Fig. 1,188.—End cell switch control for storage battery; connections showing main line open when the battery is being charged.
Ques. Where are end cell switches of large capacity located?
Ans. Generally they are placed as near the battery room as possible to avoid the cost of running the heavy conductors, and when such switches are motor driven, the usual practice is to control their operation from the main switchboard.
In [fig. 1,188] is shown the method of regulation with an end switch. The diagram shows the battery being charged with the main switch open, and the voltage of the dynamo raised to the charging pressure. During discharge the cells are connected in series, and as the voltage of each cell at the beginning of discharge is at least 2.1 volts, only 52 or 53 cells are required to give the desired pressure of 110 volts, but as the discharge continues, and the voltage of each cell decreases, the end cells, 1, 2, 3, 4, etc., are cut into circuit successively by means of the end cell switch, thereby adding to and compensating the drop in the total voltage until, at the end of discharge when the voltage of each cell has fallen to 1.8 volts, the entire 62 cells are in series to supply the required line pressure.
Fig. 1,189.—Diagram of connections arranged for charging battery in two parallel groups and discharging in series, the charge and discharge being controlled by variable resistances. In yacht lighting the limited space generally prohibits the use of a charging booster, and in such instances this method of charge and discharge control is the usual practice. In case the dynamo from which the battery is charged has sufficient range in voltage to charge all cells in series, a charging booster is not required, nor is it necessary to connect groups of cells in parallel, as the dynamo voltage may be varied as charge proceeds.
For a 110 volt circuit, the number of cells required is 110 ÷ 1.8 = 61, and the number in series when the battery begins to discharge is 110 ÷ 2.1 = 52. Hence, in a 110 volt circuit an arrangement must be provided whereby 61 - 52 = 9 cells may be cut out or switched in, one by one.
The number of end cells for any voltage may be obtained by the following formula:
Number of end cells = (E/1.8) - (E/2.1)
E = voltage of supply circuit;
1.8 = minimum voltage of cell during discharge;
2.1 = voltage of fully charged cell.