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Old 07-02-2022, 05:44 AM   #1
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Alternator Output Not Always What It Seems

In a recent thread about DC-to-DC chargers there was a discussion about alternator output. My existing one was 124 amps, which while not incredible was enough for the purpose at hand so I left it alone.

Then coincidentally it stopped alternating. I suspect it was due to a non-related repair being done incorrectly. So rather than replacing it with the same OEM alternator, I decided to bump up to the next available size for my engine, 140 amps. There were larger options, but this one required the least amount of other mods to make it work correctly from what I could see.

What surprised me was the data sheet which came with the new alternator. Listed as a 140-amp alternator, it actually has a max output of 165 amps @ 6,000 RPM. But, at normal highway speeds with my engine at 2,000 RPM or less the output is only rated at 100 amps.

To get the 140 amps listed on the box, I'd have to keep the thing spinning at 3,000 RPM, which is something I rarely see on my engine.

Seems like this is an important factor to keep in mind when sizing an alternator for a project like installing a larger DC-to-DC. That high capacity alternator in the plans might not be as capable as thought while cruising down the highway.

For now I'll stick with the 140-amp unit. The 124 seemed able to handle things so far, so hopefully the extra 16 amps will help provide headroom.
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Old 07-02-2022, 05:57 AM   #2
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The other issue is that I bet those output numbers are not for continuous output but rather for limited times. And...a set of discharged lithium batteries is going to be a continuous draw for a good while.
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Old 07-02-2022, 06:37 AM   #3
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Most vehicles don't put out their higher rated power levels at idle, no . In fact I don't think any do. Some are just higher or lower based on what the vehicle is used for. An alternator in a police car or ambulance will put out higher voltages at idle because of the commercial nature of their operation. I think this is probably more of a factor of how the voltage regulator is configured.

But generally, at 700 RPM its not doing a whole lot.
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Old 07-02-2022, 06:25 PM   #4
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Couple of thoughts.
0. The target output voltage may range from the mid 13's to the mid 14's, and not for reasons that are immediately obvious. That depends on the vehicle and the installation. It's also possible the target alternator output voltage will be pretty constant at around 13.7-ish volts. I've thought about this a couple times when people are talking about replacing their TV or trailer batteries with a different chemistry ... I haven't looked into possible side effects (and maybe there are none), but it's something I'd write my own FMEA for if I was thinking about doing it.


1. The pulley ratio comes into play. Quite often the alternator will be spinning 2 to 3.5x as fast as the engine.

2. Alternator power output varies not only with speed but with temperature. An alternator rated at 150A at 3000 (alternator) RPM, may meet that rating at an underhood temperature (technically alternator temperature) in the 20-40C range. At 50 or 60C, it may be 10-15% less, and it may be capable of producing around 170A at -30C. I'm attampting to past a link to a pretty crude chart of output vs temperature vs alternator RPM. If you pay enough money or are getting paid to write or use them, there are similar graphs with 3-5 temperatures and a similar number of speeds. I did a quick look on the web and this was reasonably close to what you'd get.
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Old 07-02-2022, 06:52 PM   #5
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Under a prolonged high output, I would also be aware of belt wear and pulley ... well, abuse is as good of a word as any ...
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Old 07-02-2022, 07:06 PM   #6
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Originally Posted by RedSHED View Post
Under a prolonged high output, I would also be aware of belt wear and pulley ... well, abuse is as good of a word as any ...
For good measure I replaced the idler pulley, belt tensioner, and belt when swapping the alternator. Seemed like a good idea to get the new alternator off with as good a start as possible, especially like you say it will need more ponies getting to it to push more electrons.
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