Normal voltage drop on 12V in different run modes?

Our 2007 had new tires put on and yesterday I went to check the inflation, which was of course not quite what the door sticker said. After setting them to exactly those values I went in to do the TPMS reset, and used IG-ON but not READY, ie, two pushes of the power button but not foot on brake. The cabin fan was off. The little voltmeter plugged in under the glove box showed 11.80V. That seemed pretty low. Took it for a drive, and the voltmeter was at 14.20 for a long time (normally it falls to 14.10 in just a few seconds), but after a while it did arrive at 14.10 and stayed there. After the car was in READY for over an hour it was parked and the hood was left unlatched last night. This morning measured these values (all in volts):

At jump point: 12.60 (~80% SOC)
Plugin Voltmeter: Unlock, open passenger door, enter: 12.28
Plugin Voltmeter: Fob in dash, press power button twice (IG-ON): 11.80
Power off, wait 5 minutes:
Plugin Voltmeter: Fob still in dash, press power button once (ACC): 12.40

I have not yet run the HF battery tester on it. For some reason that only works right when attached directly to the battery.

Anyway, these 12V batteries are puny, but are voltage drops this large normal? I'm pretty sure I have used the voltage checker in ACC before and seen values >12.40, but I need to go dig through my notes to confirm it.

 

The tire pressures were not so much wrong as strange. They were all between 33 and 35, but the highest value was on a rear tire and the lowest on a front tire. The car had sat overnight before I checked the pressures. A couple of weeks ago the same shop did the tires on our other car, and I was there to watch for that one, and at least in that case they aired it up to final pressures when the wheels were back on the car.

Found this page

Toyota Prius: How to Check the 12v Auxiliary Battery Health - PriusDIY.com

and at (5) they give some voltages for ACC and IG-ON modes. Summary from "elearnaid" (some company, doesn't seem to be a Priuschat user, don't go to elearnaid.com, it tries to download software) quoted on that link:

ACC: 11.9-12.6v as normal.
IG-ON: 12V (ish), less if headlights or fans are on, replace battery if <10.2V.

So mine was in range for ACC at 12.40, and it was known it wasn't at full SOC, so less than 12.6V makes sense. The description for IG-ON is too shot on details of load conditions to really be useful, but 11.80 is certainly not <10.2V.

I think the reason the car sat at 14.20 for so long after coming back from the tire shop is that they had it in IG-ON for a while while setting up TPMS, and maybe also while doing the alignment. After charging it up for an hour it was back to its original behavior of settling at 14.10 volts in READY. Supposedly a battery in really good shape sits at 13.something volts instead, since the inverter isn't trying so hard to charge it.

I do have an electronic battery tester. Don't trust it much though. Among other things, if a test is done at the jump point it will fail but if it is just moved over and plugged directly onto the battery posts it will pass.

 

Harbor Freight Viking 12V Digital Battery and System Tester
(HF part number 58759)

It doesn't give a test error per se, the test runs to completion, and then it says the battery is bad.

I wager 95% that the problem is inductance, almost entirely on the long heavy wire between the jump point and the battery. I get the impression that these little testers use high frequency signals to somehow emulate a real load, and high frequency signal transmission is reduced by series inductance. The ground path might play some small role too, but that is through a very complex and large surface, which isn't going to make much of a magnetic field when the current passes through it. I would also wager a 4% bet on multiple paths with slightly different delays being present in that ground return, which would also play havoc with a high frequency signal. The remaining1% would be on gremlins.