Batteries — How is capacity specified? Choose the standard unit used to express the capacity of an automotive storage battery.

Introduction / Context:Battery capacity quantifies how much charge a battery can deliver over time at a specified rate and temperature. This is crucial for sizing batteries for cranking performance and auxiliary loads in vehicles.

Given Data / Assumptions:

• Lead-acid starter battery.
• Standard discharge rate (often 20-hour rate) and rated temperature.

Concept / Approach:Capacity is expressed in ampere-hours (Ah), meaning the product of current and time that the battery can supply before reaching a cutoff voltage. Voltage (volts) indicates nominal electrical potential, not storage capacity. Amperes is instantaneous current, not duration. Weight is irrelevant to electrical capacity, and watt-seconds describe energy but are not the conventional capacity rating for starter batteries.

Step-by-Step Solution:Define capacity: Ah = current (A) * time (h) to a specified end voltage.Identify the industry convention: automotive batteries are rated in Ah (and CCA for cranking).Select “ampere-hours.”

Verification / Alternative check:Battery labels and datasheets list Ah and CCA; this aligns with service manuals.

Why Other Options Are Wrong:Volts: nominal system voltage (e.g., 12 V), not capacity.Amperes: instantaneous current only.Weight: unrelated to electrical rating.Watt-seconds: energy unit; rarely used for automotive capacity ratings.

Common Pitfalls:Confusing capacity (Ah) with cold-cranking amps (CCA); both are important but different metrics.

Final Answer:ampere-hours