Lead acid cell is a secondary cell. That means they can be recharged when cell is discharged.let’s look in to the working principle of lead cell . P and Q are two lead plates called Electrodes p is positive Anode and “Q” is
Now Suppose , we connect a battery across the electrodes. Plate “P” to positive terminal of the battery and plate “Q” to the negative terminal of the battery then the current starts from the positive terminal of the battery and enters
into the positive plate ‘P’ i.e. anode of the cell. And inside the cell the current flows from anode to the cathode and from cathode, it comes out and goes to the negative terminal of the battery. This process is called charging.
During this charging process, the hydrogen gas appears at the cathode Q and the oxygen at the positive plate ‘P’.
This oxygen at “P” reacts with this lead plate and forms the dark brown lead Peroxide ( pbo2 ) and at the positive plate, the hydrogen gas bubbles rise to the surface and escape, to the atmosphere without reacting with the negative plate ‘Q’. So that the negative plate remains the same. This charging of the secondary cell has to continue non stop for some hours so that the cell is fully charged. How can we say the cell is fully charged ? If the emf of the cell becomes 2.2. volts and the specific gravity of the electrolyte becomes 1.21 or as we say 12.10 ; then we can say that the cell is fully charged and ready to work.
Now the battery has been removed in Fig. and a resistance and Ammeter is connected in series are connected across the terminal ‘P’ and Q. Current (D.C) starts from “p” and passes through the external load resistance and ammeter, and enters into the negative plate ‘Q’ and inside the cell it travels from negative to positive.
Methods of Charging and Maintaenance of lead acid cells :
Methods of Charging : In general, There are two methods of charging batteries or Accumulators ( the name accumulator, as it accumulates, means adds or stores the electrical energy ) From d.c. supply there are two methods of charging a lead acid battery.
1) Constant current method.
2) Constant voltage method.
Constant current method :
Constant current method of charging is usually adopted for initially charging the new batteries. According to this method charging current is kept constant throughout by adjusting the external resistance R. This method of charging
is defective to the extent that it does not take into consideration the state of charge of the battery. Usually high, charging rate is required for fully discharged battery in the beginning and this rate of charging should go down as battery approaches its fully charged rate. This draw back is removed when we charge the battery according to constant voltage method.
Constant voltage method :
In this method, charging voltage is held constant throughout the charging. Charging current in the beginning is high which however reduces as the back emf of the battery increases. This is the most common method of charging. The battery is charged till the cells are gassing freely and the specific gravity of the electrolyte and the terminal voltage of cells remain constant. The fully charged battery has fallowing indications
Specific gravity of electrolyte 1260 to 1280
Voltage of cell 2.1 volts
Where specific gravity can be determined by hydrometer and voltage by cell tester accurately.
Maintenance of lead acid cells :
Care and maintenance of lead acid cells is most impotent. Other wise the batteries will get spoiled.
The following points should be observed :
1)If the voltage of the lead acid cell comes down up to 1.8 volts per cell, the battery should not be used. It should be kept aside. As we studied earlier an insoluble substance called, lead sulphate is formed which will damage / spoil
2)See that , always the plates are submerged inside the electrolyte. The level of the electrolyte should be always 15 mm above the top lever of the plates. The plates should never be exposed to air. Now and then we should pour
disfilled water in the cells to keep the level of the electrolyte constant. Because the electrolyte gets evaporated after some time.
3)Never leave the discharged battery in that discharged condition for a long time. Other wise, it completely gets spoiled.
4) Keep the cell in dry and clean position.
5) The charge and discharge should be at normal rates
6)While charging , vent plug should be kept loose for passing out of the evolved gases if any.
7) The naked flame, near the battery, while charging , should be avoided.
8)While preparing electrolyte , water should not be poured into the acid, but acid should be poured in water drop by drop.