The capacity of a lead-acid cell is measured in

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ampere-hours

Explanation: The capacity of a lead-acid battery represents the total amount of charge the battery can deliver under specific conditions. It is measured in Ampere-hours (Ah), which is the product of the current (in amperes) and the discharge time (in hours).

Working Principle: During the chemical discharge process, the lead-acid battery converts stored chemical energy into electrical energy. The total capacity is defined by the Faraday laws of electrolysis, where the mass of active material consumed is directly proportional to the total charge (current multiplied by time) drawn from the terminals until the cutoff voltage is reached.

Key Formulas: $C = I 	imes t$ — where $C$ is capacity in Ah, $I$ is constant discharge current in A, and $t$ is discharge time in h.; $E = V 	imes I 	imes t$ — where $E$ is total energy in Watt-hours (Wh).

Key Points: Ampere-hour (Ah) rating specifies the charge storage capacity of a battery.; The capacity varies depending on the discharge rate (Peukert's Law).; A standard C20 rating implies the battery is discharged over a period of 20 hours.; Temperature significantly affects the actual effective capacity of lead-acid cells.

Advantages: Provides a standardized metric for comparing battery life.; Allows engineers to calculate expected backup time for specific loads.

Disadvantages: Capacity rating is non-linear; higher discharge currents result in lower effective Ah.; Does not account for voltage drop over the discharge cycle (Energy capacity is a better metric for some applications).

Applications: Automotive starting batteries.; Uninterruptible Power Supplies (UPS).; Telecommunications backup systems.

Additional Info: Watt-hours (Wh) is a measure of total energy, whereas Ampere-hours (Ah) is a measure of total charge capacity.. Amperes measure instantaneous current flow, not the total volume of electrical charge available.

Answer: B is correct — The capacity of a lead-acid cell is defined as the total charge it can provide, measured in Ampere-hours (Ah).

Exam Tip: Always remember that capacity is a measure of charge (Ah), while total energy is measured in Watt-hours (Wh). For battery systems, Peukert's law $I^k t = C$ explains why high-drain applications reduce the usable Ah capacity.

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