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.