The present invention relates to sealed lead acid batteries having electrodes for oxygen gas recombination, said electrodes being separated porous separating means in which the electrolyte is absorbed- So-called maintenance-free batteries exist in a plurality of different embodiments to satisfy the special conditions with which a battery is designed to operate. The operating conditions are highly determinative to the way in which the cell or the battery is charged.

When a battery is discharged with a relatively small current during long time period, and the discharge is more than 90% of the total capacity of the battery, the major part of the active material is used, both in th postive and the negative electrode.

When the discharge occurs with high current during a short time period the battery is discharged to less than 50" during a relatively sho time. On such operating conditions the active material is unco pTetely us due to the fact that the discharging current is high which means that onl the outer layers of the electrode take part in the ion exchange.

When a conventional lead acid battery is charged practically all charging current in the beginning of the charging will convert the discharged active material to charged active material. Depending on the charging velocity a substantial reduction of the efficiency of the chargi occurs when the battery has been charged to about 80. - 95.%.. The higher th charging velocity and the charging current, the earlier this inefficiency will occur and it will be seen when oxygen gas is generated at the positi electrode. The negative electrode is more inclined to receive charge than the positive electrode and inefficiency at the negative electrode will be seen when hydrogen gas is generated which will not occur, however, until the whole electrode is nearly completely charged, that, is up to 95 - 100%. In practice this inefficiency at charging makes it necessary to give the battery a certain overcharge in the magnitude of 5 - 15%. In a conventional lead acid battery having the electrolyte in the form of a liquid the oxygen generated at the positive electrode will esca from the battery but for a small part which is dissolved in the electroly and possibly reacts with the negative electrode. The solubil ty of the oxygen gas in sulphuric acid is very low, however, and the faster the battery is charged the greater amount of oxygen gas will be released to t atmosphere. For maintenance-free batteries therefore sealed battery vessel have been used preventing the escape of oxygen to the atmosphere and simultaneously facilitating the contact of the oxygen gas with the negati

electrode in order to recom ne w t t e act ve mater a therein. To provide this object special porous separators are used in which the electrolyte is partly absorbed. It is essential that the separators are n completely saturated with electrolyte to permit the oxygen gas developing at the positive electrode to diffuse rapidly through the porous material from the positive to the negative electrode to recombine there. During th charging the oxygen gas developed at the positive electrode should consequently diffuse to the negative electrode to react rapidly with the active material, whereby lead is converted into lead oxide. This reaction means that the negative electrode is partially discharged and is prevente from reaching its completely charged state. This means that hydrogen gas also prevented from developing at this electrode. Since the velocity of the oxygen gas recombination at the negative electrode is greater than th velocity of the generation of oxygen gas at the positive electrode, there will be practically no loss of liquid and simultaneously the overpressure in the sealed battery will be: limited..

The disadvantage in ma ntenance-free batteries having the electroly absorbed in porous separators being only partly filled with electrolyte naturally consists in the fact that ^* at d scharge there will be a lack of sulphate ions which can react with the active material and this lack is significant at discharges with small currents when, as described above, practically all active material in the battery can be used. Batteries' of this type will therefore have a relatively low capacity compared to conventional batteries having a liquid electrolyte. Sealed lead acid batteries having split negative electrodes are previously known, e.g. from US-PS 3 457 112. In this construction the spacing between the negative electrodes is ^" not filled with electrolyte bu is only intended to enhance the active areas of the electrodes and thereb the recombination effect. The object of the present invention is, however, to provide a construction of a maintenance-free battery in which at small current dis¬ charges no lock of sulphate ions will occur and which therefore, in view of the capacity are substantially better than conventional sealed batteri having either single or double negative electrodes. This object is real zed according to the invention substantially by the fact that a porous separator saturated with electrolyte is disposed between the negative electrodes.

Thus, by doubling the ^* negative electrodes and inserting a porous separator between them an extra supply of sulphate ions in the electrolyt is created between them which results in the fact that the battery is giv considerably better discharging characteristics for small current outputs compared to conventional sealed batteries.

Naturally it would also be possible to arrange the acid reservoir between double positive electrodes, but since the oxygen gas is developed at the positive plate the recombination would decrease and the amount ^* of gas escaping from the cell would increase since only a minor amount of oxygen gas would be able to reach the negative electrode by diffusion in the porous separator. In addition, for a split negative electrode the recombination will be more efficient since a larger active area of the negative electrode is obtained.

An embodiment of the invention chosen as an example will be describ in detail below with reference to the accompanying drawing showing a battery cell according to the invention having, double negative plates.

In the Figure a lead battery is schematically shown, the electrodes being enclosed in an airtight case 1 and the positive 2 and negative 3 electrode are separated by a separator made from porous material 4, in which the electrolyte is absorbed. Since a sealed battery of this type do not permit the occurance of free electrolyte in the cell the porous separator is saturated with acid only up to about 90%.

According to the invention the negative electrodes consist of at least two electrode plates 3a and 3b, positioned at a certain distance fr each other, whereby a supply space 5 for the electrolyte is formed which is also absorbed in a porous separator.

According to a preferred embodiment the electrode plates can be provided with openings in order to facil tate the migration of acid from the space between the plates to the positive electrode. As has been indicated above the oxygen gas generated at the positiv plate will be given good possibilities to reach the negative plate throug the porous separator in order to oxidize the lead to lead oxide. For small current loads the acid surplus between the two negative plates will guarantee a high battery capacity compared to a conventional battery of t ma ntenance-free type. The small amount of oxygen gas developed at the positive electrodes which will not immediately reach the adjacent negativ electrodes will cause a certain pressure increase in the case, but this will be possible to maintain at a relatively low level, since the

larger areas of these electrodes. The distance between the positive and negative electrode plates could be varied but should preferably be about 1 mm. The distance between the negative electrodes could be varied depending on the necessary acid reservoir but should normally preferably b between about 1 and 5 mm.