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I was reading about load testing and full fielding alternators in a few different books I've got, and one of the points they all made was that to test an alternator's maximum output (or close to it) the rpm's need to be raised to around 2000 to 2500 since alternators can't provide full current at idle speeds.

This seems a bit counter intuitive to me as I would think that by design an alternator logically needs to be able to provide sufficient current at idle to power all accessories plus keep the battery charged.

As a practical example, I took some measurements on my 99 Nissan Almera 1.6L (which I assume is in good working order.) I turned on the bright lights, A/C and radio.

At idle (about 850 rpm due to idle up) I measured a DC current of 59 amperes at the B+ alternator cable, and 11 amperes coming into the battery positive cable.

I then raised the rpm to 2500 and measured again, getting a DC current of 69.2 amperes at the alternator and a DC current of 14.5 amperes at the battery. According to what I've read, normally only about 5 amperes is needed to keep the battery charged, but I'd been running some loads without the engine on for a few minutes before I did these tests so the battery probably needed a bit more charging than usual.

So clearly even with the idle up the alternator is not capable of providing all the current the system actually needs, yet at the same time it is providing sufficient current to run the system without drawing from the battery.

So what are the underlying reasons that charging systems are set up this way?

Peter Mortensen
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Robert S. Barnes
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7 Answers7

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Cost is the main reason.

An alternator that can provide full charging current would be far larger requiring bigger rotor and stator windings. This would make it more expensive and heavier.

The manufacturers are using the fact that you only spend a small amount of time idling compared to driving to their advantage. Most cars are designed to cruise at around 2krpm. This is were you spend most of the time, so to make the alternator smaller and lighter it is designed for rated output at that rpm.

Finally that mythical alternator that can produce rated current at idle would make more than the rated current at 2krpm. If the car is designed to consume 100A with everything on then the 150A the alternator is capable of producing at 2krpm is wasted.

vini_i
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If an alternator could provide its full output capacity at idle, what would it output at red line? Output amperage is proportional to rotational speed of the alternator.

Via gearing, an alternator could be made to spin faster at idle and produce its full amperage. However, at high RPM it would then be exceeding the speed at which it's designed to efficiently operate.

Thus, alternators are sized appropriately for the estimated draw of the accessories. Not more, not less.

justinm410
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Your premise is backwards. Max output and sufficient output aren't the same thing. Your own numbers show that alternators provide sufficient output to keep the battery fully charged at idle. If they didn't, cars couldn't idle for very long as the battery would drain down. As indicated in the other answers to this question, the faster the alternator spins, the greater the output. Sufficient at idle, more than needed at higher revs.

tlhIngan
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As more electrical load is placed on the alternator (i.e. more loads such as lights and heaters are turned on) the alternator becomes harder to turn. As this happens, a modern ECU controlled car will increase the idle speed to overcome this and to avoid stalling the engine. Thus your original question "Why can't alternators provide full current at idle?" is false. It depends what you define as "idle".

TopCat
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Most modern vehicles control the alternators field coil and idle RPM to optimize the alternator's output.

Most economy vehicle ECU minimize fuel consumption and thus alternator output at idle providing just what's needed to keep the vehicle running while stopped with minimal charging if any. They also use very small <75A max alternators to reduce size, weight and cost.

Heavy duty police, contractor & RV grade vehicles, with very high 12V loads, take the opposite approach using oversized >150A alternators and ECU that increase engine idle speed up to a "high idle" to provide >70% of their rated output at idle when needed.

A good example of this was the British Ford Escort police cars that came with high output alternators and ECU controlled high idle capability to run their significantly higher 12V idle loads when "All Lit Up".

So it can be done with a custom ECU & Alternator! But it's not a trivial bolt on hacker upgrade!

PS Hacking/Jacking up the field coil at idle can destroy your cars ECU and electrical system!

Be Smart, Be safe

T-Rex
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Alternator can't provide more electrical current than it's intended to. There's a regulator on the back of the alternator and if the engine is spinning at much higher RPM it literally doesn't allow the excess current to flow into battery. Generally alternator should be capable of producing enough current to keep the car running without discharging the battery.

unknown
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So the regulator providing proper voltage for the load seems to imply the load should be able to pull its required current to operate efficiently. And why is it I keep hearing the output voltage is directly related to the alt. rpms, which it may, but little mention of exciting the field coils with a little more voltage, or less as required? Isn't this the function of the regulator and the reason permanent magnet alternators can't do this, or at least not quite as simply. Just a novice so be nice.

Howard
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