Multi-amp installationsPowering multiple amplifiers: If you're installing more than one amplifier, run a single heavy-gauge power cable from your battery to a distribution block, and then connect lighter-gauge cable from the block to each amp. This arrangement minimizes potential noise problems and keeps your installation looking neat. Make sure you use power cable that's thick enough to accommodate any amps that you might add in the future.
Grounding multiple components: If you're installing several components, use a distribution block to organize the various ground cables into one larger cable. If that's not an option, try to ground each one separately, with about a half-inch of space between each of the grounding points. If you'd rather use a single bolt, place the ground for the component that draws the most current (your most powerful amp) closest to the car body. Put the ground for the component that draws the least current on top.
Turning on multiple components: If you have multiple components in your system, you may need a relay to protect your receiver. That's because the receiver's power antenna lead can only supply a limited amount of current. If the components in your stereo system ask for more current than the receiver can supply, you can burn out that circuit in your receiver. A relay connected to the power antenna lead draws a small amount of current from the receiver, but supplies enough current to turn on all your other components simultaneously.
Power demands of a multi-amp system: Some cars' systems are designed for additional current loads (trailer towing packages, for example), but most of them are not. Luckily, most manufacturers design their cars' electrical systems with a reserve capacity that is well-suited to supplying power for a sound system. Look at the tag that is riveted or screwed to the side of the alternator. This tag tells you how much current (expressed in amperes or amps, for short) the alternator is capable of producing.
Your car's alternator ampere rating determines how powerful an amplifier you can install. Multiply the ampere rating by 40%, and you'll get a rough idea of how much reserve current capacity your car's system has. Next, you'll need to calculate the approximate current draw of the amplifier you're considering installing.
To calculate the current draw of an amplifier, multiply the number of channels by the RMS watts per channel (a 2 channel amp rated at 300 watts RMS per channel would be 600 watts). Double it to account for amplifier inefficiency (600 watts X 2=1200 watts), then divide by the average output Voltage of an alternator, 13.8 volts (1200 divided by 13.8=87 amps). Since the average music signal requires about 1/3rd of the average power in a test tone, divide by 3 (87 amps divided by 3=29 amps). The result is the amplifier's approximate average current draw.
A quick way to ballpark an amplifier's current draw is to divide the total fuse value of the amp by two. For amplifiers with multiple fuses, the rating of all fuses provided with the amp must be added together. This will likely produce a significantly higher estimate than using the proper formula. Although inaccurate, this will err on the side of safety.
Finally, compare the amplifier's approximate current draw to your vehicle's reserve current capacity to determine if the electrical system can support the amplifier.
If all those numbers are a bit much, here's a simpler way to think about it: an alternator capable of producing 65 amperes is usually adequate for systems up to 270 X 2 watts RMS. A compact car with a 35-amp alternator can accommodate around 150 X 2 watts of power, while a Sport Utility with a 145-amp alternator can handle a 600 X 2 watt system.
If you want more power than your alternator can service, you will have to consider having your alternator rebuilt for higher output — or invest in a high-output aftermarket alternator. (Sorry, we can't help you with this.) Installing a second battery won't help — that only allows you to play your stereo for long periods with the engine off. But, remember that any system playing at one-third volume is drawing considerably less current than the same system at three-quarter volume, and may not need a charging system upgrade if the volume levels are kept reasonable.
If your system needs more current than the alternator can supply, it starts to grab current from the battery — not a good thing for your car or your music! Here are a couple of warning signs to look for:
your headlights dim in time to the music when you crank it up loud at night;
your bass sounds great when you first turn it on, but gets mushy and less powerful within a minute or so.
One easy, effective solution is installing a capacitor. Heavy-duty capacitors connect to the power cable (just before it reaches your amplifier) and act as a buffer zone between your amp and your car's electrical system. They store up a reservoir of power which can supply the amplifier's peak demands (like a kick drum beat) without having to pull current from the battery.