Electrical Engineering ⇒ Topic : Voltage Control Methods of Lead-acid Batteries

Voltage Control Methods

A lead-acid battery is often required to give approximately constant voltage to a circuit as in the case of a generating station stand-by battery. We know that a lead-acid battery has 2.2 V/cell when fully charged and as the battery discharges, the voltage per cell decreases. Therefore, it is necessary to use some means to keep the output voltage of the battery within permissible limits.We discuss three methods of controlling the output voltage of the battery.

(1) Rheostatic control. In this method, a variable resistance R is inserted in series with the battery as shown in Fig. (a). There is a certain amount of resistance in the circuit when the battery is fully charged. As the battery discharges, the resistance is gradually cut out of the circuit. The use of rheostat for controlling the battery voltage is objectionable particularly in large capacity installations where I²R losses would be considerable. Therefore, this method is used for batteries of small capacity and where the cost of energy is low so that energy losses in the rheostat are not objectionable.

                                                                 FIGURE (A)

(2) End-cell control. In this method, the battery voltage is controlled by changing the number of cells. Here, group of cells selected for this control are situated at one end of the battery and are called end cells. By suitable switching arrangement, the end cells are cut in or out of the battery circuit to control the battery voltage. Fig. (b) shows a simple circuit for end-cell control. The sliding end switch is made of two parts A and B separated by insulating material through the protective resistance R. This arrangement permits the end switch to operate without opening the circuit or shorting the cells during its passage from one cell to another. The resistance R limits the current in the short circuited cell at the instant the end switch passes from one cell to the next. By moving the end switch, the desired number of end cells can be cut in or out of the battery circuit to control the battery voltage

                                                                              figure (b)

Number of end cells. Suppose it is desired to get a constant voltage of 100 volts from a lead acid battery. We know that a fully charged battery has a terminal voltage of about 2.1 V/cell while a fully discharged battery has a terminal voltage of about 1.8 V/cell

(3) Booster control. A booster is a low voltage generator inserted in series in a circuit to change its voltage. A battery booster is a small d.c. generator connected in series with the battery.The function of the battery booster is to add or subtract the voltage in the battery circuit. The amount of voltage produced by the booster can be regulated by the rheostat in its field circuit. By reversing the field of the booster (i.e. d.c. generator), it may be used to discharge the battery.