Car Stereos: car alternators, rockford fosgates, maximum power output

Question
I was wondering what size alternator i will need to run two 2000.1db watt amps in my twothousand chevy tahoe and the amps are Rockford fosgates.

Answer
Hi Anthony,

I apologize for my extremely late response to your question.

It's actually pretty hard to predict just how much current capacity you'll need from your vehicle's electrical system.  The answer depends on several different factors:  the type of music you play, and how loudly you play it, your subwoofer wiring configuration, and the total impedance load on the amps will all help determine how much power is used by the system.

Let's assume you have each amplifier connected to a 1-ohm subwoofer load, and plan to push them to their maximum output.  Your amps will be producing 4000 watts. If the amplifiers are about 75% efficient, your alternator and battery will need to provide about 400 amperes of current to maintain that type of output.  You're not likely to find an alternator with that capacity.

Luckily, unless you're playing test tones, your amplifiers won't need to produce 4000 watts on a continuous basis.  When an amp is playing a musical track, the power required will be jumping up and down along with the loud and soft points in the music.  The ratio between the average power output and the maximum power output is called the "crest factor", and it varies quite a bit with the type of music you play.  For most music, the average power output is about 20-40% of the peak output.  If you're playing your music at maximum undistorted volume, and your amps are producing 4000 watts on the musical peaks, the average output will probably be around 1000-1500 watts.  In that case, the average current requirement from the alternator might be around 150 amps--and if you don't play your music at maximum volume, it will probably be a lot less.

You'll also need to consider what other electrical accessories will be used in your vehicle.  Will the headlights, A/C fan or defroster normally be in use?  If so, you might need more alternator capacity.

If you plan to spend a lot of time playing your music loud, I wouldn't go with an alternator under 250 amps; and, of course, going bigger won't hurt anything.  Be sure to upgrade your charging system wiring as well, including the main power wire between the battery and the alternator, the wire between the negative battery terminal and the vehicle chassis, and especially the engine ground strap.

Hope this helps!

Brian