With the engine off, most good batteries can provide about 40 amperes for 3 hours before discharging to the point where the engine will not start.

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Quizplus · Accepted Answer

Most good car batteries have a capacity of around 48 to 100 ampere-hours, meaning they can provide 1 ampere for 48 to 100 hours, or a higher amperage for a shorter period. However, providing 40 amperes continuously for 3 hours (which equals 120 ampere-hours) exceeds the typical capacity of a standard car battery, and such a discharge rate would likely deplete the battery to a level where it could not start the engine much sooner than 3 hours.

With the Engine Off, Most Good Batteries Can Provide About