The voltmeter plugged into the charger port under the dash on our car always shows 14.1V once the car has been on for a few seconds. Except today when I was cruising along on the highway and happened to glance over and saw 13.6V. Strange. Then I remembered that I had turned the cabin fan off on the MFD climate control screen. Turned it back on, and instantly back to 14.1V. Messed around a little and found the following: In D the 12V reads 14.1 when the fan is on, no matter what fan speed, but 13.6 when the fan is off. (Regardless if car is stopped or in motion.) In P the 12V reads 14.1 no matter what the fan is doing, whether it is off, or for any fan speed. Why would the car drop the voltage to 13.6 when the cabin fan is off and the car is moving???
My gen 3 also makes adjustments like that. It will use a higher voltage, 14+, to charge the aux battery faster if it thinks the aux battery charge is low. It seems to now and then drop the charge voltage briefly so it can gauge the aux battery charge level. Once it thinks the aux battery is charged enough, it will use a lower voltage (mine drops to 13.5 or so) while moving, probably to improve the MPG by having less power diverted to battery charging. But when stopped or in P, when MPG is zero anyway, it charges the battery more because why not? And also it boosts the voltage if enough electrical accessories (like the cabin fan) are on. I made a road trip last summer where I added a little 12-volt computer case fan under the passenger seat headrest to give me a little more airflow. That fan was very sensitive to voltage, so I could hear it ramp up and down every time the car adjusted the converter output. If I had the A/C on and the compressor was cycling, my little fan would ramp up and ramp down every time the compressor started and stopped (respectively). Because when the compressor runs, the condenser fans run, and when they run, the converter output gets boosted, so my computer fan ramps up.
Does it have more voltages? This car only does 13.6 and 14.1. If it was a load issue one might think that it would vary over some range as the cabin fan speed increased, because the current draw is wildly different at the slowest speed (<1A I think, anyway, very low) and the fastest speed (close to 20A). Nope, 14.1 for any fan speed. I should see what it does when the heater is on or the A/C. The only time I ever see a different voltage is right at startup, when it starts at something like 14.5 and very rapidly (seconds) drops down to 14.1, where it stays. If the 12V is very discharged it may hang for a while at 14.3 or 14.2 instead of 14.1.
Mine has 14.7 and 13.5 or so in its repertoire. I agree with you in dismissing a simple loading effect. It looks very much like programmed behavior with distinct target voltage steps. It adjusts the target based on the aux battery state of charge, and based on whether the car is moving or stationary, and based on which electrical accessories it knows are turned on.
Almost like it is deciding "may need up to 20A, go to high voltage so that it's there instantly if it is needed". Problem with that is that if the 12V is already fully charged 14.1 may be too much voltage for it, 13.6 might be better. Perhaps I will put the current clamp on the cable from the battery and do this again. It could be that the amount of current being drawn in these states are not as simple as I am assuming.
I suspect that whoever did the firmware programming was thinking of that. I don't always assume that people were thinking of everything. But in this case the very evidence that this interesting voltage-selection business was programmed in at all seems to be evidence that some people were very much thinking about exactly these kinds of things.
