The lead cell contains plates of lead, the positive plate being coated with lead peroxide and the negative with “spongy lead.” These plates are immersed in a solution of sulphuric acid and water.
The electrolyte or solution of the lead cell is made of four parts pure water and one part pure sulphuric acid.
By “charging” a cell is meant the process of chemically changing the plates so that the cell will deliver a current, this being brought about by causing a current to flow through the cell for a time.
By “discharging” of a cell is meant the delivery of electrical energy by the cell to an external circuit.
When a cell is placed on discharge the current is produced by the combination of the sulphuric acid of the solution with the active material of the plates. The product of this combination is lead sulphate and water. The lead sulphate fills the pores of the active material, and the water is left in the solution, causing it to become thinner. The volume of sulphate formed by this combination is greater than the volume of sulphuric acid and lead which entered into the combination. The result of this is that as the discharge goes on, the sulphate which fills the pores in the plates prevents the action of the solution on the active material. Therefore the voltage will gradually drop as more and more sulphate is formed. The maximum voltage of a lead cell is ordinarily about 2.1 volts, the lower limit is 1.75 volts. The voltage should never be allowed to drop below 1.75 volts, because after this point is reached the voltage falls off very rapidly and possible damage to the plates may result.
In charging, direct current of greater voltage than the maximum potential of the storage battery is caused to flow through the cells in an opposite direction to that which the current takes when the cell is discharging.
The sole object of charging, then, is to drive the sulphate from the plates back into the solution, thus leaving spongy lead on the negative plate and lead peroxide on the positive.
Is a cell of this type properly called a “storage” cell? No, because electricity is not stored in it. When it is being charged, electrons are forced into it, but they leave it by the negative terminal. Electrons do not leave the circuit. No new electrons are put into the cell--it is the passage of the electrons in reverse direction that affects the cell chemically. Recharging therefore
