How are Alternators duty rated?

I have two alternators in my armory, a 110amp and a 300amp unit. I have just installed am ammeter to confirm the actual demand, since I am now running 2 x pumps and a high volume fan for the interchiller, and found the draw to be 72 amps. I have trawled the internet without certainty beyond the numerous speculated claims of what my aunty uses etc. I could jump straight to the 300amp unit, however keen to understand if there is a known demand rating for 14V alternators? My previous understanding was automotive alternators had a 5-10% duty rating at nameplate, by then the crank depletion of start up had been replenished, so the alternator demand dropped to the nominal headlight/heater/a/c/ etc load.
Specifically will my 110amp alternator withstand the demand of a 70amp draw over an extended period of time? Added to this worst case would be driving at night, country miles and perhaps low range into a camp at times, so this could jump to say 90 amps with my set-up. For clarity, its either the 110 or the 300 installed, not both. They are a cartridge change out, takes about an hour to do.

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Originally Posted by PeeBee

I have two alternators in my armory, a 110amp and a 300amp unit. I have just installed am ammeter to confirm the actual demand, since I am now running 2 x pumps and a high volume fan for the interchiller, and found the draw to be 72 amps. I have trawled the internet without certainty beyond the numerous speculated claims of what my aunty uses etc. I could jump straight to the 300amp unit, however keen to understand if there is a known demand rating for 14V alternators? My previous understanding was automotive alternators had a 5-10% duty rating at nameplate, by then the crank depletion of start up had been replenished, so the alternator demand dropped to the nominal headlight/heater/a/c/ etc load.
Specifically will my 110amp alternator withstand the demand of a 70amp draw over an extended period of time? Added to this worst case would be driving at night, country miles and perhaps low range into a camp at times, so this could jump to say 90 amps with my set-up. For clarity, its either the 110 or the 300 installed, not both. They are a cartridge change out, takes about an hour to do.

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I didn't read all of it, but this article cites an international standard, so I hope it will point you in the right direction, Phil.

https://www.lifewire.com/understandi...ratings-534785

Thanks Craig,

it fills in some pieces of the puzzle, but not the ratio I am seeking, however what i am asking may be irrelevant judging by the article. Essentially the alternator will put out what its called to put out based on the load, the question is more how long can it continue to run at close to its capacity or nameplate before failure - does running at 80% of nameplate reduce the life or increase the chance of thermal failure? I guess I am doing the right things by running the Quicktifier rectifier device which is rated at 400amps when the alternator is rated at 110amps as the temps will stay low in the rectifier and these are typical heat failure points. I will continue the search, there is surely to be a qualified answer out there.

You might be chasing a pot of gold at the end of a rainbow, Phil. There are some manufacturers that sometimes overstate the rating to sell more product.

I understand your frustration as it is the same with some generators which advertise their products rating, but really is Peak Power, which can only be supplied for a short time.

There are so many factors to consider, such as the size of the pulleys, engine revs, engine bay temps, air flow, cable sizes, and position of the alternator.

I've cooked an early model alternator in a '67 XR falcon, as I fitted a sound system and a sneaky vintage klaxon horn which originally ran on 6volts.

Have you considered running two alternators in parallel? Or is there no more space in the engine bay?

Interesting to read in the Standards documents, Ambient temps are set at 20-23 degrees C. So you will have to take that into account as we know that is a cool day here in OZ.

Interesting observation from yesterdays swap out of the 105amp alternator to the 300amp unit. I undrstand the principle of the altrnator supplying the necessary current and no need to over supply. I am now wondering if the alternator actually accurately does supply the demand. Both alternators have replacement new regulators as well.

When the 105amp unit was in play, running thru the VDO ammeter, I was showing a constant current flow of circa 80amps, initially 98 amps for the first 10mins or so, which I thought would be to replace the depleted charge from glowpugs and big starter current. I do have some big 12V fans in play and two intercooler pumps, so the magnitude was not unexpected, however with 80/105 ratio was concerned at the duty rating of the 105amp unit.

Anyway, what was interesting yesterday, was after swapping the 105 to the 300, with exactly the same electrical load, the ammeter read 115amp on start up then dropped to 110amps steady state. I am wondering if the smaller unit has a preset upper limit for constant demand? The point being I could be running for 10hrs straight whilst touring and find I am out of capacity when I arrive at camp when running the 105. Now I will say this is just a theory or musing, and on the failed Madigan Trip it did not eventuate, but the number seem to suggest it is possible I guess. The scenario I am talking about is that the actual amperage has jumped to above the capacity limit of the smaller unit, being 110amp on what would have been the 105 alternator.
Keen to hear what others thinking about this. I am now comfortable I have the duty rating sitting well down on the curve of the 300amp unit, and the 105amp unit is relegated to the spare parts shelf once again.

When I updated my alternator, I was given the advice to run a larger cable to the battery. I cheated a little, by just running another cable in parallel with an 80 amp weatherproof fuse. The only problem with that is the mechanic who fitted the new engine only connected one wire onto the alternator. Got cable ties now holding the cables together.

I don't know about any recent upgrades to the voltage regulators, but the current capacity of alternators was determined by the resistance of the internal wires, not the regulator. I don't think the regulator has a current limiting function - it regulates the output voltage only. The regulator can only supply the required output as long as its input voltage (alternator voltage - internal voltage drop) is high enough. Higher capacity required thicker wire and consequently lower resistance. It does not take much resistance to drop the available voltage when you are looking at currents around 100 Amps - just 0.01 Ohms per volt at 100 Amps). Any internal voltage drop reduces the voltage available to the regulator effectively limiting the current.

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Originally Posted by Brissieboy

I don't know about any recent upgrades to the voltage regulators, but the current capacity of alternators was determined by the resistance of the internal wires, not the regulator. I don't think the regulator has a current limiting function - it regulates the output voltage only. The regulator can only supply the required output as long as its input voltage (alternator voltage - internal voltage drop) is high enough. Higher capacity required thicker wire and consequently lower resistance. It does not take much resistance to drop the available voltage when you are looking at currents around 100 Amps - just 0.01 Ohms per volt at 100 Amps). Any internal voltage drop reduces the voltage available to the regulator effectively limiting the current.

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Thanks I am reading this but not really seeing how I can apply the info. The only thing that changed was the alternator, same wiring originally sized for the 300amp unit. Anyway, I am happy to just note it as an observation and leave it as it is. I will take a passive note of what is going on over time and see if this is the normal state of affairs or perhaps the crank battery was more depleted than I thought and it took a longer period to boost it?