Station Batteries — Maintenance — Traction Distribution Notes

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Lead-acid batteries at traction substations/switching stations provide DC control supply, and demand real, precise maintenance discipline — a detailed history of every battery must be maintained, with fortnightly specific-gravity readings recorded.

Battery Room Safety

Smoking, open flames, or spark-generating tools are STRICTLY FORBIDDEN in the battery room.
The room must be well-ventilated, dust-free, with acid-proofing on walls and flooring, kept isolated from other electrical equipment, and given appropriate short-circuit fuse protection between the battery and distribution switchboard.

Specific Gravity — Real Numbers

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Exam Focus — Charged vs Discharged Thresholds

Electrolyte specific gravity should be maintained at about 1.210 at 27°C; when it drops to 1.150, the cell may be considered DISCHARGED. Per IS 1652, temperature correction to a fully-charged reading of 1.20±0.005 (corrected to 27°C) works as: ADD 0.0007 to the observed reading for each 1°C ABOVE 27°C, and DEDUCT 0.0007 for each 1°C BELOW 27°C.

Variations in specific gravity between cells (from unequal evaporation) are corrected by adding DISTILLED WATER only — never concentrated/diluted sulphuric acid, except when acid is known to have spilled out.
Hydrometer readings taken while a cell is gassing freely reflect a mixture of gas bubbles and electrolyte, NOT true electrolyte specific gravity — readings must be taken only after bubbles subside. Hydrometers should be 300mm length for required accuracy, and two should be kept per station, periodically cross-checked against each other.

Pilot Cells

One cell per row is selected as the 'pilot cell,' read frequently to indicate the state of charge/discharge as a guide to the condition of ALL other cells. Once selected, the pilot cell should NOT be changed unless it needs special treatment/repairs — and any such change must be noted immediately on record sheets. Its electrolyte height should invariably be kept at a fixed point (12mm above the top of plates), topped up with distilled water every fortnight if necessary.

Trickle Charging

Lead-acid batteries are sensitive to BOTH overcharging (positive plates shed active material quickly) and over-discharging (sulphation — whitish deposits on plates). There should be NO occasion for stationary traction installation batteries to sulphate, since terminal voltage of each cell is maintained close to 2.15V via a low, carefully-fixed trickle charge rate (milliamperes per Ah capacity).

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Memory Aid — Reading Bubble Size

A properly float-charged battery shows very small gas bubbles (about the size of a pin head) rising slowly from the plate. An OVERCHARGING battery shows MUCH LARGER bubbles reaching the surface at a HIGHER rate — a simple visual check any staff member can perform without instruments.

Cell Voltage — Real Ranges

New cells at end of full charge (10-hour rate): about 2.5 to 2.75V — this gradually decreases as the battery ages, settling to about 2.4V at normal temperature/charging rate.
No cell should ever be discharged below 1.85V, as measured while discharging at the normal 10-hour rate.

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Exam Focus — Voltage Readings Are Meaningless on Open Circuit

A cell's voltage only gives an approximate indication of its state of charge WHILE it is actually being discharged (e.g. at the 10-hour rate) — NOT when the cell is on open circuit. To get comparable month-to-month readings, voltages must always be taken with the SAME current flowing through the cell each time.

Condition of Plates — Visual Diagnosis

In healthy cells in use for more than 12 months, when fully charged, the active material in positive plates should be CHOCOLATE in colour — a simple, real visual reference point for a healthy cell that a technician can check without special equipment. Sulphated plates, lug corrosion, and cracked separators all cause a noticeable terminal voltage drop when current flows, varying with the amount of current — hence the importance of comparing readings taken at consistent current levels.