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attempted, and replacing the loss in cells occasioned by evaporation and overcharge. The frequent addition of water required to replace evaporation leads eventually to a concentration of any impurities which it may contain. As ordinary water supplies are not pure, their use is always questionable. Water from natural sources should be used only with the approval of competent chemists. Rain water should not be used if distilled water is available, as it often contains traces of nitric acid and ammonia, either of which is harmful to storage batteries. Distilled water is preferable and should always be used unless otherwise advised by the chemists. The water should be stored in carboys or thoroughly cleaned whiskey barrels. Water obtained by condensing the exhaust from engines and which may thus contain cylinder oil and other impurities should never be used for battery purposes. In cleaning batteries the ordinary tap water may be used provided it does not contain a great quantity of impurities.
In this connection, the expression “chemically pure” acid is often confused with acid of “full strength.” Acid may be of full strength (approximately 1.835 sp. gr.) and at the same time chemically pure. If this chemically pure acid of full strength be mixed with chemically pure water, the mixture would still be chemically pure, but not of full strength. On the other hand, if a small quantity of some impurity be introduced into chemically pure acid, it would not materially reduce the strength, but would make it impure. The usual method of determining the strength of electrolyte is by taking its specific gravity. The method is possible on account of the fact that sulphuric acid is heavier than water. Therefore the greater the proportion of acid contained in the electrolyte the heavier the solution or the higher its specific gravity. By specific gravity is meant the relative weight of any substance compared with water as a basis. Pure water, therefore, is considered to have a specific gravity of 1, usually written 1.000 and spoken of as “ten hundred.” One pound of water is approximately one pint. An equal volume of concentrated sulphuric acid (oil of vitriol) weighs 1.835 pounds. It therefore has a specific gravity of 1.835 and is spoken of as “eighteen thirty-five.”
Since electrolyte, like most substances, expands when heated, its specific gravity is affected by a change in temperature. For the convenience of the operator, the following table is given to show the variation in the electrolyte specific gravity at various temperatures likely to be met with in service:
The solution must be allowed to stand several hours to cool. Never add hot or even warm electrolyte to a cell, as the plates are liable to be dangerously sulphated thereby. The strength of the resultant solution should always be checked by hydrometer readings reducing the latter to 70 degrees Fahr.
Features of the Edison Cell.— The instructions given apply only to batteries of the lead plate type and not to the Edison battery, which is entirely different in construction. The Edison cell uses an electrolyte consisting of 21% solution of potash in distilled water so that the electrolyte is alkaline instead of acidulous. For 6-volt ignition and lighting service it is necessary to use 5 cells owing to the lesser voltage of the Edison batteries. The average voltage during discharge is but 1.2 volts per cell, and is not as constant as is the case with a lead battery, the voltage of which may be as high as 2.5 volts per cell.
An Edison 6.5-volt battery used for lighting or ignition may be charged completely in ten hours. A feature of the Edison battery is that overcharging at the normal rate has no harmful effects, and it is advised by the maker to give the battery a 12hour charge once every 60 days or when the electrolyte is replenished. The electrolyte must be kept sufficiently high so as to cover the plates, and any loss by evaporation must be compensated for by the addition of distilled water. Another feature in which the Edison battery is superior to the lead plate type is that the plates will not be injured if the cells are allowed to stand
in a discharged condition. The external portions of the cells must be kept clean and dry, because the container or can is made of a conducting material. The vent caps must be kept closed except when replacing electrolyte or bringing the level up to the proper height by adding distilled water. Care should be taken to avoid short circuiting of the battery by tools or metal objects, and special emphasis is laid on the precaution that no acid or electrolyte containing acid be poured into the cells. It is said that the Edison battery has a longer life than the lead plate type of equal capacity.
Repairing Exide Sealed-Type Batteries. The smaller Exide
cells, such as used in electric vehicles and for automobile starting, lighting and ignition systems, are made in two types. In one a double flange cover is depended on to keep the electrolyte from splashing out, this construction being shown in the sectional view at Fig. 25. In the other a single-flange cover, as outlined at Fig. 26, is utilized in connection with sealing compound. The double-flange cover has two downwardly projecting flanges, one fitting inside and the other outside of the cell jar. The two flanges form a channel or slot, holding the jar walls. In order to insure a tight joint a small amount of sealing compound is placed at the bottom of the slot and between the cell cover and the top of the rubber jar.
Fig. 26.—Exide Single Seal Burned Connector Type Battery.
To unseal this type of battery two stout boards about onequarter of an inch longer than the height of the jar are needed. Rest the side flanges of the jar on these, as shown at Fig. 27 A, so the cell will be raised and the weight supported by the wood blocks. Next warm the cover around the edges so that the sealing compound will soften. Of course, the terminal straps and