The High Tension Battery

Radio Record, Volume I, Issue 37, 6 April 1928, Unnumbered Page

The High Tension Battery

AN ARGUMENT FOR "SUPER". SIZES

A BRITISH wireless journal gives the result of an interesting test carried out on B_bafteries of several inakes, tending to show that, the larger sizes of B battery pay better than the small or "standard" size. . 1 "nPHE whole secret of success when dry-cell batteries are used lies simply and solely in thé installation of units capable: of ‘staiuding ‘up for a reasonable period ‘to the load’ imposed upon them. ‘This, unfortunately, is a point that isnot realised ‘by large numbers of wireless enthusiasts: ‘Dry high-tension batteries. fall as regards capacity, into three © main’ groups. . There is, first of all, what. may be called the "standard | capacity" battery built up of cells measuring on the average jin. in diameter by 2tin.-in height. " Next comes the large capacity battery whose cells average ljin. in diameter by ‘2}ih. in height. Lastly there is the ‘‘super" capacity type whose cells have a diameter of-ltin. and a-height of about gill. ‘ . Probably at least 75 per .cent.. of the batteries sold, no matter what the. receiving set with which, they are. to be used, are of the standard capacity type. In other words, the purchaser is apt to think only of the volts and not to bother about the milliamperes. When a given number of new dry cells, whether large or smali, are wired in series, the potential on open circuit will always be in the neighbourhood of 1.5 voit per cell.

RAPID VOLTAGE DROP. "Bur whilst cells of large size can deliver a fair amount of current for a considerable time without much falling off in potential, those of small _size show a rapid drop when under any but the lightest of loads. ‘In the dry cell the fall in potential is of two kinds. There is, first of all, what may be called the temporary fall, which takes place while the battery is under load, and is due mainly to the effects of polarisation. A film of hydrogen gas collects round the carbon positive element and this causes a rising resistance to he present within the cell, since the deploriser cannot get rid of it sufficiently quickly. | "When the cell is rested by being» placed for a time on open circuit the accumulation of hydrogen is removed

ee . « . :t H at and the potential tises. It does not, however, quite reach its initial figure,’ and we thus have a second or per manent falling off in the voltage. From the wireless man’s point of ‘view both kinds of voltage drop are enormously ‘important. We shall go niore fully, into. the reasons in a moment. _ "There can be no doubt that for the .single-valve set. or for the ‘twovalve or even three-valve set employed. only for telephone reception the use of. the standard capacity battery -is ‘quite sound practice. Such’ batteries Will stand up to an intermittent load of from 8:to 5 milliamperes for:a very: * long time, But matters: are rather: different. when the receiving set ig ‘required to work a loudspeaker, ‘HEAVIER LOADS. "TF anything like quality is desired i a small power valye must be used in the last holder, and for really beautiful reproduction a super-power valve is indicated. » Both power valves and super-power valves impose heavy loads upon the battery, and the current required :is likely tc be from 8 to 20 milliamperes, according to the size of the: receiving set. It is probably fair to take the current passed by the -‘ayerage loudspeaker set when the hightension battery is in good condition at about 10 milliamperes. | How will the standard capacity battery deal with this. lord? . The question is an important cue, and the best way of finding the answer seemed to be to undertake a series of tests under laboratory conditions. A load of 10 milliamperes when a battery is up to its full nominal E.M.F, is imposed when it is discharged through a resistance of 150 ohms per cell.

A STANDARD RESISTANCE. "THIS, then, was made the standard ; resistance for the tests. Since batteries differ considerably in the number of their cells and in the tappings provided, the total terminal voltage was not by any means the same in all of those tested. Some, for example, were 386-volt units, whilst others had a nominal If.M.F. of 60 volts (really 63 volts), others of 66 volts, and others, again, of 100 volts. "In every case, however, a resistance of 150 ohms per cell was used for the battery tested, so that all were subjected to exactly the same drain, ‘'o make comparisons easier, all results have been reduced to a percentage basis, an E.M.F. of 1.5 volt per cell being regarded as a hundred per cent. A battery was taken as done for when the H.M.F. had fallen to seventy per cent., or a little over 1 volt per cell.

THE TEST PERIODS. "TT was decided to run the batterics for three hours a day from Monday to Saturday inclusive tn each week, giving them twenty-one hours’ rest in each twenty-four, and a rest of forty-five hours over the week-qnd. The total service hours were thus eighteen per week, which is very much what is asked of the average hightension -battery. Only batteries of well-known type were selected for test and two of each make were uséd "Tn eyery case the batteries were ordered specially so that there should be no possibility of their having deteriorated by having been in stock for a considerable time The first process was to take the initial open-cir-cuit voltage, and this disclosed some interesting variations. . BIG DIFFERENCES. RATTERIES of the same make gave, as a rule similar readings, but there were big differences between (Continued on Page 13.)

Construction Continued those of different makes, On the aver- | | age the initial voltage was just over 1.5 ‘per cell, but the test disclosed that it is not always the battery with the highest initial voltage that gives the ‘best service. ‘For all the tests a voltmeter with a resistance of 97,50) ohms was used, | for only with a ,high-resistayce in-. struments can ‘accurate "readings: be" taken. Two of the batteries were found to have "bad spots" in them when received, and these were, im-_ mediately replaced by the makers. — Once the test was under way it prov--ed to be full of interest-and of surprises, ‘The first batteries to succumb, (both of the same make) reached the 70 per cent. mark on the nineteenth day, having given an average of fifty--six hours’ service. These had the highest. initial E.M.F,.of any tested, and .for .the first week it appeared that. they were going to do pretty well,, At the.end of that time, however,’a rapid decline set it. ‘AN, EXPENSIVE PROPOSITION. T soon became evident that a bat- .+ tery with a performance such as this. could not prove an economic. pro-’ position for high-tension supply ° -in the. multi-valve set: It wonld require renewal at. least twelve times a‘ year, and if the, average initial ‘cost. of a’ standard capacity battery is taken as twopence a volt, the-expenditure ‘per: annum would work: out at two° shillings’ per volt,or £10 for 100° volts: ~ If we take it that’ the receiving ° set: is in use for a thousand hours during the year, the rinning: costs ‘per hour for H.T. alone come to a little, under. twopence- halfpennyy. © Similar batteries tested gavé Yarying, but ‘similar, results: i ; / . SUMMING: UP. * To sum up, the ‘standard: capacity battery is quite ‘urtsuited for working any set that is required. give greater volume than that « prodne: ed either by -head telephones or. by a’ midget loudspeaker. In fairness ‘to manufacturers, it must be said tliat the’ majority of them do not recommend sinall cell types for any such parpose, whilst many state quite clearly that the maximum economical’ discharge . is’ of the order of 5 milliamperes: ‘ It is evident that batteries of Jarger’ capacity are likely to be far miore economical. Exact data upon this point’ must be given later, since the tests that I am conducting with bigger batteries are not’ yet complete. Speaking generally, though, one may sav that where the load exceeds 5 milliamperes, but does not exceed -40, a battery of the large-capacity type is likely to prove a much more profitable investment. The permanent fall in voltage is much less rapid, and the. temporary drop during a_ three hours’ run is far smaller. Figures of the running costs will be given in a later article. A DISTINCT ECONOMY. y INH a load of 10 milliamperes or more it undoubtedly pays to purchase batteries of the super type. Although the initial cost may seem rather high, there is a very distinct econonly in their use.