Do All Alternators Operate Similar ?

My car alternator charges more at faster speeds and higher rpm. Are all alternators similar in this regard or are there alternators available that operate
just as well at low speed and low rpm.
You can run your battery flat if you are stuck in a three or four hour traffic jam at night with your lights on and barely moving forward. This gets worse in cold weather.
I was just wondering if there were alternators available that charge well at low rpm.
Thanks in advance Denny B
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Denny B wrote:

I had that problem with a Chrysler. You can put a different size pulley but then you run the risk of spinning too fast at high rpm's. There are also high output alternators but I was told those sometimes can not supply any more current at low speeds. The cheapest and easiest solution was to turn off electrical loads such as the heated mirrors and such. Some have also said that a battery getting old can start to cause more of a load on an alternator as well as an alternator getting old.
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The voltage output of most alternators is well above the battery 12 volts, typically between 13.5 to 14.6 volts depending on the brand. BUT it isnt the volts that count its the amps which is a measure of volume of electrical flow, 2 different alternators both rated at say 78 amps mgith not put out 78 amps at the same speed. A decent alternator will max to the 78 amps at low rpm and a less quality unit won't put out the 78 amps until spinning at a pretty good rpm. OEM units that came with the car are usually well mated to the job they have to do. Discount parts store rebuilts which are sold at a price often may have the same amp rating but actually put out less at lower rpm. You get what you pay for. To know exactly what the alternator should be putting out you'd need the spec sheet from the manufacturer or rebuilder.
For the average back yard do it yourselfer all that can be checked is voltage output with a volt meter. Someone else pointed out that having a large load while the engine is idling in traffic can tax the alternator. A higher amp output unit may help even if the low rpm amps aren't at full capacity because it is still more than what the old unit gave. Again to know exactly you'd need specs at various rpm for both units. Knowing the amp load also helps.
-- Mike.................................................... "Opportunities are spawned from crisis"
Denny B wrote:

I had that problem with a Chrysler. You can put a different size pulley but then you run the risk of spinning too fast at high rpm's. There are also high output alternators but I was told those sometimes can not supply any more current at low speeds. The cheapest and easiest solution was to turn off electrical loads such as the heated mirrors and such. Some have also said that a battery getting old can start to cause more of a load on an alternator as well as an alternator getting old.
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Denny wrote:

Denny, you did not identify your vehicle, so it is difficult to respond in detail. As a general rule, modern alternators are closely tied to a voltage regulator function, and it is possible for a voltage regulator to be malfunctioning. A normal alternator and battery will not perform as you suggest.
However, there is another possibility to consider. Inside the alternator there are some diodes (rectifiers), and these rectify the generated AC into DC. Normally, you have three sets of them all working normally, and as a result, you get the normal amount of smooth charging current as output to the vehicle's electrical system including the battery. However, if one set fails (normally they fail open), then you will still have two sets, so they do a so-so job of generating current as output. If you put a 'scope on it, you will see increased AC ripple on the DC. This condition is easily overlooked by anybody without a good electrical system tester. If two sets open up, then you will still get some charge current output (not much), but the ripple is huge. At this point, typically you can't use headlights and run the engine at the same time. Without headlights or any other electrical load, the engine can run successfully, especially if you keep the RPM high, but you are on shaky ground. (I had to drive a car this way for over 100 miles to get it home one time.)
So, I suggest that you get your alternator checked out by somebody with a good tester.
---Bob Gross---
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All alternators will be capable of producing more (substantially more, in some cases) at higher speeds than at lower/idle speeds. From a GM service bulletin:
"Depending on the vehicle application, generator current (amperage) output at engine idle speeds of 600-700 RPM can be as low as 35 percent of the full rated output. With enough electrical loads "ON", it is easy to exceed the generator current (amperage) output when the engine is at an idle of 600-700 RPM. This is a normal condition. The battery supplements for short periods of time. Items that affect the vehicle's electrical system current and voltage at idle are the number of electrical loads being used, including add-on accessories, and extended idle times. When the vehicle speed is above approximately 24 km/h (15 mph), the engine/generator RPM is high enough and the generator current (amperage ) output is sufficient to supply the current (amperage) requirements of the vehicle as originally equipped and recharge the battery."
"As the engine/generator speed changes, so will the current (amperage) output of the generator. As a vehicle slows, engine/generator RPM slows, and the current (amperage) output of the generator may not be sufficient to supply the loads, the vehicle system voltage will drop and the lights will dim. Dimming of the lights is an indication that current is being pulled from the battery. If the battery is in a low state-of-charge (discharged condition), the driver will notice a more pronounced dimming than a vehicle with a fully charged battery. "
--
Robert Hancock Saskatoon, SK, Canada
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Denny B wrote in alt.autos.gm

It is a fact of the way electricity works, and alternators and generators are built. The faster they turn the more voltage they put out. The maximum voltage that goes to your battery will be limited by the voltage regulator, and the design of the alternator.

3 or 4 hour traffic jam? Hope you don't run into that too often. However, my advice is that when you are forced to spend that much time in traffic, when you are stopped, put the car in neutral and give it a little gas to increase the engine speed, and keep the charge rate up. It will also help to keep unneeded accesories off. No high output stereo, rear window defroster off unless actually needed, and heater fan off, or on low. Seat heaters off. Once the car is warmed up, the rear window will usually stay clear, the heater will usually not need to be going full blast, and the seats will be warm.

Probably not for your car, but if this is a common occurance you could go to a smaller pulley on the alternator. Of course, you will also have to match the belt size, along with the shaft size, but that information should be available in an engineering hand book.
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Denny,
Your vehicle was equipped with an alternator that will run all of your accessories, and provide a charge to your battery, when you are in drive and have your foot on the brake (2hrs, 3hrs in traffic. I doesn't make a difference). The alternator should supply a sufficient charge.
If your electrical stuff dims or slows-down a bit, when you are at a stop, it's normal. There can be a problem if this stuff slows-down too much.
If you are in doubt, have your battery and charging system load tested.
An old battery, that is in a constant state of discharge, can take-out your alternator. The alternator will work harder to replace the amps in the battery. Your car's battery is only for starting the car. After your car is running, it's up to the alternator to maintain a good charge to the vehicle and battery.
And yes, your alternator is rpm dependant. It will put out more amps at a higher rpm. When your mechanic dose a load test, he dose it at 1500-2000 rpm.
The volt meter in dashboards today is a toy. It gauges very little. The cars of the 50's and 60's, had an ammeter. When the needle went into "-", you had a discharge problem. When the needle was in the "+", everything was good. It's not the volts that matter most of the time, it's the amps.
The only time that volts matter is, so that your engine's computer can continue to function. Most engine/powertrain computers, shut-down at about 10.5-10.9 volts. Your engine will stop. It's not a matter of amps at this point, because all of the amps have been spent. You won't even be able to get the starter to click.
I hope that some of this information helps you, GMdude GM & ASE certified master tech.
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