Big amps and big speakers alone may not produce the awesome sound system you seek.

Some car owners pack their car with audio components only to end up disappointed. Why? They never stop to consider the vehicle's electrical charging system. As a result they get a sound system that produces more distortion than music. They also run the risk of damaging their components and draining their car battery. That's a bummer.

This 750-watt Rockford Fosgate 1501 bd Mono Subwoofer amplifier draws approximately 90 amperes of current, a significant burden for your vehicle, considering the average stock alternator produces around 60 amperes of current.

New amps and speakers, especially the big powerful ones, may require more electrical current than your car's electrical system can produce. If you add monster amps rated at hundreds of watts of power, your car must be able to produce enough "juice" to drive them, or you risk damaging your system.

Electronics experts often refer to Ohm's Law when explaining vehicle-charging systems. To paraphrase Ohm's Law, as voltage decreases, current decreases. A lack of voltage to an amplifier creates heat as the amp begins to work harder to supply the power (current) demand. Heat, it should be noted, is the mortal enemy of electronic components. An inadequate charging system won't supply the voltage needed for big amps and woofers. Therefore, the electrical current decreases — and heat increases. As a result, you get a lot of distortion or component damage caused by excessive heat.

Understanding automotive electrical systems can be awfully confusing; even technicians may have trouble explaining some aspects of it. Fortunately you don't need a degree in electrical engineering to learn some important basics about auto electronics.

Bass basics
Most modern music is bass-heavy. Rap, hip-hop, and metal music all rely on lots of hard-hitting, low bass. If that's your type of jam, then you'll probably want to add a big ol' subwoofer, or maybe several subs, to your car audio system.

Before you buy a whopping subwoofer, you first must consider your car's electrical system. That new sub will need an amplifier to power it, and the amp needs electrical current from your car to create that power. If you want your sub to perform to its potential, you'll need an amplifier with a similar RMS power rating (along with a well-designed subwoofer enclosure). If you choose an amp that doesn't provide enough power for the sub, then the subwoofer will underperform and likely create distortion.

With the engine off, current flows from the battery to the accessories. The alternator supplies nothing.

Current draw
How do you know how much current your new amplifier will need? Divide the RMS power rating of the amplifier (let's say 500 watts) by the amount of voltage your car's electrical system will produce — RMS/Voltage = Current Draw.¹ The average automotive electrical system will produce about 13.8 volts and 60 amperes of current with the motor running. So, 500 watts divided by 13.8 volts equals 36.2 amperes — your new amp will require about 36 amperes of current draw from your car's electrical system to achieve optimum performance.

When the engine is running, the alternator charges the battery and powers the accessories. As long as the accessories' current draw does not exceed the alternator's charging capacity, the battery does not supply current.

Most cars come from the factory with a charging system that is adequate for the vehicle's stock equipment — air conditioner, power windows, defogger, etc — and can provide a modest amount of extra capacity (30-40%). Note that bigger vehicles like SUVs and large pickup trucks usually have larger alternators that can handle an additional 30 amperes of draw. However, if you drive a small car, adding 36 additional amperes of current draw to a system that produces only about 60 amperes of current could be a significant extra burden, one that your stock electrical system might not be equipped to handle.

A high-powered audio system can draw more current than the alternator is capable of producing. At this point, current flows from both the alternator and the battery to supply the vehicle's accessories and the audio system.

Power Supply
Understanding the electrical system of your car will help you determine how much additional electrical current you need. When the motor is started, the car's electrical burden shifts from the battery to the alternator. The alternator creates alternating electrical current (AC) that is "rectified" (changed) into direct current (DC). The DC current runs all the devices in the vehicle which require electricity. The alternator also recharges the car's battery.

If the vehicle's electrical power demands exceed the alternator's capacity to produce juice, then power will be pulled from the car's battery. If the alternator and battery combined cannot meet all the car's power needs, then the voltage available to power each electrical device is diminished. Diminished voltage means an increase in current, and current produces heat that may damage your stereo system.

How do you know how much power your car's electrical system produces? Check out your alternator. Most alternators have a "hot" power rating listed somewhere on the unit. The hot power rating is the number of amperes produced when the motor has reached its peak operating temperature.

Perhaps you discover that your car's alternator is capable of handling the new amps and speakers you just installed. If so, that's great.

¹There are several methods for calculating current draw. The Mobile Electronics Certification Program (MECP) lists two: RMS/voltage and RMS x 2/voltage. The second method doubles the RMS power before dividing by voltage, in an effort to compensate for the power wasted by the amplifier in the form of heat (up to 50%). Many sources, including The Crutchfield Guide to Car Stereo Installation, cite the RMS x 2/voltage standard, but amplifier efficiency varies from model to model (especially with Class D design amps). The MECP states: "[The RMS/voltage method] is perfectly adequate for a system to operate safely." % end if

Power Options
But what if you need additional current? The easiest and fastest way to boost your electrical system is to install a larger battery. This solution is only recommended if you need only 10 or so amperes of current. Adding a current draw of more than 10 amperes will often weaken your battery quickly and could leave you stranded somewhere.

If your new sound system creates a significant additional electrical burden on your car, then you can consider adding a second battery to your car. Many car audio experts don't recommend a second battery because two batteries create stress for the alternator and raise other complex electrical issues. There's also the question of safety. Batteries that are not properly ventilated can be dangerous. If you're thinking of adding a second battery, talk to a professional 12-volt installer first.

Adding a second battery will increase the amount of time you can listen to the audio system with the engine off. When the engine is running, the secondary battery is an additional "load" for the alternator to charge.

A capacitor stores electrical voltage and releases it as needed. Capacitors are great if your electrical system is near its capacity and the music you are playing suddenly hits a thunderous, long bass note. A capacitor releases the needed power to meet the demand without stressing the system, then it recharges itself. There are also specially designed car batteries that work like capacitors; that is, they have standby power that can be released as needed.

Mount a capacitor as close to the amplifier(s) as possible.

Aftermarket alternators can significantly increase your vehicle's charging capacity, but installation can be complex — speak with a professional installer first. (Crutchfield does not offer alternators.)

Perhaps the best way to add a significant amount of electrical power is to install a larger, more powerful alternator. This is usually a more expensive solution, but it also may be the best solution. You should consult a professional installer if you're thinking of installing a bigger alternator. Big alternators can create big stress on small motors, so make sure your engine can handle the added burden.

It's great to have a big powerful sound system in your car, but don't rush out and buy big amps and speakers until you've considered your vehicle's electrical charging system. Without enough juice, all you'll get is a lot of static, and nobody needs more static in their life.

RFTECH Technical Resource Center for more information.