Automotive Linear Regulators M

Alternators Deliver Plenty of Power

The typical automobile alternator can deliver about 3000W. Assuming a 14V output, that allows a current draw of more than 200A. Even with the air conditioner fully on and the car radio playing at discothèque level, the alternator's capacity can supply any combination of electrical loads in the car. Nobody has to worry about quiescent currents. But, what happens when the motor is off?

The car battery can deliver all the power required—for a while! An average car battery's key specification is its capacity, measured in ampere-hours (Ah). For compact cars, a typical battery provides about 50Ah. In theory, that means that you can draw 1A for 50 hours. Doubling the current reduces the time to half. Starting the car, for example, takes several hundreds of amperes, but only for a short time. Assuming an average current draw of 300A during starting, you should have exactly ten minutes to start the engine before exhausting the battery.

Another example is lighting. Normally, a car has two front bulbs at 50W each and two rear bulbs at 20W each. Together, they consume about 140W. If you forget to switch them off when you leave the car, they will draw about 11.5A from an average 12V system. Assuming the battery is fully charged, you should have four hours before the lights extinguish automatically! In practice, the lights will extinguish earlier than that, and you will not have the ten minutes mentioned above for starting the engine either.

Typical car batteries have only 36Ah to 100Ah of capacity, depending on the car size and type. Several effects reduce the available capacity of a car battery. Colder outside temperatures, for instance, slow the internal chemical reactions. Only half the original capacity is available at -20°C. Older car batteries have less available capacity, and the battery in a car driven repeatedly for short distances (as for running errands in a town) discharges much more rapidly than that of a car used predominantly for long-distance driving. (Normally, a car must be driven for at least half an hour to recharge its full capacity.) The number of charging cycles is approximately 500 for a car that repeatedly discharges its battery by only 30%. Thus, short trips exhaust a battery more quickly and shorten its lifetime. You should expect to replace your car battery every five years.

Always-On Functions Always Consume Power

Imagine a cold winter with temperatures below zero for weeks at a time. You drive your 4-year-old car, used mainly for city driving, to the airport where you escape for a warm and sunny 3-week vacation. On your return, the car probably will not start! Its capacity has been reduced, and not only by the city driving, the elapsed parking time, and the frosty temperatures. Other, hidden energy thieves have been at work. The always-on