Lead Acid Battery Construction And Working Principle – Battery

Parts of Battery

The following are the important
parts of a battery

1. Container
2. Plates
3. Separator
4. Cell cover
5. Electrolyte
6. Terminal post


The container is a single piece
moulded part made up of hard rubber or bituminous. The container will house all the internal parts of a battery and holds the liquid electrolyte. The container is divided into six compartments for the six cells. Projections are provided on
the inside at the bottom to support the
plates. Bridges are used to prevent the
plate from touching at the bottom.


Plates are of two types viz.

1. Positive plate

2. Negative plate

The positive plate is made of lead
peroxide. This is dark brown, hard and brittle substance. Lead oxide acts as active material. Highly porous in nature. The negative plate is made of pure lead in soft sponge condition. Spongy lead acts as active material. Grey in colour. Highly porous in nature. Grids are used to hold the plates and to conduct the electrical


Separators are placed between
positive plates and negative plates to
insulate them between each other. This will prevent the short-circuiting between the positive and negative plates. The separators are thin sheets of non-conducting material made up of chemically treated wood, porous rubbers, or mats of glass fibre.The separators must be porous so that
the electrolyte may circulate between
the plates.

Cell cover

Cellcover is used to cover the
complete cell. It protects the cell from the dust as well as other external impurities. Vent holes are provided to exhaust the gases generated in the cell to the atmosphere. Filler openings are available to fill up the electrolyte.


In lead-acid battery dilute sulphuric
acid (H2SO4)is used as an electrolyte.
For this purpose, one part concentrated sulphuric acid is mixed with three parts of distilled water.


Terminals are used to connect
the charging circuit as well as the load. For easy identification, the diameter of the positive terminal is bigger than the negative terminal.

Working principle

During discharging (i.e., supplying
a current), atoms from the spongy lead on the negative plates combine with sulphate molecules to form lead sulphate and hydrogen. The combination of lead peroxide
and hydrogen at the positive electrode produces water and lead sulphate. The water dilutes the electrolyte, making it a weaker solution, drop in cell voltage, hence
specific gravity of electrolyte is reduced.

During recharging, current is made
to flow into the positive electrode of
each cell. This current causes the lead
sulphate at the negative electrode to
recombine with hydrogen ions, thus
re-forming sulphuric acid in the electrolyte and Spongy lead on the negative plates. Also, the lead sulphate on the positive electrodes recombines with water to regenerate lead peroxide on the positivenplates and sulphuric acid in the electrolyte.
Thus cell voltage is increased and the
specific gravity of electrolyte is increased.

The two way of reversible chemical
reaction (charged on the left and dischargednon the right) is given as

Lead Peroxide + Sulphuric Acid +
Spongy Lead  Lead Sulphate + Water
+ Electrolyte

PbO2 + 2H2SO4+ Pb  PbSO4+ 2H2O+ PbSO4

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