1. The fundamental purpose of a car battery is to start the engine.

2. The fundamental function of the alternator is to charge the battery and run all of the car’s electrical accessories — including the stereo system.

3. The fundamental reason to install a second car battery is to increase the engine off, parking lot listening time for the stereo system.

Conspicuously missing from the above three rules are claims that extra batteries improve the sound of the stereo system, prevent headlights from dimming on deep bass hits, and lighten the load on the alternator. Although SPL vehicles often require huge battery banks, extra batteries do not improve the sound for normal installations. Adding an extra battery will not prevent headlights from dimming. And adding an extra battery actually increases the load on the alternator, thereby leaving less available energy for the stereo system. Whew!

The Bicycle Ride
Several years ago, Kenwood U.S.A. (www.kenwoodusa.com) asked Autosound 2000 to present a special workshop for their annual installation conference. Several hundred reps, dealers, installers, and product specialists met in Dallas, TX, for the three-day event. One of our topics was the subject of “Installing a Second Battery” for a car audio system. The obvious reason for the second battery is so that the stereo system can play for prolonged periods of time without depleting the car’s engine starting battery. However, some demo and show vehicles were adding multiple secondary batteries. This led to charging problems that often required upgrading the alternator just to maintain all the batteries. Our job was to convey the physics behind batteries and alternators to the audience. My partner, Richard Clark, came up with the stationary bicycle and car alternator demonstration.

The type of bicycle is not critical. As the human motor pedals, the alternator produces around 14.5 volts DC. A small 12-volt, fully-charged motorcycle battery is connected to the alternator. We begin the demonstration by selecting the best long distance bicycle athlete in the audience. The seat and handlebars are adjusted, and the rider sets about pedaling at a modest pace. With only the small motorcycle battery connected to the alternator, the voltmeter reads 14.5 volts DC.

Also installed on this system is a “Zero Centering” ammeter that’s wired to show the direction of current flow from the battery’s positive terminal. In the beginning of the demonstration, the current from the alternator is going into the battery. But this will soon change.

The scene was getting a bit boring, so we decide to add some tunes. A car audio CD player connected to two full range separates and a 50-watt amp for a subwoofer is switched on and adjusted for moderate volume. As soon as the stereo is activated, the cyclist’s pace slows and the voltmeter dips quickly. To overcome the load of the stereo system, the cyclist must pedal harder. As soon as he is back up to speed, we increase the volume of the stereo system. Now our cyclist is really working hard to keep the current going into the battery as well as into the stereo system. But it’s getting dark, so we decide to turn on a headlight. When the headlight is turned on, the cyclist really lugs down to a slow crawl. He must stand on the pedals to keep up. But what has happened to the direction of current flow? With the motorcycle battery, the cranking stereo system, and the headlight, our human motor can no longer keep the ammeter swung to the charging side. Energy is no longer going into the motorcycle battery. In fact, on deep bass hits, the ammeter swings far into negative territory and energy comes out of the motorcycle battery. We can’t continue with this situation very long because the battery will become depleted and our bicyclist will wear out. So, what do we do to help?

From a flurry of hands in the audience, some say, “Add a second battery.” We just happen to have a fully charged, brand new, car battery sitting on the stage. So we connect the second battery into the system to help our poor overworked cyclist. Guess what happens? The bike stops. Standing on the pedals, the alternator just barely moves, but there’s no way to produce enough energy from the alternator. Most of the energy for the headlight and the stereo system is now coming from the two batteries.

Lighten The Load
To get things rolling again, we must lighten the load. The first accessory we drop is the headlight. With the headlight off, our worn-out cyclist can once again pedal fast enough to get out of the battery discharge condition. Adding that second battery increased the load on the system beyond the capacity of our human engine. The only way to get the stereo system powered by the alternator again is to disconnect the second battery. As soon as the battery is out of the circuit, the ammeter swings to the positive direction and energy again flows into the motorcycle battery.

The point here is that a battery is a load to a working alternator. Just ask one of the volunteers who participated as riders in this demo. You can bet they don’t take adding batteries lightly! Adding many batteries in parallel increases the load on an alternator. This means that there is LESS energy for the stereo system. Large capacity batteries present a greater load than a small motorcycle battery. The moral of this story is that when a second battery is added to a system; don’t install a larger battery than is necessary.