preferred measurement ratios. With a separator sheet thickness of 0.25 mm the ribs 2 have a height of 0.75 mm and the ribs 3,3&#39; a height of 0.3 mm, so that a total separator thickness of 1.3 mm results. The distance of the opposing ribs from each other (measured from rib centre to rib centre) is 2.5 mm, the distance between adjacent groups of three ribs (measured from rib centre to rib centre of the centre rib) is 13 mm; the ratio of the distances is consequently 1:5.2. The width of the ribs 2 and the opposing ribs 3,3&#39; each measured at the foot of the rib is approx. 0.6 mm for each. As already explained above, the compressibility and elasticity of the separator can be influenced inter alia by the distance of the opposing ribs from each other. However a rib may never meet back to back with an opposing rib, because then the compressibility would be limited t that of the material, which is extremely low. It is only through the bending deformation that there results the necessary compressibility and elastic or recovery force with the separator according to the invention. 
     FIGS. 3 and 4 show another embodiment of the separator according to the invention, in which two ribs 12, 12&#39; are arranged as a pair to an opposing rib 1. Here too the opposing rib 13 runs in the centre of the rib Pair 12, 12&#39;. The size ratios are the same as for the embodiment in FIGS. 1 and 2. 
     In the embodiment of FIG. 5 the separator sheet thickness is increased in comparison to the area 21 outside of the ribs between the ribs 22, 22&#39; arranged in pairs opposite to the single ribs 23; the increased thickness is 0.4 mm. A higher recovery force is obtained in this manner. 
     If desired, it is possible to provide further ribs in the intermediate spaces between the rib groups on the front and/or rear side of the separator without impairing the elastic deformation in the areas structured according to the invention. In particular with the additional ribs on the front side one can prevent the separator sheet coming into contact with the positive electrode plate. These additional ribs can also fulfil their purpose then when they have a smaller height than the other ribs provided on the front side. 
     The manufacture of the separators according to the invention can take place in the same manner as with conventional polyethylene separators. In these the synthetic substance which contains fillers is extruded through a slit and then fed through two calender rolls, by which means the ribs are produced and the separator sheet is reduced t the desired thickness. In the manufacture of a separator according to the FIGS. 1 and 2 the calender roll for the front side can remain unchanged from the prior art, for the rear side however a modified roll is necessary to produce the opposing ribs that are arranged in pairs. 
     Tests have shown that the separators according to the invention have the necessary compressibility and elasticity, to compensate for the variations of thickness of the electrode plates and separators occurring in manufacture. With these separators, manufactured plate packs can therefore be inserted without spacers into the battery containers; the plate packs are each then squeezed together to such an extent that they can be slid into the appropriate cell compartments and remain firmly clamped in them. 
     While the present invention has been described with reference to its preferred embodiments, it will be obvious to those skilled in the art that other variations and embodiments may be made and it is intended to cover such variations and embodiments in the appended claims.