Alternators and Charging Systems


Upon reading and answering many questions about car audio, one question always pops up:
"Do I need a high output alternator or another battery, or just a capacitor?"


If you want a killer system, you'll need more than just big amplifiers and huge speakers. Adding a bunch of car audio components to your vehicle without the proper electrical system may lead you to disappointment, distortion and damaged equipment.


Let's say you install a 1,200 watt amplifier in your ride. Your ride's charging system must produce enough electrical current to run the amplifier and still power the rest of the car's electrical requirements such as the ignition system, power accessories, headlamps, and so forth. Now how do you know how much current is needed? Simple:

step 1: Find out how much contnuous (often labeled "RMS") wattage your amplifier will produce based on the load presented by your speakers.
(This means if you have a monoblock sub amp, and a dual 4-ohm coiled sub, with the coils wired in parallel, you'll present a 2 ohm load to the amp, as an example.)

step 2: Divide this number (watts) by 12 (volts)

step 3: Take the resulting number and multiply by the factor below, based on the amplifier class:

Class AB (typically full range, multi-channel amplifiers): *1.4

Class T (tripath) or D (digital): *1.2

step 4: Add the totals together if there are multiple amplifiers in your system

step 5: add your stock alternator's HOT current rating to this figure, or if this figure is unknown, just add 50 (amperes) .
The total you get is the amount of current your car and audio system will draw at peak output.*


*Here is where things can get a bit tricky, so I'll explain how this works. The "RMS" pak output of an amplifier is measured by the manufacturer on a test bench, using a regulated switching power supply, and a resistive anechoic dummy load, at full output (just below clipping) using a full 20Hz-20KHz pink noise tone generator.


OK, What's all that mean to me?


The truth is, many factors will affect how much power you really use versus what you are told is the rated power of an amplifier. Your speakers, for example, are an inductive load, and subject to fluctuating impedance based on factors like the frequency being played, the efficiency of the speaker enclosure at that frequency, impedance rise of the enclosure, cabin gain and transfer function, the source material (music) you're playing, and at what level that music was remastered and recorded, and many other big words that don't really matter. The point of this is, while your amp says "1200 watts" on the case, the reality is that at moderate to loud volumes (see my Power to Volume ratio paper for more on this) you're really only using a fraction of the power that amplifier can actually produce.


OK, but again, what does this mean for me??


Well, while your class D 1200 watt @ 2 ohm amp may be able to draw 120A of current on a test bench, in your car, you may never see two-thirds of that actual current draw, and not for any sustained period of time, at that. So, when you figure you're going to need a 170A alternator using the calculations above, the truth is, for a daily driver setup, you'll probably be fine with a 130A alternator, or possibly even smaller than that. The only way to really know for sure is to install your system and watch it with a voltmeter to see if voltages drop below 12VDC. If they do, then you want to start looking at a larger, high output alternator.


Most competitors in sanctioned bodies entering SPL events will run their systems with the car's engine turned off, so at that point it's all about having enough battery power to drive the system long enough to burp and be measured in the lanes. If you plan to have the engine running while you listen to music, chances are you'll be in the car at the time, thus you won't be playing a high end audio system at full blast for fear of hearing damage and all that, so again this will also limit the current draw on your electrical system if you listen responsibly.


Most vehicle electrical systems are 12.6VDC at idle, and between 13.8 to 14.4VDC above about 900-1200rpm but we use 12 volts as the standard for these calculations because many amplifiers today use a regulated power supply, so they get 12VDC regardless of input voltage.

1,200 watts divided by 12 volts equals 100 Amperes of current.

If this is a class D amplifier, it'll be ~80% efficient, so we'll add 20% of 100A to the total, and we get about 120A of current demand continuously at peak output.

This means that the electrical system of the vehicle will need to produce an extra 120 Amperes of electrical current to power your amplifier and subwoofer at peak levels. Keep in mind that the factory electrical system is designed to produce enough charging power (alternator and battery) for the vehicle's stock equipment, and was not designed to accomodate high-powered audio systems.


The biggest mistake made by consumers (and many installers) is failing to beef up the charging system to handle the extra load of the audio system. First, you need to understand how the electrical system operates. This must be one of the most mis-understood systems of the entire vehicle, so here's a brief summary:


So, how does the electrical system in my car actually work?


Turning the ignition key begins the process of cranking the motor.

The battery supplies the power to get things started. Once the engine is running, the electrical burden is shifted over to the alternator, and the battery then goes into a charging state, with the voltage bias of the alternator being greater than that of the battery itself.

At this point, the battery only functions to filter and stablize the DC voltage from the alternator since DC voltage regulators only do so well at rectification of AC voltage which is what the alternator supplies.

The alternator uses the engine's mechanical power to produce electrical (AC) voltage. The AC voltage is passed through a rectifier and changed to DC voltage by the DC voltage regulator to smooth out and set the voltage rails for the car . The alternator also has the duty of recharging the battery after starting the vehicle by providing a forward bias voltage higher than that of the battery.


Everything works perfectly so long as the power requirements of the vehicle do not exceed the capabilities of the alternator. If the peak output is surpassed due to excessive load, then power will be pulled from the battery. If the alternator and battery combined cannot meet the demand, then the vehicle's voltage rails, and subsequently the electrical devices are diminished (dimming lights, spark plug misfires, audio distortion and amplifier clipping, or even the engine stalling.)


The first place to look to determine if your charging system is up to the task is the alternator itself. If possible, look for the HOT RATING on the alternator. IF you can't see it easily, call a local auto-parts store or car dealership and ask them to look up the stock alternator size, or rating for your vehicle. The hot rating will tell you the amount of power the alternator will produce once the engine reaches it's operating temperature (this is a lower rating than the cold rating). I suggest using your stock alternator unless you experience problems. That's how you know if you need to upgrade, since there is no concrete way to tell if a stock alternator has enough reserve to handle your additional burdens. Now, if you do need a new and larger alternator, after finding the stock rating, then allow your alternator about 10 Amperes credit or buffer area.


One more thing to remember is that all of this is conected as a giant electrical circuit, nd for all of this current to get from the alternator, or battery, to your stereo system and amplifiers, you need the right wire. The same size wire should be used for everything, including the power line run(s) to the amplifier(s), the ground lines, and the "Big 3" (covered in here.)


The size wire you will need depends on the current draw of your combined amplifiers, and the length of wire in the total circuit (battery to amplifier to ground)

The following charts should assist in determining the ideal wire gauge for your application.



For more information on charging systems and alternators, see here:
BCA&E page on Charging systems
BCA&E page 2 on charging systems

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