It takes a lot of amps to power the lights, electric defrosters, wipers, power windows, power seats, power liftgates, the ignition system, fuel injectors, fuel pump, cooling fan, A/C compressor, heater fan, heated seats, audio system and navigation systems on today's late-model vehicles. Since alternators can generate only so many amps, their output may depend on engine speed.

Cramping the Cranking Loads
At idle, when the alternator is turning slowly, output is reduced and may not be able to keep up with the demands on the charging system. Consequently, the battery has to make up the difference. The reserve capacity of the battery is like a bank account. It stores amps so that the electrical system can withdraw amps when electrical loads exceed the charging system's output. Cranking the engine pulls amps out of the battery. So do high electrical loads when the engine is idling. The amps are put back into the battery when the vehicle is driven and the charging system is able to generate more amps than what are needed to keep things running.

Trouble is, the vehicle may not be driven long enough to make up the difference. Short trips of 15 minutes or less ‹ especially in stop-and-go traffic ‹ may not be enough to keep the battery fully charged. This may cause hard starting or a no-start condition if the battery doesn't have enough power to crank the engine ‹ especially during cold weather. Chronic underchargi...