What Should Alternator Voltage Be At Idle?

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The car alternator is a key component of the car as it subserves the function of providing the electricity that not only charges the car battery but also runs other electrical accessories in the car.

There is however a normal range of voltage a good alternator is expected to put out at idle as well as an amount that is considered too low or too high.

So, what should alternator voltage be at idle? The amount of charge a car alternator puts out at idle depends on a factor.

There is however a stated normal range for every normal or functional alternator irrespective of these factors. The expected voltage output for every working car alternator should fall within the range of about 13.8 to 15 volts.

However, this value is dependent on how old the car is, the amount of load that is placed on the alternator, and the idle speed. 

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How to test for alternator voltage?

To test the alternator voltage, the alternator is saddled with lots of loads by putting on the car headlights, its air condition is set at the coolest temperature with fans rotating at the highest speed, the wipers are turned on amongst other electrical accessories.

With all this load placed on the car alternator, the voltage output will understandably become reduced, this should however not fall below the lower limit of the normal range, that is 13 volts.

As the electrical load saddled on the alternator is reduced, for example, by putting off the headlights or turning the car air condition off, there should be a visible increase in the voltage output of the alternator noticeable on the voltmeter connected across the battery terminal.

There is something you should know about the car alternator voltage output, the amount of voltage generated by the car alternator is depend on the engine idle speed.

The car engine generates mechanical energy which brings about the magnetic field the alternator converts into electricity.

Therefore, the amount of revolution is central to the alternator’s voltage output. This is usually set to about 500 to about 600 revolutions per except for modern computerized vehicles which are automated and complicated programming languages and algorithms.

Regardless of the conditions mentioned above, that is, however old the car is, or however great the amount of load placed on the alternator is, the ideal alternator must not have a voltage output less than about 12 volts, else, it won’t be able to charge the car battery adequately.

Therefore 12 volts is the minimum voltage output an alternator should have when there is no load on the car battery, that is when the car engine is running, but accessories such as the headlights and others are turned off.

An alternator in perfect shape is capable of putting out a voltage of about 14 volts or more. Whenever the engine revolution is increased, the alternator voltage output will also show a commensurate increment.

The amount of voltage the alternator puts out depends on the load placed on it. If the alternator sends out the same amount of voltage it does when dealing with a full load when a light load is placed on it, it could lead to catastrophic outcomes. Thankfully, this does not happen as its voltage output is well regulated by the voltage regulator.

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Causes of a reduced alternator voltage

 Whenever the voltmeter shows an alternator voltage that is below the lower limit of the normal range, this could bed as a result of alternator failure.

Aside from using the alternator with a weak car battery, there are other reasons for alternator failure. However, rather than ditching the alternator to find a new replacement, you should first make sure the alternator is the source of the low voltage as other factors could be at play.

When an alternator begins to fail, its voltage output falls short of the normal range causing the car engine to stall and the battery not to be adequately charged.

Other electrical accessories of the car are equally affected. The headlights are usually the first indicators of a failing alternator as they become dim.

Aside from a failing alternator, the alternator belt, as well as wiring issues, could also explain the low alternator voltage.

The normal alternator voltage output should fall within the range of 13.5 to 15 volts. Values below this range are considered low, especially if the voltage output is below about 12 volts, this will be grossly inadequate to charge a car battery to full capacity.

While different manufacturers have different specifications in terms of the normal alternator voltage output, 12 volts seam to be the minimum voltage output for any given alternator when the voltmeter is connected to the battery terminals while the car engine is running but the headlights are other electrical accessories that an electrical load on the car are all turned off.

  • The alternator drive belt could be responsible for the low voltage output noticed with the alternator. The alternator could be working fine, a problem with the alternator belt, however, which caused it to become stretched or slippery can lead to a low alternator voltage.

The alternator belt under frictional effects over time and may become worn out or cracked at some point. It should therefore be inspected regularly to make sure it remains in a good state, able to produce the needed tension.

  • Aside from alternator failure and belt problems, the wiring within the chagrining circuit could also be the primary reason for the low alternator voltage. The wire connections in the electrical system of the car should be checked to make sure there are no loose connections, disconnections, or any form of corrosion over the battery terminal.

Battery terminals can get easily corroded over time, in this state, the voltage sent out is usually decreased unless the terminals are repaired of this.

What alternator voltage is considered too low?

First, it is important to note what the manufacturer’s specification is for the alternator in question, that is, what is stated in the manual as the normal range for the alternator’s voltage output.

Aside from the general value of 13.8 to 14.2, there may be slight variations across different brands and models.

When the car engine is idling, the car battery can be hooked to the test lead of the voltmeter with one lead on the positive terminal and the other on the negative terminal. an alternator in good working condition should give a reading that is less than 0.2 volts and no more. 

If this is not the case, rule out other possible causes such as corroded terminals, disconnections, loose connections, and issues with the alternator drive belt.

If none of this explains the low alternator voltage output, then alternator failure may be the right diagnosis, necessitating a replacement.

A voltmeter reading greater than 0.2 volts may be indicative of a serious underlying problem and warrants proper investigation to find out what this is.

Ideally, the drop in the alternator voltage output should not be greater than 0. 2 volts, the ideal range of values should either be zero or short of 0.1 volts.

If the alternator is working efficiently, it should be able to keep the battery voltage within the range of about 13.9 volts to about 14.8 volts.

The optimum battery voltage is about 14.2. these values are indicative of the charging capacity of the car’s alternator. Irrespective of the amount of load saddled on the battery, even with electrical accessories such as the headlights, radio, fans, and air conditioners as well as wipers all turned on, the least battery voltage should not be short of 13 volts.

Always note that a low voltage output does not necessarily translate to an intrinsic problem with the alternator. It could be a result of other possible problems as mentioned earlier like the belt or wiring.

If the voltage reading of the alternator at idle falls outside the range of 13.8 and 14.5, the car should be taken to the mechanic or car electrician for a proper diagnosis of what the problem might be. This could be a result of alternator failure of due to other causes.

What alternator voltage is considered too high?

how about a charging voltage about the upper limit normal range, does this pose any challenge to the car battery or any of the components of the charging system? There is usually a catastrophic outcome associated with a greatly increased voltage output that overcharges the car battery.

In most of the cases involving a high voltage output that overcharges the car battery, that is voltages above 14.5, the acid content of the battery is transformed into vapor and boils out giving off hydrogen gas. The acid then corrodes whatever it touches in its immediate surrounding, creating a huge mess.

So, for a 12-volt battery, the recommended voltage the car alternator should be putting out is about 13 to 14 volts on average.

What Should Alternator Voltage Be At Idle – Conclusion

The article is a concise attempt in explaining what the ideal alternator voltage should be at idle, what alternator voltage is considered too low or too high, and the possible consequences of any of these scenarios. Always test the alternator voltage to make sure the device is in good working condition.

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