The importance of keeping storage batteries clean can not be overstressed. Aside from the unsightliness of a dirty battery, the accumulation of dirt, moisture and acid on the battery shortens the life of the battery by producing a leakage path for the current between terminals. This leakage current would not be the same for all the cells, with the result that it would be difficult to keep all the cells at the same condition of charge. Therefore some of the cells would have to be overcharged to fully charge the entire battery. Corrosion of the terminals would spoil the electrical connections, and might cause disturbing noises in the reproduction of sound.

42. Adding Water to Batteries. -Water must be added to each cell regularly, and the electrolyte should never be allowed to get below the top of the separators. They should never be filled higher than just below the bottom of the filling tubes, because there should always be room above the liquid .for the gases to escape freely. The electrolyte expands when heated and its level rises. If the cells are filled too high, some of the electrolyte will be lost when the batteries gas, as they always do when being charged or discharged. Therefore, the best practice is to keep the electrolyte level about one-quarter of an inch above the plates by frequent fillings and thereby avoid all possibility of losing electrolyte. The frequency with which water should be added depends on how much the batteries are used. When used continuously they should be inspected daily. The water should be added just before charging, as this causes the water to mix thoroughly with the electrolyte.

Use pure distilled or other approved water only. Ordinary water often contains minerals and other impurities harmful to the batteries. When in doubt about the quality of other water, use distilled water.

Water used for filling storage batteries should not be stored in any metallic vessel (lead excepted). Glass, earthenware, rubber, lead or wooden receptacles that have never been used for any other purpose are satisfactory. If water is drawn from a tap, it should be allowed to run a few minutes before using it.

Never add acid under any circumstances. The acid is not "used up" by the chemical reaction in the battery and never needs replenishment.

43. Checking the Condition of Charge of Storage Batteries.- The most reliable means of checking the condition of charge of a storage battery is that of measuring the specific gravity of the electrolyte. The specific gravity of any substance is the ratio of weight of a given volume of the substance to the weight of an equal volume of water. The specific gravity of pure sulphuric acid is about 1.83 ; that is, a quart of pure sulphuric acid weighs 1.83 times as much as a quart of water. If water and sulphuric acid are mixed together, the specific gravity of the solution will be something between 1 and 1.83 depending on the percentage of acid in the solution. The percentage of acid in the solution can therefore be determined by measuring the specific gravity of the solution.

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