ACCUMULATOR CHARGING

Radio Record, Volume II, Issue 1, 20 July 1928, Page 13

ACCUMULATOR CHARGING

1 order to charge an accumulator, it is necessary to pass a current through it in the reverse direction to that in which the cur.ent flows when it is lighting the filaments of our ralves. . The filament current consumption of the average home broadcast receiver is about 4 ampere, and, assuming. that we-use a six-yolt accumulator, we shall thus have a voltage of six driving 2 4-ampere filament current through the filaments. Now, the measurement of power in watts is obtained by multiplying the voltage by the curgent, the formula for power being W equals I x E, where W is the power measured in watts, I the current measured in amperes, and 5 the yoltage. It is obvious from this simple equation that if we know the yalue of any two of the quantities expressed it is easy to find .the value of the missing one. Therefore, it is equally true to say that 1 equals W-E and E equals W-I. Maximum Charge and Discharge Rates, TF our receiver requires a current of 4-Ampere, We must choose an aceumulator whose normal discharge rate as specified by the maker is not less than ‘this value. For safety we shall probably choose an accumulator which can be discharged at a considerably greater rate without suffering injury; in this manner we shall also gain the advantage of being able to add an extra valve or two to our set without the necessity of purchasing a new accumulator. Now, the normal discharge rate of any accumulator depends on its am-pere-houy capacity, which itself depends upon certain physical characteristics of the accumulator which are fixed by its makers. In general, the greater the ampere-hour eapacity of an accu | mulator the greater its bulk and weight. The normal discharge xaite, | which is also the normal charging rate, is the maximum safe rate at which the accumulator can be charged or discharged without risk of injury to it; this rate is normally one-tenth of the) actual ampere-hour capacity of the accumulator. Thus if we have an accumulator possessing an actual amperehour capacity (hereinafter referred to as the A.H.C.) of ten, the rate of discharge must never exceed one ampere. There is nothing to prevent us from charging or discharging at a lower rate than. the normal one, ‘Therefore, we can charge, for instance, by passing a current of 1 ampere through the accumulator for 10 hours, 4 ampere for 20 hours, } ampere for 40 hours, etc.; the same rule holds good when discharging. Thus it follows that if we know the total filament current taken by our receiver and the A.H.C. of the .accumuator, we can predict with certainty the length of time during which our accumulator will operate our receiver before it requires recharging. The formula for this is T equals I-A.H.C, where T equals the time expressed in hours, I the current in amperes, and A.H.C. the actual amperehour capacity of the accumulator. By. using the same formula we shail know exacily how long our accumulator will take to charge if we connect it in series with any apparatus through which a current of given value is flowing. It will be again obvious that if we know any two quantities in the above equation that we can find the third, and that I equals A.H.C-T and A.H.C, equals I x T, Most experimenters are aware that if a low-voltage accumulator, say 6 volts, is re-charged from a high-volt-age main, say 230 volts, D.C., by means of resistances, most of the energy drawn from the mains is wasted in the resistance. Of course, if the charging is done in the evening, when the electric lights are on, and the current through the latter is passed also through the accumulators, this is a yery efficient, although not always a eonvenient system. With alternating-current main supply, however, it is always possible, by means of a transformer, co step-down the voltage to the right value, or thereabouts, when the wastage can be reduced to a small quantity. Some form of rectifier is, however, necessary in this case, so as to pass eurrent through the accumulator in one: direetion only.

Charging from D.C. mains, when only one battery is dealt with, is a far more expensive process than charging from A.C., on account of the great loss of current dissipated by resistances, and which has to be paid for. New Accumulators, A NEW accumulator should not have the acid put in it till just before charging, If it is likely to have only occasional use or indifferent care, 1-10th less decimal point grayity of acid (ie, 1,19 if 1.20 is recommended) will be safer, and efficiency not noticeably less; 1.18 is not’ too low. After one or two charges and tischarges some active material from che plates will generally be found in the bottom of the case. This is seen mostly m the corners; and if at any time, either on charge or discharge, or even when not in use, it should touch any two plates, positive and negative, it will prubably ruin the whole accumulator. When seen to have reached a dangerous point it must be got out. This sediment ruins more cells than any other thing. Excessive rate of charge and discharge causes most of it. Removing Sediment. DRILL a 8-16in, hole in the corner of the lids of each cell, then take out yents, and empty acid in a ¢lean jar, and let it settle. Rinse out the cells with soft water until all that will dislodge has come out. ‘Fhe small holes you. drilled in the corners will perhaps take a knitting needle, and = stir up any that has got solid, and will enable you to get out the last bit of dirt. Then put in the cleanest of the acid again, and at onee have it charged. Plug the little holes you made with celluloid cement or a rubber plug. Whether there is any dangerous sediment to come out or not, this will often improve the capacity of the accumulator. As to the sympathy of the charging man for your cells, you can only hope. fox the best. | The top of the accumulator mnst be | kept as free as possible from acid, and the terminals and all metal parts kept. coated with vaseline. To ease a sulphated terminal, warm it, and if you have to use pliers, go carefully. Clean) it well afterwards, and vaseline. , Final Hints. } pont discharge at more than onetenth of the rated capacity, or even less than this for fairly constant. work. And reckon one-tenth of a re-_ puted 40 amp. aceumuiator as 2amp., and other sizes accordingly. Exceeding this, except for short periods, is not good for the plates. The same applies to charging. This is why the more valves you use the larger should be the accumulator. . Beware of the acid getting on the clothes, It burns holes at this strength. Liquid ammonia kills it in the cloth, but is worse than the acid itself on the skin. A discharged accumulator standing by is like a dry battery at work. It is eating its inside away. , A broken lug should be burnt on, but it may be soldered, or mended with a brass plate, or threaded brass wire and nuts. In either of these last two cases be sure to coat the mend well with paraffin wax or celluloid cement before replacing. Always keep the dielectric well up, and make up for loss in evaporation only with pure distilled water. Ordinary house tap water may cause peculiar and dangerous sediments and deposits to collect. Finally, remember always that an accumulator is a chemical and not a mechanical device,