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What engine RPM = maximum alternator output? #5289338 12/09/19 11:41 AM
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Ed_Flecko Offline OP
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I like listening to the AGCO Auto weekly podcast (http://www.agcoauto.com/content/Radio).

I heard Louis Altazan saying that short trips can be really hard on car batteries because your alternator never runs at sufficient speed to maintain a full charge. Louis said that at low engine RPM combined with short trips, it's likely that most of the car's electrical needs are being supplied by your battery and not your alternator.

I have a 2019 Toyota Tacoma with the V6 motor and I was curious at what engine RPM my factory, 130 amp alternator achieves maximum output...so I e-mailed Toyota. They called me back and said the only data they had freely available said something like, "Initial output starting speed - RPM maximum 1,300."

If that means that my alternator produces 130 amps at 1,300 RPMs, that means that my truck is always producing maximum output if I'm driving anywhere (because my RPMs are always above 1,300) and the only time it isn't is when the motor is idling.

Does this sound correct or am I misinterpreting what Toyota told me?

Ed


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Re: What engine RPM = maximum alternator output? [Re: Ed_Flecko] #5289344 12/09/19 11:49 AM
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I think that's a bit off base. Once the engine is started and alternator is turning, the battery's job is over. Most modern alternators can achieve max output at idle and use the regulator to determine how much to actually produce to both recharge the battery from starting and keep the engine and everything powered. The alternator is usually turning 2.0-2.5x the engine rpm due to pulley ratios. It's true if you short trip it enough that the battery may not get fully charged back up from the power needed for starting the engine.

In the old days, you had to get the engine up to 2000+ rpm to spin the alternator enough to make full power. My old Bronco was this way. You could see the headlights brighten a little when revving from idle to 2000 rpm and dim a little when going back to idle. A larger crank pulley solved that problem.


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Re: What engine RPM = maximum alternator output? [Re: Ed_Flecko] #5289346 12/09/19 11:52 AM
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Ed_Flecko Offline OP
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"I think that's a bit off base. Once the engine is started and alternator is turning, the battery's job is over".

So you think that Louis is wrong when he says that if you're idling, the battery is likely supplying the car's electrical needs?

Ed


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Re: What engine RPM = maximum alternator output? [Re: Ed_Flecko] #5289349 12/09/19 11:57 AM
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RDY4WAR Offline
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Originally Posted by Ed_Flecko
"I think that's a bit off base. Once the engine is started and alternator is turning, the battery's job is over".

So you think that Louis is wrong when he says that if you're idling, the battery is likely supplying the car's electrical needs?

Ed


Yes. If you have a voltmeter / gauge in your instrument panel, you'll see voltage jump up to 13.5-14.5 volts as soon as the engine is started. The battery isn't suddenly jumping up 2+ volts. That's alternator.


"He who is without oil, shall throw the first rod." - Compressions 9:1
Re: What engine RPM = maximum alternator output? [Re: Ed_Flecko] #5289351 12/09/19 11:59 AM
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Dave9 Offline
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While vehicles are now using more power than ever due to more electronics inside (except possibly those with LED lights), a proper vehicle design has the alternator providing enough power at idle to power "practically" everything, meaning headlights on, factory stereo on, A/C blower, etc. I don't know about defrost grids but normally they are only on a few minutes.

This leaves regenerating the amount of current needed to start the vehicle. Alternator output depends on the pulley diameter but you don't want too small a pulley for more RPM or you'll wear out the alternator faster. "Usually" alternator output has a sharp increase when IT'S RPM goes through the idle to about 2000 RPM range. You can measure your alternator pulley to determine its RPM ratio to that of the engine.

After about 2K RPM, it continues to rise in output more slowly, all the way up to the max, though a recent alternator output report I received, didn't plot that past 5000 RPM, probably because it is just guaranteed to output what is specified and it did before that point. That alternator is also 130A and at 1300RPM it produces about 40A. Hmm, I can't find that slip, but attached is another very similar.

However, some vehicles today, possibly your Tacoma too, have more advanced charging algorithms means to eek out a slight fuel economy improvement by reducing drag, that aren't just relying on a steady alternator controlled, max charge possible per RPM. You could get one of those lighter outlet LED voltage displays to try to chart what your particular vehicle does in various driving scenarios.

Yes if you are driving with RPMs above 1300, it is likely your battery is fully recharging if your trip is more than some single-digit # of minutes, unless it is very cold outside and this greatly increasing the amount of cranking time to get it started, as well as slows down the charge rate at any particular alternator output, but if you aren't driving fast, engine heat should warm the battery a bit if driving long enough for that to happen.

As you can see there are several variables so it is best to just compare your peak, engine off battery voltage after a longer trip, say at least 15 minutes, to that after a short trip to see if the battery is getting topped off.

The battery does continue to provide an important function once the engine is running which is buffer and pull down the voltage a bit so the electrical system doesn't see larger spikes.

[Linked Image from i.imgur.com]

Last edited by Dave9; 12/09/19 12:04 PM.
Re: What engine RPM = maximum alternator output? [Re: RDY4WAR] #5289359 12/09/19 12:15 PM
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Originally Posted by RDY4WAR
I think that's a bit off base. Once the engine is started and alternator is turning, the battery's job is over.


Ed you are miss quoting him read it again.


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Re: What engine RPM = maximum alternator output? [Re: RDY4WAR] #5289367 12/09/19 12:32 PM
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Dave9 Offline
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Originally Posted by RDY4WAR
Most modern alternators can achieve max output at idle and use the regulator to determine how much to actually produce to both recharge the battery from starting and keep the engine and everything powered.


That (max output at idle) is unlikely, since alternators produce more current the faster they spin. It would be impractical (wasteful of cost, space, and weight) to put in an alternator so over-spec'd that it could produce full rated (~100A or whatever it may be) at idle then be capable of multiple times more than that at higher RPM, and then much above idle RPM it would wear out faster due to spinning at a higher RPM than needed, and be producing more drag due to higher RPM and more mass, which is the opposite of the trend with modern charging systems.

Last edited by Dave9; 12/09/19 12:36 PM.
Re: What engine RPM = maximum alternator output? [Re: Ed_Flecko] #5289399 12/09/19 01:32 PM
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On one of our vehicles, it shows the output at certain RPMs on a sticker on the alternator.

Haven't checked the other vehicle.


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Re: What engine RPM = maximum alternator output? [Re: Ed_Flecko] #5289466 12/09/19 02:40 PM
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If your alternator is producing a voltage of at least 13.8 Volts at idle, with modern alternators producing approx. 14.5 volts at idle, then your battery is being charged and all automotive electrical systems are receiving current from the alternator only.

Anytime the alternator voltage is greater than the resting battery voltage then the battery is being charged.

Quote
I heard Louis Altazan saying that short trips can be really hard on car batteries because your alternator never runs at sufficient speed to maintain a full charge. Louis said that at low engine RPM combined with short trips, it's likely that most of the car's electrical needs are being supplied by your battery and not your alternator.


If that were the case, then your alternator or overall charging system has a problem.


Last edited by MolaKule; 12/09/19 02:44 PM.

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Re: What engine RPM = maximum alternator output? [Re: Ed_Flecko] #5289477 12/09/19 02:50 PM
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Alternators do have an inherent current limit which you see as the flatting of the curve

This occurs because the current flowing in the stator wires adds magnetism along with the magnetism from the rotor, and the resistance from the impedance increases with the frequency. The core saturates and the current begins to level out. Hot the output decreases about another 10-20% depending on the air temps starting and ending. You will not see that on an alternator tester unless you go to endurance mode if it has one.
30 years ago many alternators were too small, this is no longer the case.

Rod


Last edited by ragtoplvr; 12/09/19 02:51 PM.
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