The connections for one of the simplest methods is shown in fig. 1,185. The battery is divided into two halves, which are connected in series for discharging and in parallel for charging. Since the voltage of each cell at the end of a discharge should not be lower than 1.8 volts, a battery intended for use on a 110 volt lighting circuit will require 110 ÷ 1.8 = 62 cells. The voltage necessary, however, for each cell at the end of a charge is about 2.6 volts, or a total of 2.6 × 62 = 161 volts for the battery, a value which is far above the line voltage. By dividing the battery into two halves and connecting them in parallel only 80.5 volts are necessary for charging. The excess voltage of the line, 29.5 volts is taken up by the resistance, which also controls the output of the battery on discharge.
=End Cell Switches.=—These may be used to advantage in small installations where there is not demand for current during the day, or where the charging is done by means of _boosters_.
[Illustration: FIG. 1,186.—Diagram of connections of a battery equipment for a residential lighting plant. In the diagram the voltmeter and voltmeter connections have been omitted. The bus bars on the battery panel are connected directly to the bus bars on the dynamo panel. In this installation the dynamos are run during the afternoon on discharge, being regulated by means of an end cell switch. On charge, the pressure above that of the bus bars, required to bring all cells up to full charge, is supplied by means of a motor driven charging booster, the voltage at the armature being suitably varied by changing the field excitation.]
Ans. A form of switch employed in connection with a storage battery in order to control the end cells for regulating the voltage.
=Ques. Describe the construction of an end cell switch.=
Ans. This is shown in fig. 1,187. The switch contact arm is made in two parts, A and B, which are insulated from each other as shown, and connected with each other through the protective resistance R. The end cell contacts are so spaced that when the main current carrying part A of the switch arm is squarely on one end cell contact such as X, the part B, does not touch any other contact such as Y, but when the switch arm is advanced for cutting into circuit another end cell, the part B, reaches the contact Y before the part A, leaves the contact X, thus keeping the battery circuit closed, while the resistance R, limits the current in the short circuited cell at the instant the switch arm passes from one end cell contact to the next.