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scale can be employed for testing storage batteries while the ampere scale may be utilized in determining the strength of dry cells. A fully charged, fresh dry cell should show a current output of from twenty to twenty-five amperes. If the cell indicates below six or seven amperes, it should be discarded as it is apt to be exhausted to such a point that it will not furnish current enough to insure energetic or reliable ignition. Dry cells
Fig. 72.—Showing Construction of Storage Battery Plates. Grids at
Left of Illustration are Not Filled with Active Material in Order to
should always be stored in a cool and dry place, so that the electrolyte will not evaporate. If moisture is given an opportunity to collect on the top of the pitch seal it will allow a gradual loss of current due to short circuiting the cells. In applying an amperemeter, care should be taken to always connect the positive terminal marked with a plus sign against the carbon terminal. In the indicating meter shown at B, it is necessary to use only one contact point which is pressed against the screw passing through the carbon rod. The case of the instrument is placed in contact with the zinc terminal to complete the circuit. A flexible wire is usually included in order to test the amperage of a group of cells should this be thought necessary. When dry cells are used for automobile ignition, they should be carefully packed in box made of non-conducting material, such as wood, and securely covered so there will be no chance for water to enter the container. If placed in a sheet metal case, care should be taken to line the box with insulating material and also to pack the cells tightly so they cannot shake around. The best practice is to use wedges or blocks of wood which are driven in between the cells to keep them apart. In no case should a dry cell be placed directly in a steel box, as the binding posts on the zincs might come in contact with the walls of the box and tend to short circuit the cells, producing rapid depreciation. A battery box should always be placed at a point where it is not apt to be drenched with water when the car is washed or should be watertight if exposed.
Storage Battery Defects. The subject of storage battery maintenance was thoroughly covered in a paper read by H. M. Beck before the S. A. E. and published in the transactions of the society. Some extracts from this are reproduced in connection with notes made by the writer and with excerpts from instruction books of battery manufacturers in order to enable the reader to secure a thorough grasp of this important subject without consulting a mass of literature. Endeavor has been made to simplify the technical points involved and to make the exposition as brief as possible without slighting any essential points. In view of the general adoption of motor starting and lighting systems on all modern automobiles, the repairman or motorist must pay more attention to the electrical apparatus than formerly needed when the simple magneto ignition system was the only electrical part of the automobile. The storage battery is one of the most important parts of the modern electrical systems and all up-to-date repairmen must understand its maintenance and charging in order to care for cars of recent manufacture intelligently.
A storage battery, from an elementary standpoint, consists of two or more plates, positive and negative, insulated from each other and submerged in a jar of dilute sulphuric acid. The plates consist of finely divided lead, known as the active material, held in grids which serve both as supports and as conductors for the active material as at Fig. 72. The active material being finely divided, offers an enormous surface to the electrolyte and thus
Fig. 73.—Part Sectional View, Showing Construction of Exide Starting
and Lighting Battery.
electro-chemical action can take place easily and quickly. Two plates such as described, would have no potential difference, the active material of each being the same. If, however, current from an outside source is passed between them, one, the positive, will become oxidized, while the other remains as before, pure lead. This combination will be found to have a potential difference of about two volts, and if connected through an external circuit, current will flow.
During discharge, the oxidized plate loses its oxygen and both plates will become sulphated until, if the discharge is carried far enough, both plates will again become chemically alike, the active material consisting of lead sulphate. On again charging, the sulphate is driven out of both plates and the positive plate oxidized and this cycle can be repeated as often as desired until the plates are worn out. Thus charging and discharging simply result in a chemical change in the active material and electrolyte, and the potential difference between the plates and capacity is due to this change.
In taking care of a storage battery, there are four points which are of the first importance:
First-The battery must be charged properly.
Third-Short circuits between the plates or from sediment under them, must be prevented.
Fourth- The plates must be kept covered with electrolyte and only water of the proper purity used for replacing evaporation.
In the event of electrical trouble which may be ascribed to weak source of current, first test the battery, using a low reading voltmeter. Small pocket voltmeters can be purchased for a few dollars and will be found a great convenience. Cells may be tested individually and as a battery. The proper time to take a reading of a storage battery is immediately upon stopping or while the engine is running. A more definite determination can be made than after the battery has been idle for a few hours and has recuperated more or less. A single cell should register more than two volts when fully charged, and the approximate energy of a three-cell battery should be about 6.5 volts. If the voltage is below 6 volts the batteries should be recharged and the specific gravity of the electrolyte brought up to the required point. If the liquid is very low in the cell new electrolyte should be added. To make this fluid add about one part of chemically pure sulphuric acid to about four parts of distilled water, and add more water or acid to obtain the required specific gravity, which is determined by a hydrometer. According to some authorities the hydrometer test should show the specific gravity of the electrolyte as about 1.208 or 25 degrees Baumé when first prepared for introduction in the cell, and about 1.306 or 34 degrees Baume when the cell is charged.
The appended conversion formula and table of equivalents will be found of value in changing the reading of a hydrometer, or acidometer, from terms of specific gravity to the Baumé scale, or vice versa.
145 Sp. Gr.=
at 60° F. 145 — Baumé degrees The following table gives the corresponding specific gravities and Baumé degrees :
18 19 20 21 22
3 4 5 6
8 9 10 11 12
1.000 1.006 1.014 1.021 1.028 1.035 1.043 1.050 1.058 1.066 1.074 1.082 1.090 1.098 1.106 1.115 1.124 1.132
25 26 27 28 29 30 31 32 33
1.141 1.150 1.160 1.169 1.178 1.188 1.198 1.208 1.218 1.228 1.239 1.250 1.260 1.271 1.283 1.294 1.306 1.318
Either voltage or gravity readings alone could be used, but as both have advantages in certain cases, and disadvantages in others, it is advisable to use each for the purpose for which it