Construction Continued

Radio Record, Volume I, Issue 43, 11 May 1928, Page 13

Construction Continued

ee THE LIFE OF THE B BATTERY

SOME INTERESTING FIGURES

! RECENT article dealt with the | cost of running B batteries, demonstrating the advantage of using the larger or ‘"‘super’’ sizes rather than the small or ‘‘standard" sized cells where the load is more than 5 milliamperes, or a three-valve set. Claims are sometimes made that these small cells will stand up to a heavier drain, but they cannot be substantiated. A British radio journal gives further light on the subject. A test was made of a "standard" dry B battery upon the label of which it was stated that the normal discharge rate was 10 milliamperes. Run for three hours a day upon week-days only and kept at a constant temperature of 62 degrees Fahrenheit, this battery proved to have a useful life of just under eleven davs, or 324 hours of service, and this, with an initial drain of 10 milliamperes. which, as the voltage fell, was reduced to 7 milliamperes. THE average potential drop of this battery during its daily three hours’ run was nearly 12 volts. The actual rate at which the voltage drop occurs during a three hours’ run is also a matter of considerable importance. It might be thonght that it would take place quite regularly, but this is not the case. During its period of rest the battery recuperates the potential rising to a reading much above that obtained at the end of the previous working period. GRID-BIAS ADJUSTMENT. S soon as the battery is placed under load again a rapid falling off is seen owing to rising internal resistance. The current declines in proportion, and when a point is reached at which the discharge rate is such that the battery can cope with it fairly | well the reduction of potential becomes much more ‘gradual | ‘ Records of curves from readings taken every quarter-hour show that a heavy fall in voltage is to be expected | during the first hour, a smaller one_ during the second, and a still smaller one during the third. When, therefore, small batteries are used under a fairly heavy load, a reduction of the grid bias may be desirable at the end of the first and second hours of each run. It shonld, however, be remembered that any such -reduction will mean an increase in the current and, therefore, a still more rapid fall in the voltage ANOTHER point worth attention is the amount of recuperation shown by the battery. On some days, in the case of the one whose history is plotted in Fig. 1, this was as much as 12 volts, or rather more. Now, rapid recuperation, especially if it means a quick fall in the voltage at the beginning of the next working period, is not altogether a virtue; in fact, one would much prefer a battery which showed a smaller temporary recovery, and a subsequent voltage drop of less amount. Another point which emerges from a study of Fig. 1 is that voltage readings taken after a rest period, and before the battery is placed under load, may be entirely misleading. On the eleventh day, for example, a reading taken prior to the working period would have shown a voltage of 84.2; one might, in fact, have thought that the battery was by no means in bad condition At the end of the first hour, however, the voltage was 72.8 and at the end of the second, 70.4 A reading should never be taken untii they have been for at least an hour under their ‘normal working load. BATTERIES OF LARGER TYPE. | addition to the standard capacity battery two other sizes are upon the market. These are the ‘large capacity," made up of cells 1jin. in diameter by 2tin, in height, and the "super capacity,’ whose cells measure on the average liin. in diameter by rather less than 84in. in height. What performances are to be expected from these under a load of 10 milliamperes? The life history of a good quality largecapacity battery (by no means the best ‘of those tested) over a period of eigh‘teen weeks, the conditions of the test being the same as those previously described, shows a discharge at the rate

of 10 milliamperes for three hours daily on week-days. HEAVY INITIAL DROP. THIS curve is most instructive. It shows that,.as m.ght be expected, a fairly heavy fall takes piace during the first week, after which the battery settles down and maintains a comparatively steady If.M.F., with a very gradual falling off. ‘The point at which it begins to steady down is approximately 93 volts, thus showing that about 9 milliamperes is a load with which it is well able to deal, On a conservative basis their average useful life may be estimated as at least six months. This means that two renewals will be required in the course of a year at a cost of roughly fourpence per volt. The annual cost thus works out for 100 yolts at £3 6s 8d., or .8 penny per hour for 1000 working hours , AN IMPORTANT POINT. A VERY important point to notice ‘about the life curves of the largecapacity battery is the small fall in E.M.F:; which takes place during a working period. On the last day of the first week, for example, the average fall during the three hours’ run was just over 3 volts out of the- 100, and matters were very little worse after eighteen weeks.. It follows that no noticeable distortion is likely to occur during an evening’s reception if the low-frequency valves are properly biased at the beginning; it will be seen that there is very little need to bother about the grid bias, provided that it is correctly adjusted at the end of the first ten days or so. The life of the average small grid-bias battery is about nine months, and its own fall in E.M.P. will be almost sufficient to allow for that which takes place in the high-tension battery. THE "SUPER" CLASS. We come next to the super battery, usually made up in nominal 43or 50-volt units from eleven to twenty pounds apiece ‘The actual weight depends largely upon the nature and amount of solid insulating imaterial used ‘These batteries entail rather a large initial outlay, but experiemce shows that whenever the average load is 10 milliamperes or more they are well worth the extra money. They will stand a drain of 25 to 30 milliamperes for long periods, and one of their great virtues is that they very seldom become noisy even when their yoltage has fallen to something very low indeed. The provision of super batteries represents an initial cost of about sixpence per volt. Under a 10-milliampere load at least a year’s working on an average of three hours a day is to be expected. The annual cost for 100 volts, therefore, works out at £2 10s., or .6 pence en hour for a thousand hours of work. It will be realised that the life of any size of battery depends (provided that it is of reasonably good quality and is kept in a suitable place) mainly upon the number of hours per week that it is run and the average load imposed upon it. USING TWO B BATTERIES. THE use of two separate B batteries is then discussed, showing that a small battery may be used for all but the power tube, and for that a medium or large sized battery supplied the current. Should the first low-frequency valve, either of the ‘first stage L.I’."’ or of the "R.C." type, be resistance-coupled, it may also be seryed by the small battery, for the total load for the whole of tins portion of the set is in this case not likely to exceed 8 or 4 milliamperes. ‘he valve in the last stage may then be provided with a battery of its own of the medium capacity type if it is of the small power kind, or of the largest type if it is a super-power valve. Besides leading to economoy in working, the use of a separate battery for the last low-frequency valve has other advantages, which cannot be dealt with here. Let us take a concrete example of the saving produced by using separate batteries. On measuring the total current taken by a three-valve set we find that the amount is, say, 10 milliamperes. On switching off the last valve, however, the current falls to 3. The high-frequency and rectifying valves are thus passing 3 milliamperes and the power valve in the last stage 7. ANNUAL COSTS. REFERENCE to the life euryes in Vig. 6 shows that under a 3-mili-ampere load about half a year’s working may be expected from the small battery, whilst at 7 milliamperes one of medium capacity should last for some thirty-five weeks. The small battery will thus have to be replaced twice a year, the annual cost being thus about £1 for H.F, and rectifier H.T. supply. The medium-capacity battery will need renewal about once every nine months. If, therefore, 100 volts are uSed, the annual cost will be about £2. We thus get a total outlay of £3 per 100 volts, which for a thousand hours works out at .72 pence pcr hour. Now, if a common battery of the medium capacity size were used to supply the 10 milliamperes needed for the set, its useful life, as the curves show,. would be about half a year. The annual ex-

penditure would thus be, roughly, £4, or .96 pence per hour for a thousand hours, There is thus a very distinct economy in such a case in using two batteries. (End of Construction.)