If the alternator fails, how far can you drive on the battery? How capacity, current draw and weather determine the answer.
Even with a failed alternator, a car can keep going on its battery for a while. MSG Equipment says the distance depends on battery capacity and condition, the electrical loads switched on and the weather.
The article tabulates the current drawn by headlights, fan, heated seats and rear window heating. The calculation is simple: battery capacity (Ah) divided by total current gives approximate running time. With a 60 Ah battery and about 23 A of load, for example, the theoretical figure is 2.6 hours.
That applies only to a fully charged, healthy battery and drops noticeably in winter. The company advises switching off unnecessary loads and heading to the nearest workshop.