Voltage drop on 12V when power windows engaged?

I have been driving around for a while with a voltmeter display stuck in the power outlet under the glove box. It typically reads 14.3 V or 14.4 V for a second or two after the car starts and then falls quickly, like less than 5 seconds, to 14.1 V where it sits the rest of the time. Doesn't matter what sort of driving, fast, slow, sitting at a light, accelerating, decelerating, always 14.1 V. However, I recently noticed when parking the car (not moving, parking brake set, the ICE has turned off, transmission may or may not be in P, car still in READY) that if both front windows are engaged "up" at the same time it drops to 13.7 V. Do only one window, either one, and it only drops to 13.9 V.

Seems odd to me that the front window motors can cause a voltage drop like this when all sorts of other changes during driving don't.

Anybody else see this voltage drop under the stated conditions?

Thanks

 

Thanks.

I knew this 12V had very little capacity, but it still holds voltage OK overnight, and still has enough oomph to get the car to READY.

This voltage drop struck me as strange because, since it is in P, the inverter is still operating - it won't let the 12V run down, and if hobbit is right, I could probably unhook it entirely and the car would still be in READY. So I assume what what the voltmeter is seeing is the output of the inverter, and not the voltage on the 12V battery alone. If it was the latter one would expect that the voltage would start falling back towards 12.7 (ish) as the car sat, and that doesn't happen. However much these little motors pull, it cannot compare with, for instance, the cabin fan, and turning that on and off doesn't change the voltage reading. Honestly I thought the most likely explanation was that the charger port and the motors were in the same or somehow related circuits, so that the observed voltage change was due to the voltage drop across the motors, or something like that.

 

Exactly. It made no sense that the cabin fan, which can draw a lot of current, didn't affect the reading, whereas the window motors did, if the reading was at the battery. Makes me wonder where the MFD is sensing the 12V value it shows in testing mode. Probably on yet another circuit, most likely its own 12V input, and not on the thick wire that connects the positive battery post to the fuse box under the hood.

 

Well, hand waving, ballpark 10A for either one window motor or the blower (at some speed). Hard to figure it out from the fuses for the window motor, as multiple fuses seem to be in play, and the owner's manual description of the fuses doesn't sort it out. HTR is a 40A fusible link in the engine compartment, and that one, I believe, protects the cabin blower.

On a somewhat related tangent, the 12V (OEM battery) was reading 12.2V today, so attached it to a charger (max 2A, in reality rarely above 1A) and 4 hours and 45 minutes later the charger thought it was full. That's at most 4.75 Ah, changing SOC from 34% to 100%. Yet when the battery was tested before and after recharging (with a little HF tester, right on the battery) these were the numbers:

SOH % 70 79
SOC % 34 100
Ah 39 42
volts 12.21 12.78
R mOhms 9.41 8.86

If the charger kept track of what it was putting in it would have rated this battery with a capacity of around 7.2 Ah, but the testers always show high Ah values, like here. I wonder if anybody makes a charger which keeps track of the charging profile and outputs an equivalent set of numbers that result from the time charging? The little electronic battery testers seem to be extrapolating the Ah from just a few seconds of discharge, and then assume the battery will keep going at that rate.

 

The plugin voltmeter has two USB sockets on it, rated at 2.4A each. Never used them before. Today plugged in a small flip phone, and the voltmeter display then alternates between voltage and current, which are 12.0 V and .75 A respectively. Unplug the phone and the voltage instantly jumps back up to 12.1. So oddly current through the device itself is enough to change its voltage reading, no window motors needed.

 

Seemed a little high to me. .1V/.75A = 133 milliOhms.

Electrical connectors, if not dirty or corroded, are rarely more than a milliOhm, and often much less. Let's allow for 5 of those at 1 milliOhm, so 5 milliOhms. The copper wire from the 12V battery to the fuse box in the engine compartment is a large gauge, not sure which exactly, but let's say 15 ft at 6 gauge, which would only be another 4 milliOhms. 9 milliOhms so far. Presumably the path back through the body to the negative post should also be on that scale. 13 milliOhms. That leaves the smaller wires from that fuse box to this meter, and then from there to the nearest ground. 10 ft (?) of 14 gauge(?) copper, adds another 25 milliOhms and a total of 38 milliOhms. It is order of magnitude the same as 133, but high by 3 X (ish).

On a 17 year old car I would guess that the extra resistance is probably dirt or corrosion at some of the connectors, or if there is a relay in there, the contacts might not be the best shape. Nothing terrible, just not what it was when it rolled out of the factory. Definitely not enough to bother trying to find it. Especially because it might not really be .1V, it might be .03 V and 14.1 was really 14.07 rounded up and 14.0 was 14.04 rounded down, and .03/.75 = .04, which would be very close to the hand waving total resistance estimate.

 

Good point, but I don't know which fuses are involved. Littelfuse

https://m.littelfuse.com/~/media/automotive/datasheets/fuses/passenger-car-and-commercial-vehicle/blade-fuses/littelfuse_atof_datasheet.pdf

gives (Amps, mOhm) values of: (10,7.7), (15,4.8), (20,3.4), (30,2.0), (40,1.4)

The power outlet circuit is certainly not less than 10A, so no more than 7.7 mOhm more. There might be a bigger fuse upstream of that, but it would only add a few mOhms more.

There is the big one at the battery which is I forget, 100A(?), so it should be very low resistance, at least if the general trend for "bigger amps lower resistance" holds for this other type of fuse.