The capacity of a battery is expressed in terms of

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ampere-hour rating

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

Working Principle: Battery capacity is defined by the electrochemical active material available within the cells. According to Faraday's laws of electrolysis, the total mass of the active material converted determines the charge (Q) available. If a battery provides a constant current I for time t, the capacity C is calculated as C = I 	imes t, reflecting the total quantity of electric charge stored.

Key Formulas: $C = I 	imes t$; $Q = \int_{0}^{t} i(t) dt$

Key Points: Ampere-hour (Ah) is the standard commercial unit for battery capacity.; Higher Ah ratings imply the battery can supply current for a longer duration.; Actual capacity is temperature-dependent and affected by the discharge rate (Peukert's Law).; Voltage rating defines the potential difference, not the total energy storage capacity.

Advantages: Provides a simple, industry-standard metric for comparing battery life.; Allows for easy estimation of backup time in UPS and solar systems.

Disadvantages: Does not account for depth of discharge (DoD) limitations.; Ignores voltage drop over the discharge cycle.

Applications: UPS (Uninterruptible Power Supply) systems; Electric vehicle battery packs; Solar photovoltaic energy storage

Additional Info: Standard testing is usually done at a '20-hour rate' ($C_{20}$), where the battery is discharged until its voltage reaches a cutoff level.. Voltage rating (Option B) specifies the potential difference between terminals, while current rating (Option A) refers to the maximum safe current draw or output, but neither measures the total stored capacity.

Answer: C is correct — The capacity of a battery is defined as the total charge it can store, expressed in Ampere-hours (Ah).

Exam Tip: Always remember that capacity (Ah) $	imes$ nominal voltage (V) gives you the total energy in Watt-hours (Wh), which is a better metric for total energy storage capability.

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