166 Electrical Power Systems Technology
rent to a load connected to it, the chemical process can be expressed as:
PbO 2 + Pb + 2H 2 SO 4 → 2PbS O 4 + 2H 2 O.The sulfuric acid ionizes to produce four positive hydrogen ions (H+)
and two negative sulfate (SO4–) ions. A negative charge is developed on
the lead plate when an SO4– ion combines with the lead plate to form lead
sulfate (PbSO 4 ). The positive hydrogen ions (H+) combine with electrons
of the lead-peroxide plate and become neutral hydrogen atoms. They next
combine with the oxygen (O) of this plate to become water (H 2 O). The
lead-peroxide plate thus becomes positively charged. A fully charged lead-
acid cell has an electrical potential developed between electrodes of ap-
proximately 2.0 volts.
After a cell discharges by supplying current to a load for a certain
period of time, it is no longer able to develop an output voltage. The cell
may then be charged by causing direct current to flow through the cell in
the opposite direction.
The chemical process is thus reversed as follows:
2Pb SO 4 + 2H 2 O → PbO 2 + Pb + 2H 2 SO 4.Thus, the original state of the chemicals is reestablished by the charg-
ing action.
Specific Gravity of Secondary Cells
The amount of charge may be measured by a specific-gravity test us-
ing a hydrometer to sample the electrolyte. The specific gravity of a liquid
provides an index of how much heavier than water the liquid is. Pure sul-
furic acid has a specific gravity of 1.840, while the dilute sulfuric acid of
a charged fully lead-acid cell varies from 1.275 to 1.300. During discharge of
a cell, water is formed, reducing the specific gravity of the electrolyte. A
specific gravity of between 1.120 and 1.150 indicates a fully discharged cell.
Recharging is accomplished using battery chargers.
Ampere-hour Rating of Secondary Cells
The capacity of a battery composed of lead-acid cells is given by an
ampere-hour rating. A 60-ampere hour battery could theoretically deliver
60 amperes for 1 hour, 30 amperes for 2 hours, or 15 amperes for 4 hours.
However, this is an approximate rating dependent upon the rate of dis-