Cover - Train Lighting - Indian Railways Institute of Electrical ...

4. BATTERIES
4.0 Conventional Lead Acid Cells for TL applications are governed by IS 6848.
4.1 Type of cells in use for train lighting and coach air-conditioning are as under :-
Capacity of battery in AH
at 27 Degree C at 10 Hr Rate
Type of coach
where generally used
120 110 V, BG coaches
450 MG AC Coach
525 Jan Shatabdi Non - AC coaches
800 II AC BG Coaches (Old) (Under-slung type)
1100 II AC BG Coaches (new)/AC 3 Tier Coach
4.2 Principle of Operation :
In a charged lead acid cell positive active material consists of lead peroxide (PbO2)
and the negative of spongy lead (Pb). Dilute sulphuric acid (H2SO4 + H2O) serves as
electrolyte.
The overall reactions inside the cell during discharge and charge are represented
most -conveniently by a reversible equation as follows :-
PbO2 + Pb + 2H2SO4 2PbSO4 + 2H2O
During discharge, the lead peroxide on the positive plates as well as the spongy lead
on the negative plates are converted into lead sulphate (PbSO4). In this process, sulphuric
acid (H2SO4) is consumed and water (H2O) is formed. Consequently, the specific gravity of
the electrolyte falls, the extent of fall being proportional to the ampere-hours taken out. The
process causes at first a slow, and then a faster voltage drop, until a permissible lower limit
(final discharge voltage) is reached, which depends on the rate of discharge current. The
amount of ampere-hours (constant current x time) taken out is called the capacity of the cell
at this rate.
The chemical process during charge is the reverse of that during discharge. The
lead-sulphate on the positive plates is reconverted into lead peroxide and the lead sulphate in
the negative plates into spongy lead. Sulphuric acid is formed and the water consumed. The
specific gravity of the electrolyte rises. There is at first a slow, later a faster rise of cell
voltage. From 2.4 volts onwards gassing sets in due to a strong decomposition of water into
hydrogen and oxygen. Violent gassing is injurious to the plate material. So after reaching this
gassing voltage the rate of the charging current must be limited to within safe permissible
values.
GENERAL SERVICES : TRAIN LIGHTING 43