Storage batteries are now used in so many central stations that station engineers should be familiar with their peculiarities. When a battery is being charged it is important to know when to stop charging. If a battery is only partially charged it is not being used to the best advantage because there is no use in having a battery of large storage capacity if this capacity is not made use of. On the other hand, it is equally true that a battery is not being used to the best advantage if it is repeatedly overcharged.

The question then, is to determine the point at which charging should be stopped. When a battery is discharged, the voltage of each cellfalls, and it is never advisable to discharge the battery to such an extent that the voltage falls below 1.7 volt per cell. In every-day work it is advisable to keep the voltage above this point, say 1.8 volts, because it is always well to have some reserve capacity in the battery.

We will assume then, that the voltage per cell is about 1.8 when the charging begins. A* the cells become charged, the voltage gradually rises until it reaches about 2 6 to 2.7 volts per cell. In passing, let us state that these voltage readings are taken when the normal charge or discharge current is flowing, as the case may be. In storage-battery work readings of voltage taken while the current is not flowing are of little use,. The normal current is usually taken as the current which will discharge a fully charged battery in 8 hours. Batteries should be charged at about the 8 hour rate. It is not advisable to charge at the maximum rate except in cases of emergency.

After the voltage has come up to about 2.5, or 2.7 volts, it remains stationary, and further charging at the normal rate does not increase it. Of course if the charging current is increased above normal, the voltage will increase somewhat, but under ordinary conditions the charging current should not be forced above normal, particularly at the charging where the voltage attains a fixed value, and does not increase during, say 15 minutes, it is a sign that the battery has become fully charged. The exact value that the voltage will attain depends somewhat on the age of the plates and also, as stated above, on the value of the charging current. The point to note is not so much what the final value of the voltage is, but whether it has reached a stationary value or not.

Another indication of full charge is the specific gravity of the electrolyte, an acid solution in the cells. As the battery becomes charged this solution becomes denser, because of the formation of sulphuric acid. The specific gravity, which is readily measured by means of a battery hydrometer, therefore gradually increases. The specific gravity is usually about 1.2 when the cells are charged, but the point to be noted is that when the electrolyte ceases to increase in density. When this occurs, it is another sign that the cells are fully charged.