Where a battery is charged from a fixed voltage, on the other hand, this voltage must at least equal that at the end of the charge; hence where worked to full capacity under these con these conditions is approximately 1.95 it fol lows that the energy capacity of a conserva tively designed battery for portable service (as in Fig. 24) is about 9 watt hours per pound; for the lightest types in commercial use (Fig. 26) about 11.
It is a prevalent and quite natural idea that because the ordinary storage battery is made of i lead, it is therefore unduly heavy. But when it is stated that each pound weight of battery can store up 24,000 foot-pounds, it may be readily appreciated that the electric storage battery of to-day is by far the most effective piece of mechanism known for storing energy. Put in slightly different form, it may be stated that the modern battery of conservative and substantial design, as in Fig. 24, can give out, in the space of five-hours, electrical energy sufficient to lift itself approximately five miles high.
Care and To give a complete treatise on battery operation, covering the whole varied field to which batteries are applied, would be beyond the scope of this article; but a few fundamental principles which are applicable to all cases may be briefly stated.
1. Care of Electrolyte.— Evaporation of the water of the electrolyte is constantly taking place, especially while the cell is gassing freely at the end of charge; this must be made up by periodic additions of water. Since the amount of water thus added during the life of a bat tery is many times the original amount con tained, any impurities in the water accumulate quite rapidly. Hence it is very important to use only pure water for this purpose, and among the impurities commonly present chlorine and iron in any form are especially to be avoided. Addition of acid should be avoided.
2. Regulation of Charge.— If a battery be habitually charged too little, the active material becomes gradually more and more converted into lead sulphate, until in time it ceases to function. Very long continued charge is then necessary to restore it to a working condition. If on the other hand it be charged too much, the gas bubbles liberated from the plates give rise to a softening and eroding action upon the positive material, which detaches it from the plates, and in time leaves the grids bare, and no longer workable. Experience has demonstrated that best results are obtained when each charge exceeds in ampere hours the previous discharge by about 15 per cent.
Several methods are in use for determining the correct amount of charge, as follows: (a) The "ampere hour" metre shows directly, both the current withdrawn on discharge and that put in on charge, from which the latter may be regulated; a very generally effective method when the discharge current is not too low, say 10 per cent of the normal. (b) The 'battery may be charged till the voltage ceases to rise; one of the older and less reliable methods. (c) The best indication of the state of charge is that based upon specific gravity of the electro lyte. If the specific gravity be read at regular intervals during charge, it will be found to rise steadily for a time, and then become constant.
When three successive readings covering a period of about an hour show no change in gravity, it means that chemical action between plates and electrolyte has ceased, and hence that the charge is complete, and should be stopped.
This method of determining charge is far the most reliable, and should be used wherever possible; and in any case should be used from time to time, to check up and make certain that charging is being done correctly. A single cell is usually selected as a °pilot' for taking read ings with this method.
While the life of a battery may be much in creased by careful regulation of the charge, according to the above principles, yet many hundreds of thousands of batteries are in suc cessful operation, where the only care observed in charging is to arrange that it is ample, re gardless of other considerations.
The strength of electrolyte used in storage batteries is not standardized, hut varies with different makes and designs, and even with the individual cells of a given battery. In general where weight is a prime factor, higher gravity, usually about 1280 specific gravity, is used; where weight and bulk are not important lower gravity, 1.200 or even 1.180, are preferably em ployed. In the former case the volume of acid is small, and the drop of acid during discharge is correspondingly large, so that in discharged condition it may be 1,150; in the case of a stationary cell, where there is no close limit to bulk, the acid which reads perhaps 1,200 on charge will drop during discharge to about the same point as the other, namely, 1,150-1,170. It is thus impossible to give any generally ap plicable values for the specific of the electrolyte, but the theory of charging till maximum is reached holds universally true.