The lead-acid cell should never be discharged beyond

Question

Examoogle · Accepted Answer

1.8 V

Explanation: A lead-acid battery should not be discharged below 1.8 V per cell because deep discharging leads to the formation of hard lead sulfate crystals on the plates. This phenomenon, known as sulfation, is irreversible and permanently reduces the battery's capacity and overall service life.

Working Principle: During discharge, the active materials (Lead Dioxide on the positive plate and Spongy Lead on the negative plate) react with sulfuric acid to form Lead Sulfate ($PbSO_4$). If the voltage drops below the critical threshold of 1.8 V, the Lead Sulfate crystals grow into a coarse, hardened state that cannot be converted back to active material during the charging cycle. This effectively leads to internal short-circuiting or high internal resistance, rendering the cell unusable.

Key Formulas: $V_{cell} \approx 2.0 	ext{ to } 2.1 	ext{ V}$ — Nominal voltage of a lead-acid cell; $Pb + PbO_2 + 2H_2SO_4 \leftrightarrow 2PbSO_4 + 2H_2O$ — Chemical reaction during charge/discharge

Key Points: 1.8 V is the recommended end-of-discharge voltage for load testing.; Sulfation occurs when the battery is left in a discharged state for prolonged periods.; Specific gravity of the electrolyte also drops significantly at 1.8 V, typically falling below 1.15.; Deep cycle batteries may have slightly different cut-off thresholds, but 1.8 V is the standard limit.

Advantages: Prevents irreversible permanent damage (sulfation).; Maintains electrochemical efficiency of plates.; Ensures ability to retain charge after subsequent recharge cycles.

Disadvantages: Frequent deep discharge cycles significantly reduce total cycle life.; Higher internal resistance develops if allowed to drop below limit.

Applications: Automotive starter batteries.; UPS and emergency backup power systems.; Solar photovoltaic storage banks.

Additional Info: The open-circuit voltage of a fully charged lead-acid cell is typically around 2.1 V to 2.2 V.. Option B (1.9 V), C (2.0 V), and D (2.1 V) are well within the normal operating range and discharging to these values is considered safe/normal for standard usage.

Answer: A is correct — The lead-acid cell should never be discharged below 1.8 V to prevent permanent sulfation of the electrodes.

Exam Tip: In competitive exams, remember that 1.8V is the 'Cut-off' voltage for Lead-Acid, while Ni-Cd cells have a lower cut-off, often around 1.0V to 1.1V.

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