Alternator Selection Criteria
by Ed Baker
Introduction
There are two major components in the charging system of our cars-the battery and the alternator.
The battery has two primary functions. The first should be obvious-it supplies the power required to start the engine, and secondly, it serves as a buffer to provide additional power temporarily if the alternator is unable to keep up with momentary excess load demands. A good battery is required in order to have proper alternator operation.
The alternator also has two primary functions. One is to provide the power required by the car during normal operation (ignition, fuel pump(s), fan(s), radio, and lights. The second is to maintain/recharge the battery to restore the power used during the starting process or the momentary periods when the car's demands exceeded the capability of the alternator to meet them. If the battery is not in good condition, the alternator will be overtaxed trying to charge it and this will shorten the life of the alternator.
The factory alternator is more than capable of meeting the demands of the cars as delivered. If we add enough additional electrical items to the cars such as dual fans, two fuel pumps, serious sound systems, etc., there may come a time when a larger alternator is required than the factory unit.
So what does it mean?
Many people are familiar with pressure and volume when it comes to fuel pumps, water hoses, or air compressors and they understand that as volume consumption increases, the pressure will decrease (or vice versa).
A Walbro 340 will flow approximately 58 gals per hour at 40 psi, but, it flows about 50 gals per hour at 60s psi.
Volts and amps work the same way. Amps are the electrical equivalent of fluid volume and volts are the equivalent as fluid pressure.
Ohm's Law tells us that Voltage = Amperage X Resistance and that Power (watts) = Volts x Amps.
Resistance is an important factor as it is directly related to voltage and amperage. Wire size is similar to hose size. As hose size is increased, more fluid will be able to flow thru it if required. As wire size increases, more electricity can flow thru it easier when required.
Don't be fooled by claims based solely on amps, or volts, as the power (volts X Amps) required to operate the components that comprise the system properly is what is important. Power is the constant and if volts go up, amps come down, and, vice versa.
How does the charging system work?
Most charging systems that are working properly should produce a charging voltage of about 13.8 to 14.2 volts at idle with the lights and accessories off. Manufacturer's specs are usually provided in the shop manual.
When the engine is first started, the charging voltage should rise quickly to about two volts above base battery voltage, then taper off, leveling out at the specified voltage as the alternator warms up.
The exact charging voltage will vary according to the battery's state of charge, the load on the vehicle's electrical system, and temperature. The lower the temperature the higher the charging voltage, and the higher the temperature the lower the charging voltage. The "normal" charging voltage on a typical application might be 13.9 to 14.5 volts at 77 degrees F. But at 20 degrees F. below zero, the charging voltage might be 14.5 to 14.7 volts. On a hot engine on a hot day, the normal charging voltage might drop to 13.5 to 14.2 volts. This happens because the voltage regulator is tied to temperature. On a cold day, the internal resistance of the battery increases, and, it takes more voltage to charge the battery than it does on a warm day. You can observe this voltage drop by measuring the voltage at the back of the alternator when you first start the engine and then watch it drop as the alternator begins to warm up and the battery is charged.
When do we need a larger than stock unit?
So, how do we know when a larger alternator than the stock unit is needed? First we have to determine if the stock unit is working properly and is putting out the amps listed in the specs for the unit. Our Buicks came with CS144 alternators that are capable of delivering 120 amps if needed. The factory matched this alternator quite well for our cars and it can handle the load in most instances. Factory specs are around 75- 80 amps @ idle (750-850rpms) during normal operating temps and 120amps at 1800rpm (55-60mph) cruise. This can vary a little so don’t go out and buy a new one because you can only get 74 amps at idle out of yours.
This is a good place to state that amperage specs for alternators are normally shown versus alternator shaft rpm. Given that the alternator pulley is smaller than the crank pulley, the alternator is turning faster than the engine rpm. This ratio may vary from car to car, but, the normal ratio range is usually between 2.5 and 3.0 to 1. On our cars, it is close to 2.8-1 when one takes the time to measure the diameters of both. This means an idle speed of 800 rpm will produce an alternator shaft speed of ~2240 rpm. Keep this in mind when looked at the graphs provided by the alternator manufacturer and don't be misled by claims otherwise.
In order to determine if you need a higher output alternator than currently installed, you have to measure, or calculate, the normal current draw of the various components powered by the alternator.
In order to measure the current draw, you will need an ammeter. The most common, and affordable, will normally be of the clamp on variety that you clamp over a battery cable. There is an inline version that goes between the battery cable and the battery, but, one must be careful that the unit employed is rated to carry the anticipated load as many of the affordable units will not handle a high load. They can be helpful in determining a battery drain, but, always check the specs to be sure they are meant for this. Like all things, quality may vary from unit to unit and brand to brand....some can be very expensive.
Okay, install the ammeter and start the engine.
At idle with nothing turned, you will see, on the average, an amperage draw of ~30 amps when the stock fan and fuel pump (Walbro 340) are running (ignition 5 amps, fuel pump 7 amps, and fan 17 amps on high) Note that various items are rated as being X amps at Y voltage. Power (watts) is = to VxA. Therefore, if a fan is rated at 20 amps at 13 volts, then the amperage will drop as the supply voltage increases. So the same fan at 14 volts will pull 18.5 amps (13 x 20 divided by 14)
Now as you flip on each accessory this number will climb. Headlights on low beams will add ~7 amps. Add high beams and another ~9 amps will be required.
Brake, tail, parking, marker, and instrument lights may add another ~15 amps.
Turn the wipers on high and add ~7 more amps.
Put the AC on max high and another ~18 amps is required.
Crank up the factory stereo system and add another ~5 amps.
That totals ~91 amps and some of the above are at voltages between 12-13 ratings so a good alternator with proper output will see a bit less than this. Using an ammeter is the best way to get a precise number but the amperage may also be estimated by using guidelines available online such as this one from Delco.
http://www.acdelcotechconnect.com/html/tas_alt_charging_typload.jsp
There are also a couple of other easy ways to determine if a new, or better, alternator would be a benefit beyond the measurement with an ammeter, or the use of a chart.
The quick and dirty way is the oldest way. On a car that has been running awhile, turn the headlights on "Bright" and rev the engine slightly. If the lights get obviously brighter, then the alternator is not getting enough voltage to them at idle. Remember that a bad/weak battery will take a lot more amperage than a good one that holds its charge properly. Also, bad cables, connections, etc. all affect the voltage delivered. Check the voltage at the back of the alternator by putting a probe on the battery terminal on the back, and the other to the alternator case insuring that you are making good contact. Then take a a voltage reading across the battery posts and compare to see if they are very close to being the same number. The engine should be idling, of course. If they are not very close, then it is time to find out why. Sometimes we have a significant voltage drop and it is not the alternator that is at fault.
The second way is an extension of the above. After driving the car a bit to recharge the battery loss caused by starting, and, to allow the alternator voltage output to stabilize with the temperature, connect the voltmeter to the alternator and note the voltage. Then turn on the headlights to bright and see what the output voltage does. If it stays close to 13.5, or above, then the alternator is handling the load okay. If it drops into the 12's, then something needs to be addressed. The same comments regarding battery condition, connections, etc. addressed above apply here as well. High resistance connections cause excessive power to be used.
Now, let's look at the graph provided by Delco for its CS series alternators. As mentioned before, this graph depicts amperage vs alternator shaft rpm and not engine rpm. For our cars, multiply the engine rpm by 2.8 and use that number on the X-Axis of the below graph.
These curves show amperes vs shaft rpm at 14.0 volts. Our stock alternators are of the CS120 amp variety.
Other items, such as power windows, power seats, etc. are not really material as they are easily powered momentarily, in the case of excess draw, by the battery.
Looking at the above graph and taking the the 2.8-1 multiplication factor into account, we can see that a properly working 120 amp alternator should be putting out approximately 75 amps as stated prior. This means that the battery will supply additional required amperage as it was designed to do if the requirement is above 75 amps at idle. Remember that the 91 amps calculated above would be in a case where we are stopped at idle with the headlights on bright, foot on the brakes, and the wipers running on high with the factory radio cranked up. Look at the graph and notice how fast the amperage increases as one increases the rpm driving away. By the time the engine reaches 1000 rpm we have made up the deficit.
If you are in the habit of standing still under the above conditions for extended periods of time, then you are a candidate for a higher output alternator as the factory unit is going to be working hard to replace the power pulled from the battery when you do drive off. If you are more typical and the above scenario only occurs at stop signs during rainstorms, then not enough time elapses to cause a potential problem.
Now, let's consider upgrades to the car beyond the stock equipment and the Walbro 340 included above. You may add dual fans. My Flexlite dual fans actually draw less than the stock fans, but, the popular Spal dual 11" high performance fans draw 25 amps running at 13 volts. Therefore they add approximately 8 amps to the numbers used above. This is not a major addition.
Now, if you are running a serious aftermarket sound system with large amplifiers, then you may well have charging issues at idle if you are parked under the conditions listed above. Remember that most system specs are rated near maximum volume and you may not normally operate in this range. Still, it is wise to do some research and determine exactly what is suggested for similar set ups and plan accordingly.
If you are running an original alternator, or one with a lot of years and/or miles on it, it is probably not meeting the original specs and if you are running a lot of additional equipment, it would probably be a good idea to look at an unit with more potential output when a replacement is made.
If one determines a need for a new unit, there is a factory replacement available that is ideal for most "improved" cars at a very reasonable price from most reputable suppliers.
In 1993, GM made some revisions to the CS144 series and added a new model rated at 140 amps. See the above graph. Note that this unit is capable of approximately 105 amps at 800 engine rpm (~2240 alternator shaft rpm) which some 30 amps more at idle than our original units.
This unit was installed on the following cars:
1996-94 Buick Roadmaster 5.7L, 1996-93 Cadillac Fleetwood Brougham (RWD) 5.7L, 1996-93 Chevrolet Caprice 4.3L& 5.7L, 1996-95 Chevrolet Impala 5.7L, 1995-92 Chevrolet Lumina APV Van 3.8L, 1995-92 Oldsmobile Silhouette 3.8L, and the 1995-92 Pontiac Trans Sport 3.8L
This alternator was also installed on police cars, and fleet vehicles such as taxis where normal operation may include lots of non-moving use with heavy radio traffic.
It is listed as a Lester no. 8112 which may be helpful at wrecking yards, or such places.
This model is a standard stock item at major parts stores like Autozone and Napa. It is also available new online from most reputable online dealers. Prices generally range from $130-$170 although I have seen it for less. It pays to shop around. Check our vendors to see if they have one on the shelf.
This unit is a direct bolt on when used on our cars. It will work with our wiring assuming that it is in good shape when simply being used as a stock replacement. However, if one has a strong sound system, or some other add-ons that pull a lot more current, then one should definitely upgrade the wire from the alternator to the battery with a larger gauge (something in the 2-6 gauge range). John Spina of Caspers Electronics offers such for our cars.
Given that the price of a brand new unit is not much more than a factory 120 amp replacement, I believe this is the logical choice when replacing a bad factory unit. My first preference would be to check with Pick-a-Part and see if I could get one for $50, or less as most should have several years of life left and parts are readily available for a rebuild if, and when, it needs it. Alternators are easy to rebuild. That should be kept in mind.
Make sure you understand that a 300 amp magic alternator does not put out any more amps than a factory 120 amp unit unless the car has something installed that demands the power. Alternator rating is meaningless unless there is something installed that will actually pull the current. On a normal car with normal equipment, the only thing that a mega-huge alternator does is create a large draw on your bank account.
Now let's finish with the exotic ultra high amp alternator systems. We all know they are commonly used on ambulances, emergency vehicles and 2000 watt ricer stereo systems.
As mentioned above, you need to figure how much amps you need. You just can’t see a 240 amp alternator in a magazine or webpage and say, “That’s what I need”. First, chart out all of your equipment. Then add this to the basic given load mentioned earlier. If your using 80-120 amps now you will only have 120 amps left on a 240amp alternator. Most idle specs I see are between 110-160 amps. Higher would be the more expensive commercial units. Many all out stereo systems can draw 100-200 amps alone. See a problem here? You need to hit the car stereo WebPages and ask the gurus what system you will need to operate effectively. I see many of those guys with two and three alternators and two batteries. One of the reasons is high amps create high heat. That will destroy a high amp alternator with a small case. Emergency vehicles use special CS 144 units with external extreme duty dual rectifiers to handle the load. This is on 250amp or higher alternators. There is a reason for this. It is to combat heat which is the primary killer of alternators and the rectifier is usually the first thing to go according to the experts. If you have a serious system in your car, then consult the stereo gurus and pick one that will live in your application.
There is one other application that should be mentioned and that is the small, light weight alternators that are often seen on race cars. These generally have a very limited guarantee and limited life. The small cases cannot handle the heat and they die quickly in day to day use. Buyer beware!
In closing, do some research before you waste hard earned money on parts that are not needed, and provide no benefit. If your research proves it’s required, spec out all the sources and compare outputs at idle and cruising. Demand a output statement/chart/graph at idle and cruise from the vendor. All real output charts are based on alternator shaft speed, not engine rpm’s. Our Buicks came with stock pulleys having a ~2.80 ratio. So divide that into the alternator shaft speed to get your idle and cruise speeds. Our cars have an average idle at 800rpms and cruise around 1800 rpm engine speed so use the chart/graphs to get your actual output with your personal idle/ cruise specs.
Do a search online and see what kind of reputation the vendor has. There are plenty of vendors so pick one that has been in business for awhile and who does not have a number of complaints about his service and quality. There is no need to risk a lot of money with fly-by-nights working out of their garage. Buy from one that has the ability to back up his product.
I gathered all this info from hands on testing, service manuals, tech webpages and 40 years in the automotive field. Some output amounts can vary by condition of your battery, alternator, wiring and electrical parts. So don’t take my numbers as an absolute fact as they may vary from car to car. Generally, if you use an ammeter and actually measure the current draw, you may well find that I am conservative in my numbers and your total draw may be less than what I used in some cases.