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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?

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    A couple of potential misconceptions... (1) The system draws what current it will draw. As the limits of the supply (alternator) are reached, voltage will drop. As long as system voltage meets or exceeds the battery's open-circuit voltage (something like 13.2V if memory serves), the alternator is meeting load requirement, although there may be nothing left to charge the battery. If voltage drops below that point, current is flowing out of the battery, which in combination with alternator output, meets the load. [continued]
    – Anthony X
    Commented Jun 28, 2016 at 23:28
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    (2) Alternators are paired with voltage regulators to try to normalize output over a wide speed range. Alternators have both a stator and rotor winding. One winding (the excitor) is fed current to form a magnetic field; that current is controlled by the regulator. The other winding delivers power to the system. If the regulator senses that the output voltage is too low, it increases current to the excitor so that, for the same speed, the alternator can deliver more power (which it does at the expense of more input torque requirement), up to some design limit.
    – Anthony X
    Commented Jun 28, 2016 at 23:35
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    I've heard that for cars that have a high accessory load (for example, police cars that often need to operate lights, radios, computers, etc for long periods at idle), they'll use a smaller alternator pulley to let the alternator run at a higher rpm even at low engine RPM's.
    – Johnny
    Commented Jun 28, 2016 at 23:52
  • Another factor is that the battery recharges within a minute or two from the drain of starting the car. The alternator doesn't need to provide a lot of current at idle, but as long as not every current-drawing load possible is connected, the battery will be back to full very quickly. Why lug around an over-sized alternator that is only needed very occasionally?
    – user15009
    Commented Jun 29, 2016 at 2:08
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    Why do you equate "sufficient current to power all accessories plus keep the battery charged" and "maximum output"?
    – Criticizing Israel not allowed
    Commented Jun 29, 2016 at 6:42

7 Answers 7

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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.

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  • Would cooling be an additional factor? An alternator putting out a certain amount of current at idle would by my understanding generate as much electrical resistive heat as one putting out that same current while rotating faster, but would have less air flowing over it to cool it. Is my understanding correct?
    – supercat
    Commented Jun 28, 2016 at 16:26
  • @supercat It's hard to say. Because of the alternators larger size to accommodate full current the heat would be spread over a larger surface area. Also a larger alternator would allow for a larger internal fan that could move more air at lower speeds. It could be a problem or it could be a wash.
    – vini_i
    Commented Jun 28, 2016 at 21:16
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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.

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  • The question isn't about outputting at the alternators full capacity at idle, it's about why alternator's aren't designed to supply the system's full amperage needs at idle. Not the same thing.
    – Robert S. Barnes
    Commented Jun 28, 2016 at 16:14
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    "aren't designed to supply the system's full amperage needs at idle" what car is this? I think the entire question is fundamentally invalid.
    – justinm410
    Commented Jun 28, 2016 at 17:13
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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.

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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".

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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

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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.

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    The regulator adjusts voltage, not current. The amperage generated will be dependent on load.
    – SteveRacer
    Commented Jan 4, 2019 at 3:36
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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😀.

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    An alternator uses the stator coils to provide the output, the rotor coil controls the field to control that output and the regulator controls the current through the rotor coil...
    – Solar Mike
    Commented Jan 28, 2019 at 22:12

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