How Many Amps Should The Alternator Put Out?

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 An alternator is designed to generate the electricity that charges the car battery and runs the rest of the car’s electrical components.

The rate at which the electric current flows is known as amperage and is measured in an SI unit known as ampere (short form; amps). 

So, how many amps should the alternator put out? The rating assigned to the alternator by its manufacturer is the expected amps the alternator should put out and nothing less. The amps the alternator puts out on the type of vehicle in question. 

Back in the days, that is, as far back as the ’80s, most of the cars had alternators with amperage within the range of 50 amps, however, modern cars such as cop vehicles and ambulances have a higher electrical current requirement and therefore make use of alternators with a higher amp.

The cop car for example makes use of alternators with amperage in the range of 120 amps while ambulances or fire trucks have much higher ones.

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How to determine how many amps your alternator put out?

to determine how many amps of electricity your alternator puts out, the first step would be to learn about the mathematical relationship that exists between the rate of electric flow (amperage) and the alternator voltage, that is, how volts are related to amps.

The alternator voltage has to do with the capacity of the electric current generated by the alternator, while the amperage implies its current’s power.

When an alternator is unable to produce enough voltage, the car battery will become discharged, leading to damage to other key electrical components of the car.

Similarly, when there is an inadequate amount of amperage, all the electrical components are deprived of electric current and get burned out.

Alternators are of different sizes and designs and are placed differently in the car. The size of the alternator as well as how it is placed in the car are some of the factors which can help you determine the number of amps such alternator is putting out.

There are alternators whose design is such that the voltage regulator is not situated inside the alternator but rather mounted on its exterior, this variation in design can explain the variation in the amperage of alternators.

In terms of size, large or heavy-duty vehicles have a higher electrical demand than regular size cars, therefore, they are fitted with high output alternators to meet up their electricity demand.

For such vehicles, the high output alternators usually make use of about 200 to 350 amps while regular size cars with average electric current requirements make use of about 100 amps.

the car’s electrical accessories have fuse ratings. Such fuse rating also gives some idea regarding the amperage the alternator operates on.

The fuse box or its cover usually has a diagram that indicates the assigned amperage by the manufacturer. These figures can be added for all the electrical accessories to come up with the total amperage the alternator operates on.

Alternatively, the amperage can be calculated using a mathematical formula that shows the relationship between volts and amps.

The general formula is given as “amperage = watts/volts”. The alternator’s watt can be found written on it by the manufacturer, if absent, it can be found written in the guide book for the alternator.

All car owners need to learn how to test their car alternators for power, that is, both for the alternator voltage as well as the amperage or risk getting stranded in the middle of nowhere as a result of alternator problems.

Without a functional alternator, the car battery won’t be charged nor will any other electrical component of the car function properly.

To test for the alternator’s power, you will two basic devices: the voltmeter and the amperage meter or ammeter.

Alternatively, rather than two devices, you can make use of a multimeter, a device capable of measuring both the alternator’s voltage as well as the amperage.

Always note that a low alternator amperage or voltage as the case may be is not compulsorily a problem of the alternator.

This could be a problem with the battery and not the alternator. Always check the car battery first to rule out its role in the low amperage or voltage.

To do this, the car engine is turned off while the positive and negative leads of the multimeter are connected to the respective battery terminals. The ideal battery voltage should be within the range of 12.7 volts. A reading lower than 12.4v may necessitate a battery replacement.

Here is a step-by-step guide on how to check how many amps the alternator is putting out

Step 1

To check for how many amps the alternator puts put, the positive and negative terminals of the amperage meter or those of the multimeter are commented across the positive and negative battery terminals.

Every electricity consumer in the car is then turned on including the headlights, the radio, Wipers, and all other electronic accessories.

Step 2

The car engine is then allowed to stay idle at an rmp of 12000. 

Step 3

Read the amperage meter or multimeter and compare the reading with the maximum alternator amperage given the alternator manual.

Ideally, this reading is expected to be close to the stated value on the alternator manual. For example, a 120 amp rated alternator should produce at least 188 amp. Alternator reading greatly below what is stated by the manufacturers is indicative of a faulty alternator requiring to be replaced.

Aside from the alternator amperage, the voltage can also be measured as well but using a voltmeter. The procedure is similar to that of the amperage measurement. 

After connecting the leads of the voltmeter across the battery terminals, the car engine is turned on without applying pressure in the gas, a working alternator will give a reading of 13.8v to about 14.2 volts. Values less than these may imply trouble somewhere.

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How many amps should a car alternator put out at idle?

The amount of current the alternator puts out at idle is quite different from when operating at full cruising speed. Do not take the low current output of the alternator as idle as a problem as it does not put out much when the car engine is idle.

This is part of the reason for the two 140-amp alternators found in the ambulance. This is so that the electric medical equipment in the ambulance can get the needed electric current even while the ambulance sits idle.

Given that the electric output of the alternator is usually low when the car engine is idling, measurements taken at this time may be mistaken by car owners who do not understand this fact to be abnormally low.

Other occasions when this may be the case is when an electric current is measured by someone who does not understand how to use the ammeter properly.

The amount of current an alternator puts out depends on the amount of electric current that is placed on it. Therefore, when the electric current is measured when not much load is placed on the alternator, the electric current will expectedly come out low and could be mistaken by some to be an abnormality with the alternator.

The alternator amperage is not constant but rather changes with the amount of load placed on it amongst other things.

The lighter the load the lower the amperage and vice versa. Therefore, the reading of the amperage meter can only get close to the maximum amperage rating of the alternator as stated by its manufacturer when all the potential electric load is turned on. This is slightly difficult to achieve.

What does the output rating of the alternator mean? 

All alternators are rated in terms of amperage and voltage. The ratings differ depending on the manufacturer. However, you should not expect these values when you measure the alternator amperage with the amperage meter or multimeter.

Their readings should show a value close to what the alternator is rated for when all the electrical accessories in the car are turned on.

Irrespective of the reading you got from the amperage meter, so long as the alternator is still able to bring up the charge battery light on the dashboard each time the car keys are turned on, and the car engine can be turned on once without hassle, then the alternator is functional.

Are our high output alternators necessary?

The alternator is a necessary component of all vehicles as it produces an alternating current which the diode rectifiers convert the direct current that runs the battery and every other electrical accessory in the car.

The amperage rating of alternators has changed over the years from 80 amps back in the days to about 120 to 150 amps in modern-day vehicles as the electricity demand increases.

Whenever there is a need to get the car alternator replaced such as when the previous alternator fails, the replacement should ideally have the same rating as the original alternator that came with the car.

However, for a vehicle with multiple histories of alternator failure, it is best to go for a high output alternator. They become necessary in this case, especially when the car owner installs accessories with high current demand.

How Many Amps Should The Alternator Put Out – Conclusion

Lately, alternators used in modern cars are designed to produce about 120 amps to 155 amps of current. These values are however not constant and vary significantly with the amount of load placed on the system.

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