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.
I don't see how to reconcile the observations in this thread with those here: https://priuschat.com/threads/mpg-and-the-12-volt-battery.250285 where they report a wide range of battery voltages, presumably with the fan on most of the time. Maybe not? The 12V in our 2007 is now a bit over 4 years old. The voltmeter under the dash has been reading 14.1 V forever, with the exception of this fan business, or when the windows are moving. In the other thread dolj reports voltages like 12.8 and 13.2 for a good battery, and 14.4 for a weak one. Well, today I charged the 12V (with the positive leads detached from the battery - so the car completely out of the picture) and got it up to 12.85V and 100% SOC, measured after those wires were plugged back in, with a battery tester. (The HF charger was doing some really peculiar things, including reaching "fully charged" at 14.9V and sticking there. It was screwy on our Accord battery too a week ago. I think time for a new charger.) The car at start up is going to see a fully charged 12V battery. Yet, turned it on, and the usual behavior: starts at 14.3 and falls down to 14.1 in a couple of seconds and stays there. How could the car possibly determine that this is a "weak" battery in so little time when the battery starts out fully charged? The battery tester did show the capacity as 40AH rather than 45AH, but it took around 5 seconds to measure that, whereas the car goes to ready in around a second. I suppose I might be able to spot a voltage collapse at startup using an old analog (needle) voltmeter in that 1 second, before the inverter kicks in and hides the true battery voltage, and the car could in theory do so too. Anybody know how far the 12V dips during startup with a good battery? It's a common measurement on cars with starters, never heard of it being used on a Prius though.
I think there is a thread around here where somebody (it may have been RGeB) posted a graphed datalog of converter output voltage during a drive, and it showed regular, repeated, brief drops in voltage down to no higher than you'd expect from the battery itself. It looked very much like the car taking regular, brief samples of the unhidden, true battery voltage, to inform its future decisions.
This one? More on Hybrid 12V (auxiliary) battery charge control | PriusChat The first image shows in that vehicle that nothing much happens with voltages for the first 15 minutes and then that car (not a Prius) starts doing fancy things with the 12V battery. In that first 15 minutes it is either discharging or charging the battery - not sure what the sign means on the current. One of the traces is said to be the current at the negative battery terminal, something that couldn't be read on a 2nd generation Prius without attaching a clamp ammeter as there is no sensor there, just a ground cable to the body. Anyway, after the 15 minutes the car is bouncing the voltage up and down so that the net current into the battery is zero. Or thereabouts. In the second image it looks like the integral over the positive part of the current cycle is very close to that of the negative part (other than sign), even though the former is a longer interval. The voltmeter constantly plugged into our 2007 has never shown anything like the 1 minute long rectangular wave that was present after 15 minutes. In terms of reading the actual 12V battery voltage, that would be done where the current trace crosses zero. At that point, with no current through the battery, it is effectively open circuit. Anywhere else the voltage will be skewed by that current. In the list of sensors in Torque there is one for Auxiliary Battery Voltage, but not one for its current. There could be a current sensor on the main cable that goes to the fuse box under the hood, and the car could be using that (somehow) to log a true battery voltage. Or maybe it just shows the voltage with no correction?
There are two cables at the 12V positive post, a thick one that goes to the fuse box and a thin sense wire (what is this one called?) which goes to the inverter (I think). If there was no continuity on that sense wire, would the car throw a code, or would it maybe just crank the charging voltage up?
