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Charge Excess

Avoid the voltage drop

Mike Petralia Feb 7, 2006

A car's electrical current is its lifeblood. Without it, no car could survive and the more juice you got, the better your car will run. Just like our brains firing the stimulus needed to keep our bodies alive, the car's charging system keeps it in motion at all times. Without "juice" there would be no power, no lights, no tunes, and no go. In the early years of auto electric, few people understood how a car's charging system worked and what was required to effectively improve it. Besides, back then all you needed were a few amps to run the lights and recharge the battery, but things have changed and we need to keep up or our futures will fade to black.

In the dark ages of auto electric, a common remedy for below-standard voltage was piling more batteries into a vehicle. And it was common for racers to believe that removing the alternator and running two batteries would save them power. They forgot that the extra battery added around 40 pounds to the car and that with a properly charged single battery, the alternator hardly takes any effort to spin at all. Also, after people got wise to the need for larger power cables. Eight gauge, then four, and now 1/0 has become the main power conduit of choice. And big wires, although heavy, also add a layer of reliability to your ride.

Most enthusiasts understand that alternators are the source of the charge, but many don't know just how important they really are and the alternator still remains an afterthought to most who are building a car today. The alternator creates the amperage and voltage that powers every electrical component in the car. And they do a pretty good job with their task. Upon closer examination, however, there is some inefficiency to overcome. Anytime energy is converted from one form to another, some of it is lost, usually in the form of heat. The whole process of creating energy in a mobile environment is, in itself, an inefficient transaction. Basically, the alternator first transforms the engine's rotating mechanical energy to AC (alternating current). It then changes AC to DC (direct current), which the engine and its accessories need to survive. But, engines and alternators get hot and it's that heat that causes the problems. It's a lose-lose situation we need to overcome.

So how do all the electrical components work together? Batteries and capacitors are just simple storage devices that hold electrical energy for use on demand. Batteries are large reservoirs, which, due to their internal resistance, react with a slight delay. Batteries also act as a load for the charging system until their services are required by the electrical components. Capacitors, on the other hand, are like small electrical reservoirs that react very quickly to demand and can deliver high bursts of short-lived energy.

The relationship between amps and volts is difficult to understand. Amps are sometimes easier to comprehend because you can actually calculate how many are needed. To get a feel for the current draw in a system, add up the fuse rating for each component you're running. The system may draw more than this total during peaks, but the greatest draw will be somewhere around this number.Voltage can be thought of as the pressure that pushes amperage through the electrical system. You can't force a component to take amperage, but you can force-feed it voltage, i.e. running 16-plus volts in a 12v system, which will make it pull excess amperage. The result is often smelly white smoke. Too little voltage and the components will starve for current, which may also cause damage. When the vehicle is running, the alternator controls voltage as long as it has enough amperage capability to keep up with the demands of the electrical components, voltage will be maintained at whatever level the alternator determines necessary. When components draw more energy than the alternator can supply, system voltage will drop down approaching that of the battery voltage. If this situation continues, and since battery storage is a finite amount, voltage will continue to drop until something quits.

Alternators are made up of four main components, the rotor, stator, rectifier, and the regulator. The rotor and stator are the components responsible for creating amperage. Since both of these are made up of coils of wire, the heat from the conversion process increases resistance and decreases the alternator's capabilities. On average, a hot alternator produces 10 percent less output than a cold unit. After the alternator creates the current, it flows through the rectifier. This component is usually made up of six or eight diodes that convert three-phase AC into somewhat crude DC. The battery helps clean up the DC later downstream.

The alternator's regulator is its brain. There are two different styles of regulators, externally and internally mounted. External regulators can allow for easier cooling and usually have a voltage set point adjustment somewhere on or inside their housing. Most regulators have an output delay built into them to minimize the shock to the vehicle's engine when a sudden load becomes present. Regulator response times vary between 2 and 10 seconds and often require as much as a full volt drop before an alternator will step up its output.

Many performance parts have non-regulated power supplies that could benefit from higher system voltage. For years, exotic external regulators have been available, with dashboard adjustable voltage set points that were designed for ambulances and emergency vehicles, but caution must be taken with these regulators because improper adjustments can result in dangerously high voltages. Besides, these are not very common on the street, although they have arisen in car-stereo shootouts.

Alternator designs have seen only minor advancements in past years, but recently Powermaster introduced a digitally controlled alternator called the XSvolt. In addition to its PWM and MOSFET technology, it offers internally adjustable voltage and up to three in-car controllable voltage presets. Possibly the most impressive feature of this digital system is its nearly instant reaction time. The XSvolt will react in as little as 100 milliseconds, nearly 2 seconds faster than previous designs.

Today, alternators are available with outputs from 50 to 350 amps. Be careful though, as bigger is not always better in this case. Just like when swapping an intake or camshaft, maximizing high-rpm alternator output will sometimes sacrifice performance at the low end. And performance at idle is what most of us need from an alternator. For street use, a 200-amp alternator may offer the highest idle performance, while extreme 300 amp alternators usually offer less output at idle than stock OE units do. Vehicles must have an alternator that has enough idle performance output to sustain itself.

Alternators must spin faster than the engine. The speed of the alternator is a function of the pulley ratio between the crankshaft and the alternator pulley. This ratio from the factory is usually 3:1, which means that the engine's crank pulley is three times the diameter of the alternator pulley. Stated another way, at 1,000-rpm engine speed, the factory alternator spins 3,000 times a minute. Alternator idle performance is proportional to the pulley ratio and the engine idle rpm. Because high output alternators have a very steep idle performance curve, it is critical that the alternator spin at a minimum of 2,400 rpm. If alternator rpm drops below 2,400, output capability will quickly decrease.While underdriving a vehicle's accessories can boost performance at higher engine RPM, it will sacrifice charging at idle. By keeping a small pulley on the alternator, 10 or more additional amps (at a given engine rpm) can be created. March Performance and others offer V-belt and serpentine drive pulleys that are as much as 15-percent smaller than original equipment. Using overdrive pulleys, as opposed to underdrive pulleys, will improve the charging system's performance, but keep in mind that it is recommended alternators operate below 18,000 rpm, so if your engine spends a lot of time past 6k, keep your pulley ratio lower than 3:1.

Alternators only produce enough amperage to supply the car's electrical component's requirements. An alternator rated at max 200 amps may only produce 30 amps at a given time. Voltage is a different story. Most automotive electrical components will not tolerate much more than 15 volts for very long. For this reason, it is recommended that the alternator's voltage set point be kept at or below 15 volts, unless it's a drag-only car that sees running times for about a minute at a stretch. A dedicated system can be driven to higher voltages as long as the battery and equipment tolerances are not exceeded, such as running a 16v fuel pump and ignition system, but keeping things like fans and water pumps below 15v.

There are advantages to upgrading any car's electrical system, though maximum performance can only be achieved by improving all the system's components. Alternators may be expensive relative to batteries; however, if you want to keep your car running strong and cruising long, think of the good ones as a necessity.


Hot electrical systems need more power to charge. We know you'd never skimp to save a buck on your ignition system. So why would you cheat yourself when it comes to your charging system, too?

Powermaster supplies a performance data chart with its alternators to let you know if you've got what it takes to light up the night.

Alternator drive ratio is very important. This shot of a high-rpm circle track-style drive system would not work well on the street.

Today's batteries are stronger than ever, but they still need a recharge.

Note the much larger pulley on the crank that drives the small alternator pulley. This is how it needs to be done. Keep alternators overdriven around 3-times faster than the engine and you'll be in good shape.

If your alternator has a lot of power to charge, don't give it any resistance by installing too small of a charging wire. Run at least an 8 AWG wire from the alternator to the battery. Go bigger, (lower AWG number) like 6- or 4- AWG if your battery is in the trunk.

Want to figure out how many amps you'll need before you buy your alternator? Simply add up the fuse ratings of all you accessories, and that's the number to go by.

Powermaster has developed an adjustable output voltage alternator that anyone can install. Its internal voltage regulator is adjustable from the back of the alternator in a range of a couple of volts.

Here's a shot of co-author Coy Hudnall's SPL (Sound Pressure Level) competition-winning '91 Camaro Z-28 and the twin alternator setup he designed and built in-house. It takes somewhere around 400-plus amps to drive his system. He could weld the Brooklyn Bridge with that thing!

It's those crazy car-stereo competitors that the rest of us can thank for making the aftermarket come up with ultra high output alternators and batteries.


March Performance
Naples, FL 34110
Optima Batteries, Inc.
Milwaukee , WI 53209

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