Amps, Alternators, and Batteries: Upgrading Your Electrical System
Author: Wayne Harris Originally appeared in the July/August 1988 issue of Car Stereo Reviewmagazine.
One of the most overlooked aspects of a high-powered auto sound installation is its effect on a cars electrical system. No amplifier, no matter how sophisticated it is, can change the laws of physics. Energy can neither be created nor destroyed; it can only be changed from one form to another. This is exactly what an auto sound amplifier does: It takes power from the electrical system in your vehicle – the battery and alternator, primarily – and converts it into usable power to drive your speakers. When installing an amplifier into your system, remember that it cannot produce more power than you make available for it to convert – so you need to know what your electrical system is capable of, and how to upgrade it.
Say you have a 1,000-watt audio system and you want to know if your cars electrical system can drive it. At full output, the system will produce 1,000 watts of power to your speakers at about 62 percent efficiency. This means that the input power to the amplifiers is 1,000 / 0.62 or 1,612 watts. The standard equation for power is
P = V x I
P is the input power in watts
V is the input voltage
I is the input current in amps
If we assume that the batterys voltage is the typical 13 VDC, then the input current, or alternator power, required by your system can be calculated from the above equation. Rearranging the equation, we get
I = P / V
By plugging in the numbers and solving the equation, we get...
I = 1,612 / 13 I = 124 amps.
Most stock alternators produce only about 60 amps maximum. Of this, about half is used to run the car itself. So this leaves you with about 30 amps for your system, 94 amps less than the 124 you need. Where will the other 94 amps come from? The battery will try to supply it, but it will be able to do so only for short periods before the voltage drops to an insufficient level. Your system will not be able to play at its full potential for any significant length of time.
There are several things you can do to increase the performance of your electrical system. First, you can replace the stock alternator with a high-output alternator, the single most important modification you can make. There are many high-output alternators on the market, with outputs ranging from 105 to 190 amps at full load.
When comparing these alternators, there are several important questions you should ask, the first probably being: Will it fit? Some high-output alternators replace OEM equipment bolt for bolt. If this isnt the case, youll have to construct custom brackets; this is a very involved and time-consuming task.
The most important specification to query is the alternators current rating. Make sure all ratings are for a "hot" alternator; output current usually drops as the unit heats up to its normal working temperature. Find out what the output current is at both idle rpm and highway rpm. When comparing alternators in this manner, make sure both units have pulleys of the same size and thus the same rotor speed.
The last question you will have to ask concerns regulation. Some alternators have "internal" regulation, while others have "external" regulation. I recommend using external regulation, if possible, since this type makes it easier to adjust the electrical systems voltage.
Using multiple batteries will also improve the performance of your electrical system. The extra battery or batteries will supply additional power to the system when the current demands of the amplifiers exceed the full output capability of the alternator. Multiple batteries also allow you to play your sound system longer when youre parked and the engine is shut off.
Ideally, you should opt for a high-output alternator and additional batteries. A good rule of thumb is to use the largest alternator you can fit on the engine and an additional battery for every 500 watts of amplification.
There are many types of batteries available, of course – which one is right for you? For audio installations, experience has shown that the best results will be obtained using deep-cycle marine batteries. These batteries can be obtained from almost any dealer for approximately $60. Lead-acid batteries are preferred over calcium-based (maintenance-free) batteries because of their superior internal characteristics. Maintenance-free batteries are usually designed to provide a large amount of current for a short period of time. This is great for starting your car, but not so great for running a high-powered system.
Deep-cycle marine batteries, on the other hand, are designed to provide a moderate-to-large amount of current over a long period of time. The plates in each cell are thicker, and they are made for deep, cyclic use. This is important because you can severely drain this type of battery without damaging it, which is ideal for car audio installations.
There are three ratings you should check out when selecting a battery. The first is the CCA (cold-cranking amps) rating. This indicates a batterys ability to provide a large amount of current for a short period of time at cold temperatures. (If youre looking for a battery that will be used just to start your car, this is the rating you would be most interested in.)
The second rating, and the one that is most important in terms of your system, is the AH (amp/hour) rating. This rating simply states how much current the battery is capable of delivering for a period of time. The larger the rating, the better. Typical deep-cycle marine batteries have AH ratings of 85 to 105.
The third rating you should look at is reserve time. Deep-cycle marine batteries really shine here. Reserve time represents the batterys ability to recover and produce electrical energy after a discharge cycle – without re-charging. The longer the reserve time, the better.
Installing the extra battery or batteries in parallel will result in the most efficient use of its (their) power. When using this method of connection, your total AH rating is the sum of the AH ratings of all of your batteries. Note that batteries installed in this manner should be identical (the same make and model). If theyre not the same, their internal impedances will differ and, eventually, one battery will discharge the other.
When wiring the electrical system, dont skimp on large-gauge wire. Welding cable is preferable because of its tough outer jacket, fine wire strands, and good flexibility. The bottom line is that you should never use less than No. 4 wire. Also, dont forget to install 150-amp circuit breakers at each end of the power cable running from the front to the rear of the car. The circuit breakers will protect the vehicle in case of a short somewhere along the length of the cable.
Its important to take the time to plan out your electrical system. A properly designed and installed system will provide a good foundation for your auto sound components while maintaining reliability and safety. The audibly superior performance will be well worth the added time and expense.