How do you calculate approximate runtime for a 12V battery given a current draw?

• A. Runtime ≈ Load Current (A) Divided By Battery Capacity (Ah).
• B. Runtime Equals Battery Capacity (Ah) Times Load Current (A).
• C. Runtime Equals Voltage Times Capacity.
• D. Runtime ≈ Battery Capacity (Ah) Divided By Load Current (A).

Answer: (D)

Runtime is the time a battery can supply the load at a given current. The capacity rating in amp-hours tells you how much current can be drawn over time. If you draw a steady current, the time available is the capacity divided by the current: t ≈ Ah / I. This works at 12 V because the voltage cancels when using the Ah rating (energy approach: t = (V × Ah) / (V × I) = Ah / I). So a battery with 100 Ah drawn at 10 A would run about 10 hours. If you draw more current, the runtime decreases proportionally. The other formulations don’t yield time, since they either multiply values or place current over capacity, which doesn’t match the units of time.