46. Pilot Cell. -It is not necessary to take specific gravity readings of all the cells of a battery. The reading of one cell will give an indication of the state of charge of the entire battery. When two batteries are connected in series as used in Photophone equipment, the condition of both batteries can be determined from one cell. Select one cell for this purpose and always use it when taking readings.

It is practically impossible to take readings from a cell continuously without losing some of the electrolyte. Therefore, about once every three months select a new pilot cell. In this way the amount of electrolyte lost from all cells will be about equal.

When selecting a new cell for use as a pilot cell, charge the battery until the cell shows no further increase in specific gravity for a period of one hour (take half-hourly readings) and use this value as the "fully charged" indication for all future specific gravity readings taken on that cell.

47. Condition of Charge of the Batteries as Indicated by the Hydrometer Readings. - The specific gravity of the electrolyte of a fully charged battery is high, and becomes lower as the battery is discharged. If the charging current is continued after the battery has been fully charged, the specific gravity will not increase. The cell will gas freely because the only transformation which will take place in the cell, is the transformation of water into its component gases, namely, hydrogen and oxygen. A battery should never be fully discharged, that is, it should never be discharged until it will no longer deliver current or the battery will be harmed. In case of an emergency when it is necessary to continue the discharge of the battery below the limit set for routine operation, it should be recharged immediately. A battery left in a discharged condition deteriorates rapidly.

48. Equalizing Charge. -Storage batteries should be periodically given what is called an "equalizing charge." An equalizing charge is a continuation of the regular charge (but at a lower rate) until the cells gas freely, and until half-hourly readings of the specific gravity of the pilot cell shows no further increase over a period of one hour.

49. Factors Affecting the Charging Rate. -A battery may be charged at a fairly high rate when it is in a discharged condition without detrimental effects, but when a battery is almost fully charged a low rate of charge is desirable.

The battery should not get hot when charged. The temperature of the battery increases when it is being charged, but it should never be allowed to exceed 115 degrees Fahrenheit. (This temperature is just a little above that which is termed "luke-warm.") Feeling the outside of the battery does not give a good indication as to the temperature of the battery, because it requires considerable time before the outside of the case is heated by the electrolyte.

When a battery is almost fully charged it will gas strongly. This gassing is very similar to boiling, and causes the chemicals of the plates to loosen and drop off. This loosened matter settles to the bottom of the battery in the sediment space provided. If the rate of charging is reduced the gassing is reduced, with the result that only a very little of the chemicals are lost as compared with what would be lost if the batteries were charged at a high rate.

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