what happens to the output of an automotive alternator as its temp increases?

 

Chief,
That's an interesting question, and I don't really have a good answer for you.

One of the reasons is that there are a huge number of different manufacturers and models of alternators.

Another is that the operating environment is quite dynamic; the output of the alternator more or less depends upon the load placed on it.

It would help a great deal to narrow such a broad question down, to just a specific make and model of alternator, and then test it under known load conditions at a constant rotational speed, varying only the temperature, in order to observe the results.

Your results will likely vary between different manufacturers and models.

But usually, a typical alternator will output around 13.8v to 14.4v when the engine is above idle speed.

 

To try to answer your question better....

The engineers who designed the voltage regulation system try to ensure that the voltage output will be consistent over a wide range of operating conditions, and be reliable, yet capable of being mass-produced at a low per-unit cost.

 

what happens to the output of an automotive alternator as its temp increases?

The alternator voltage regulator is designed to reduce its voltage output as its temperature increases and vice versa for temperature drop.

This is because, as a lead acid battery gets hotter, it requires a lower charge voltage, as compared to the required charge voltage at room temperature, to avoid overcharging it.

Because an automotive alternator is usually in close proximity to the battery (viz: under the hood) the regulator designer assumes that the alternator and battery will experience similar ambient temperature changes.

This can cause charging issues in a non-automotive environment (eg: a boat) where the alternator and batteries may experience widely differing ambient temperatures and thus the charging curve may be compromised. This can be overcome on a boat by utilising an alternator controller (external regulator) which senses inter alia battery temperature.