||Installing a Second Battery|
|Learn the practical theory behind adding a second battery to a 12-volt install.|
|by David Navone|
|By the traffic on the CAR SOUND Troubleshooting Forum, its evident that some car audiophiles may not completely understand the function of a car battery and alternator. Lets begin with Autosound 2000s First Three Rules of Basic Automotive Electronics as applied to normal car audio installations:
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 cars 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
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 thats wired to show the direction of current flow from the batterys 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 cyclists 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 its 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 cant 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 theres 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
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 dont 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; dont install a larger battery than is necessary.