DRY BATTERIES

Sun (Auckland), Volume I, Issue 203, 16 November 1927, Page 14

DRY BATTERIES

THEIR USE IN RADIO. POINTS TO BE OBSERVED. In the year 1868, Leclanche, a French inventor, perfected a battery or cell which had such useful characteristics ; that it has become one of the most comj mon batteries in modern use. The ori* : ginal cell which he devised was a wet ; cell. It contained two electrodes, a I positive and negative, both placed in a ! suitable jar, which was filled with electrolyte or active liquid. The two electrodes were a zinc and a carbon rod surrounded by manganese dioxide, while the electrolyte was ammonium chloride or, as it is popularly known, “sal-ammoniac.” ADVENT OF DRY CELS. Some 20 years later the dry cell was i made on the same principles. The only difference between a wet cell and a dry cell lies in the fact that the electrolyte is made up in the form of a paste so that it is not spillable. The zinc electrode, instead of being made in the form of a rod, is made to serve as a container so that there is a pot of zinc, a central rod of carbon surrounded by a large volume of the manganese dioxide depolarizer, and the intervenvening space is filled with a paste containing sal-ammoniac as the active part. The voltage developed by a dry cell lx not quite so great as can be obtained with a wet cell because of the somewhat higher internal resistance, but the dif- * ference is usually less than 0.1 of a volt. As the cell runs down, however, < chemical changes take place, and this causes a difference in the potentials existing inside the cell. For one thing, the electrolyte is no longer ammonium chloride, but contains a large proportion of dissolved zinc chloride. Secondly, the manganese dioxide has largely been used up in removing the bubbles of hydrogen gas round the carbon rod, and the resulting mixture of carbon and reduced manganese dioxide has not such a large potential difference to the electrolyte as it had before. The actual figures are as follows:—* Carbon-manganese dioxide to electrolyte, 0.4 volts; electrolyte to zinc 0.3 volts, giving a total voltage of 0.7 volts instead of 1.5 volts. Of course, when the cell reaches such a condition as this it is useless for further work. In practice, therefore, the conditions required are that the cell shall give as long a service as possible before the internal chemical change takes place which causes the drop in voltage. For radio receiving purposes there are two separate requirements. First of all there are cells equipped for lighting the filaments known as A batteries, and secondly there are batteries for providing B battery and grid bias voltages, known respectively as B and C batteries. The conditions of service are somewhat different and it will be ns well to discuss the two classes of cells separately. FILAMENT LIGHTING BATTERIES. The requirements of the filament battery are that it shall give a reasonably large current for a considerable period. The current supply is comparatively intermittent, lasting from two to five hours a day on the average. If a battery stands for some time without any current being taken from it very small chemical reactions take place inside the cell, largely due to the presence of minute impurities in the chemicals, and ultimately this will cause local action at the zinc electrode, resulting in the zinc being dissolved by the sal-ammoniac, although no current is being passed. This in time will cause holes to appear in the zinc container, and by the time the action has proceeded to this stage a considerable internal change has taken place inside the cell and the voltage has dropped considerably. Thus if a cell has been kept long on the shelf ** w |ll lose its voltage automatically and become useless. The best conditions under which io use a battery depend upon its shelf life. The less a battery Is used the longer becomes the life, up to a point where the internal action causes the voltage to drop irrespective of the current taken from it. USE OF B BATTERIES. When it is a question of B batteries, life of the battery is not such a serious matter, because the current taken from the battery is much smaller, and it is shelf life which is of importance. The most important requirement is that the internal resistance be conS il an ** With the smaller cells inside the it is not possible to keep the actual internal resistance anything’ like as low as a filament batterv. It( is important, however, that this resistance be kept constant, as otherwise whistling and howling in the amplifier) may be set up, and if very bad continual crackling will result. Particularly with this type of battery there is a considerable difference between an intermittent and a continuous discharge, owing largely to the smaller available quantity of active material in a small battery. In order to keep the shelf life long and to keep the internal resistance, constant great care has to be taken in the insulation between the individual cells. If this is not done, then continual small leakage currents will flow, which will cause the battery to run down -very quickly, quite apart from the unpleasant noises which will be set up in the cirrents by tbe °* t^ese leakage cu rIn conclusion, therefore, it will be seen that the design or choice of at battery depends entirely upon the adequate balance between the capacity and the shelf life Obviously the largest battery possible should be chosen for any given load, provided that it obtains a reasonable shelf life. There is no economy in using large batteries if thev lose their efficiency during the period of recuperation. These pofnts are usuln m,nd , by the manufacturers and as a result of exhaustive rework there are now a number of batteries on the market specially dcsigned for the stringent and exacting -onditions of radio service.