Choosing a "B" Battery

Radio Record, Volume VII, Issue 6, 18 August 1933, Page 25

Choosing a "B" Battery

An ‘explanation of the principle on which the dry cell works, together with some practical hints on the pitfalls to avoid when choosing a "B"’ battery.

HE "B" battery, which is perhaps the most expensive item in the upkeep costs of a battery set, often receives the least attention. The aver age listener usually gives much care ful thought to the choice of a loud. speaker and the care of his valves, but when it comes to buying a "B" battery he is content to see the voltage checked ‘by the dealer, and then to connect it-up and hope for the best. The construction of a dry battery and the principle on which it operates is not very difficult to understand, and such knowledge can be turned to very practical use in both choosing and using a "B" battery so that running costs will be kept down as far as possible. Practically every. "B" battery on the markat fa a ‘'davalanmant af

MACS AWE Are Mw TOA VY OO eee ere the Leclanche primary cell ’ (Figure 1). . This consists of a zine and carbon rod immersed in a solution of ammonium chloride’ or salammoniaec. This develops a voltage of 1,6 volts, and if we connect some sort of load across the terminals, current will flow. The energy for this current is obtained by a chemieal action between the salammoniac and the zine, as a result of which the zine road .gradually becomes’ eaten away: and the ammonium chloride changes to zinc chloride. . If we endeavour to make nea anf a pall anf +hic tuna far

ete eal ---eEeeEe ee ees lh EP le eT any of time, however, we find that the current, instead of being maintained, rapidly falls away to nothing. If the load is removed and the cell allowed to stand we find that it recovers its voltage and we can once again obtain a current from it. Clearly this is of no use to us in practice, and we must find what is causing the cell to fail in this manner and overcome the difficulty if possible. This effect is actually caused by formation of hydrogen gas around the surface of the carbon plate. ‘This is a result. of the chemical action in the cell, and since the gas is an insulator it prevents the current from flowing through the liquid out through the carbon rod,

We overcome this defect by surrounding the carbon with what we term a de-polariser. This is a substance which has a. strong affinity for hydrogen, such as manganese dioxide. This de-polariser immediately attacks the hydrogen as it is formed and absorbs it so that the current has free access to the carbon. With this device, provided the current is not execessive, we are able to take current for long periods, but it is a peculiarity of the cell that the de‘polariser cannot completely absorb the hydrogen so that after it has been in use for some time the voltage falls and recovers again when the cell is allowed to stand idle. The extent to which this is so de-

pends upon the construction of the cell and the facilities provided for the depolariser to come into contact with the carbon. Now in a dry cell, as it is called, we dispense with the liquid electrolyte and use, instead, a paste. The com: position of this paste varies with the different makers, one form consisting of flour, plaster-of-paris, sal-ammoniae and zine chloride, in suitable proportions, mixed with water to form a paste. Reducing Resistance. "4 In many cases gelatinous paste fs made up and in any case the amount of paste used is fairly small in order

to, reduce the external resistance of the cell, The de-polarising agent is usually contained in a sack of a porous nature built up around the carbon rod. And finally, instead of using a zinc rod in a glass container, the container itself is made of zine, the whole cell being as shown in Figure 2 in general construction. This, of course, has the disadvantage that as the cell is used up the zine is eaten away, and once this has happened, the cell is completely useless and must be discarded. Local Action in Cells, Having described the general construction of the cell, let us consider some of the finer noints. such

as suitability for different elasses of work. First of all, a difficulty which has to be avoided in _ practical manufacture is that of local action. -If the zine contains impurities, then it is found that chemical action takes place between the zine and the electrolyte: around the neighbourhood of such impurities, whether the cell is being used or not. Clearly, therefore. unless such a cell is in continuous use it will be very unsatisfactory, for it will be deteriorating the whole time, whether current is being drawn from it or not.

Mercury Amalgam. This difficulty may be avoided by using very good quality zinc, or by amalgamating the zinc with mercury, forming a very fine zinc-mercury amalgam on the surface, and, thirdly, by using different chemicals instead of sal-ammoniae for the active agent in the cell, Manganese chloride, for example, is often used, and other chemicals are employed by different manufacturers with the object of reducing this local action. Another important point about this form of cell is that with certain proportions of chemicals in the active agent and the de-polariser, the cell will operate best at a certain rate of discharge. (Continued overleaf.)

With a heavy current the de-polariser must be more effective than with a small current, and it is clearly uneconomical to have a small percentage of de-polarising agent with a large percentage of active f(sal-ammoniac) material, and vice versa. Best Discharge Rate. Therefore, for each particular construction of cell, there is.a definite alischarge rate at which the best life will be obtained: As the cell is used and the chemicals become exhausted, so the voltage across the termijals of the cell falls and the use life of ‘the cell or battery is determined by the time taken for the voltage to fall to a pre-determined value. This pre-determined value is generally taken as between one-half and two-thirds of the original voltage, for when the cell has reached this condition, its internal resistance will have grown so high that difficulty will be experienced with its use. Consequently, although a cell will continue to give voltage" until it is completely exhausted, it is not desirable to use it beyond this point. If -a small-sized cell is used with a current greater than the optimum figure, it will not have due time for recuperation, and it will become exhausted at a relatively more rapid rate. This is to say, that if a cell will last for 200 hours at a discharge rate of 5 milliamperes (giving 1000 milliampere hours), it will not last for 100 hours at a discharge rate of 10 milliamperes. It will only last for perhaps 80 hours, giving us a 20 per cent. loss of efficiency, If, on the other hand, we use a cell at a smaller. rate of discharge than the optimum value, then, again, we shall not obtain the best life because of the local action which takes place. This effect is often referred to as shelflife, meaning the capability of the cell to maintain its properties when standing in stock on a Shelf, A battery, such as a grid-bias battery, which is not required to give any eurrent, but merely to maintain its rated voltage for long periods, is specially designed to avoid local action and, therefore, to have a long shelfife, '- Excessive Local Action. A power battery, on the other hand, ig designed to give large current intermittently, and is intended for use. That is to say, it will not last indefinitely in stock if not used. Similarly, if we do not utilise a battery to its full eapacity, then we get an excessive proportion of local action, which means that the battery becomes exhausted before we have had the full discharge eapacity from it. And so, when buying a "B" battery, it pays first to determine the current consumption of one’s receiver, and then to buy a battery which is rated to supply approximately that amount of-cur-rent for its optimum life. A doublecapacity battery does not cost twice as much as a. single-capacity type, but it will last twice as long, and similarly with a triple-capacity battery. The get’s requirements in the way of total. plate current can be easily ascertained in either of two ways: firstly, by placing a milliammeter in the B- lead; and, secondly, by referente to the 8 leaflet accompanying each: valve, and which gives the plate current taken by the valve at the plate voltage employed. Of course,

in the case of screen-grid and pentode valves, the current taken by the screen must be added to the plate current. Several Maintenance Hints. With intelligent use the modern drycell battery is a reliable and economical source of plate current, but it should not be overlooked that it is essentially a delicate organism which is easily open to abuse. It should be installed in a cool place away from radiators or hot-water pipes, and short-cireuits, of however short a duration, must be avoided at all. costs. From time to time the set should be tested for leakages by inserting a sensitive milliammeter in the B+ lead with the "A" current switched off; in any case, it is a good plan to remove the plug connections from. the battery if it is to be left standing for any considerable time. Iinally, good inter-cell insulation is always: worth paying for. and a few more shillings on the initial cost of a battery so constructed is invariably justified by the performance of. the hattery toward the end of its life,