Choosing an "A" Battery

Radio Record, Volume VII, Issue 4, 4 August 1933, Page 25

Choosing an "A" Battery

_ Some sound practical advice on the choice and maintenance of ditferent types of ""A’"’ batteries

NCE a wireless set is made or purchased, if the owner does not happen to be of an experimental turn of mind, little is done in the way of alterations or renewals, The batteries, however (assuming that the set is a valve receiver employing batteries), do require renewing, sometimes inconveniently often, and there arises the chance either of making a mistake or of spending wisely and improving reception. The results obtained from a valve sef can be made or marred by carelessness with regard to batteries. Take first the case of the "A" battery, which has the task of supplying the filament current of the set. Obviously, the more valves there are, the bigger should be the battery. And, of course, it must be of the correct voltage to suit the valves. The most popular type of battery valve is the 2-volt, because its efficiency is virtually the same as 4 or 6-vyolt types, and it requires 3 battery only one-half or one-third of the size of the other types.

Choosing an Aceumulator. Suppose then, we are going to use 2-volt valves. What size should the"*accumulator be? To decide this, we must know the amount of current to be used. For an §.G. valve this will probably be about .15 amp., and for a detector 1 amp. Ordinary audio valves also take about .1 amp. The exact figures for your own yalves are available from the leaflets contained in the valve cartons. ‘To find the total filament current of the set; simply add the respective figures together. A. typical case for a three-valve set would be :- Sereened-grid R.F. yalve .. 15 amp Detector... ee eee nconee 1 amp Power valve ...... eeeeeeee 2 amp Total filament current .45 amp. Thus the set will take a little less than half an ampere from the "A" battery all the time it is running. The next question is how long will that be’ You know approximately how much the set will be used, so there is no difficulty in this. Taking an average casc again, let us suppose you will be using the set for about four hours per day, which gives you an average of 28 hours per week. We now know ths total current required from the "A" battery in a week. It will be, in this typical instance, 45 amp. for a period of 28 hours. You will notice that both "amperes" and "hours" enter into the calculation. So let us multiply one by the other and get it down to that very handy unit, the ampere-hour. All we have to do is to find the product. 45 « 28 = 12.6 ampere hours per week. This is the heart of the matter, Now that we Jnow in this particular instance we shall want about twelvye-and-a-half

ampere-hours per week, we can sce what the dealer has to us, Actual and Intermittent Ratings. We find that 2-volt "A" batteries are available in various ampere-hour capa-cities-20 ampere-hours, 30 amperehours, ete. But you may note in referring to them the dealer may say, "This is a 20-actual’; "This is a 30actual," ete. What does that "actual’ mean? Simply this: The ampere-hour capacity of an accumulator is very much less when it is discharged continuously for hours on end-as you will want to discharge it, for radiothan when it is discharged momentarily and then allowed to recover. In fact, its actual discharge rating, under radio conditions, will be only about half of its "ignition" ratingmore usually called the intermittent discharge figure. So you want to know the actwal-ampere-hour rating. All good batteries give this figure, and all reputable dealers refer to it when they speak of a battery to be used for radio purposes, Knowing that you are going to use a set which takes about 123} amperehours per week, your next concern will be convenience in the charging. £ you get a 20-actual ampere-hour battery it would last between one and two weeks-a rather awkward period of time. How about a "30-actual?’ Two weeks’ current at 123 is about 25 ampere-hours-that leaves a little margin for emergencies, A "30-actual" accumulator, then, would last two

weeks before it needed recharging, and you would easily remember to get it charged at the regular interyals. Another advantage would be the margin-25 ampere-hours’ use again a 30-ampere-hour rating. So you would always be on the right side if you wanted a little extra current for some specially good programmes. Thus the 30-actual accumulator would be a good choice under the foregoing circumstances, and other, requirements may be worked out in the same way. But there are several considerations to bear in mind. Don’t be tempted to get too big an accumulator for very infrequent recharging. It will deteriorate if it is not kept actively working, regular discharges and charges being the ideal treatment for it. On the other hand, too small an accumulator will often be liable to become run right down, which is yery harmful, though there is much to be said for two fairly small accumulators, one running the set while its fellow is being recharged. In fact, if you carry your own to the charging station this is generally the most convenient method.

For Country Listeners, Battery set owners who live in the country, away from charging facilities, and who do not wish to make their own primary cells, have a choice between two other types of "A" batteries, the 13-volt dry cell, and the "sir-cell." The former are comparatively cheap, but they have to be renewed every few months, the actual period depending, of course, on the amount of current taken by the set and the length of time the set is in use. The air-cell, which is a comparatively recent invention, has a rather heavy initial cost, though it possesses the big advantage that it lasts from eighteen months to two years without any attention, except that it should be "topped up" oecasionally with water. Next week we will discuss the choice and maintenance of "B" batteries, which are just as important as the "A" in ensuring the best results from a receiver.