Accurately reading voltage

[Hopeless Diamond]

So, it's not a critical issue, more of a question on what is accurate and how to measure it.

I have 3rd Gen 4runner with the stock alternator. I added a GM diode to the exciter circuit to raise the output voltage a little bit. I've added a "house" AGM through a solenoid and wanted to try and give a little better chance at getting a better charge. I run an android head unit with Torque connected to the OBD2 and read the system voltage from there (the diode did raise it by ~.4 to .5v, now ranging from 14.1 to 14.6 depending on temp, loads, etc.).

I also have a Midland Dual Band Ham radio, connected directly to the starting battery. It has a read out for voltage, and it has always read about .3 to .4v lower than the OBD2. (13.9 to 14.1) It seems to be in a tighter range but then I can't watch it as easily as the OBD2 readings.

I then changed out the cigar lighter with a USB dual power port, that has a voltage display. This reads lower than the OBD2 by .5 to .6v, and the radio by about .1 to .2v.

All this got my curiosity up and I got out my VOM. It comes back with readings different than all the others, typically in the middle. This is at the battery posts (both batteries read the same when the solenoid is closed, seems my wiring works right).

End of the day it's not a big deal, and as long as they read it consistently, that's fine. The OBD2 reading seems high, and the power port and radio readings seem low to me. As I expand on the house side system (nothing on it yet), I do want to have the ability to check voltages when it's isolated for fridges, night time lighting, device charging, etc., figure I'll end up with yet another reading. It's rare that we stay at one location for very long, so theoretically that system would recharge during travel, but even then with short trips and enough loads it may not get topped off completely and then could run into problems. Knowing a baseline reading would help. What the display says is less important than what it means, assuming it's consistent (it may read 11.9v, but in reality it is actually 12.5v). Short of $$ for a calibrated Voltage meter, how do you accurately read a "volt"?

 

[john61ct]

You can buy a cheap but very accurate voltage reference gadget

and then calibrate your cheap meters yourself, only buy ones that can be calibrated.

Quality used Fluke DMMs go for reasonable prices on eBay.

Finally realize that different point on a circuit will have different voltages, especially on smaller gauge wires as current rises.

That is why quality charging gear, including high end alt / VRs will have a dedicated pair of voltage sense wires separate from the thick power leads.

 

[DaveInDenver]

The voltage anywhere in the harness is subject to voltage drop based on load. It could vary a lot. The measurement point makes a difference.

The one that matters is the voltmeter measurement at the battery but the quality of the tool makes or breaks it. If you don't know the quality, condition or calibration of your meter then use a brand new 9V battery to check it. Generally an absolutely fresh "9V" will have to be 1.58V/cell coming off the line so will measure 9.48V with whatever test load they use to be considered good.

They will slowly discharge with age but in my experience a brand new from the package "9V" will indicate 9.5V +/- 0.1V and that's certainly close enough for government work. You know your meter has less than 2%, and probably within 1%, error anyway. If you measure grossly lower or higher than 9.5V with a brand new battery try a second battery. If it shows the same then your meter is suspect. I just checked two I had here still in the packaging. The Duracell (expiration 03/2023) measured 9.59V and the Energizer (expiration 02/2022) showed 9.54V, both no load with a Fluke 87 series III last calibrated about 3 years ago. Both batteries made in Malaysia so could be from the same slave labor camp for all I know.

The cigarette lighter measurement is a curiosity, not likely to be correct or consistent but you never know. I had one cigarette lighter meter that bounced between 12.7V and 12.8V when the DMM showed 12.78V, so it was exactly what you'd expect it to show. But the cigarette lighter plug itself could be quite off from the battery voltage.

The OBD is probably fairly precise but on a 3rd gen 4Runner not necessarily accurate and I think sampled at a fairly slow rate, maybe once a second, so it'll probably bounce around so may not be consistent. It would be a data point and intended for powering ancillary circuits. So Toyota wouldn't have any reason to go to great lengths in measuring it accurately since it wouldn't control anything. The alternator regulates itself independent of the ECU, for example. The difference would probably be right, IOW the voltage bump you see with the diode is really ~0.45V.

The radio running from the battery with sufficient sized leads might be accurate but will depend on the internal measurement circuit. It should I'd think be consistent at least with the radio in the same condition (e.g. receiving only, backlight off, etc). The voltage measured at the radio while it's transmitting will probably be lower unless you ran very large wires. I used 10 AWG with a 50 W radio and get around 0.5V drop when transmitting. You should I think see a consistent offset if there is one between the voltmeter reference and the radio indication.

 

[Hopeless Diamond]

Thats a good idea on the 9v, thanks. I just want to find a good baseline that I can then figure an offset for mainly the OBD2 reading to monitor from. I actually expected the OBD2 reported voltage to be lower, assuming it had more wiring convolutions getting to it.

 

[Hopeless Diamond]

interesting, never messed with OBD but once. Is voltage reported from the car software ?
I assumed the OBD reader just measured voltage at the OBD receptacle.

Good point. it's a '96, not much in the way of on-board software, it barely has the OBD2. It's probably more of a function of the reader (USB hard wired into head unit for input to the the Torque app).

 

[rayra]

Different meters, different readings. Take the measurements at all those points with the same meter, if you think you've got something odd going on. Consider the reports of several different devices nothing more than a rough guideline.

 

[DaveInDenver]

interesting, never messed with OBD but once. Is voltage reported from the car software ?
I assumed the OBD reader just measured voltage at the OBD receptacle.

Not sure on the 3rd gen 4Runner but in my 2008 Tacoma the ECU has a field to report the voltage it reads and the OBD software I use can also read the plug voltage if the adapter measures it. Can't say I know the SAE or EPA rules for what must be standard. I don't run Torque and the adapter I have is true junk so other's mileage may vary.

 

[67cj5]

If you want it to read 100% spot on then buy one of the Fluke models that come with a Calibration Certificate , I got fed up wasting money on DMM's and none of them displaying matching numbers, When I went to the Fluke Importers they took me down to their Calibration Shed and set up a machine with a 10 digit readout and the Fluke DMM I wanted was their top of the line 189 Now replaced by the 289 and it was so accurate right down to 1.0000v. there is none better if you want the best.

 

[rayra]

A Fluke 289 is ~$600. Pointless waste of expensive 5-digit accuracy on a vehicle's ~~12VDC systems.

 

[67cj5]

A Fluke 289 is ~$600. Pointless waste of expensive 5-digit accuracy on a vehicle's ~~12VDC systems.

I agree Ray, But I bought about 3 or 4 DMM's and none of them were right So I bought the top of the line 189 "at the Time" more out of anger than anything else, I was just hell bent of getting one that was accurate So I just asked them for the best and most accurate meter they make, when you work out what I spent on the other 4 that would of covered the cost of the Fluke, I still have it to this day and it is a great piece of kit, I know that I can use it to check all my other meters and it is perfect for measuring Solar panels,

 

[john61ct]

As I mentioned above, one of these

AD584KH High Precision 4-Channel 2.5V/7.5V/5V/10V Voltage Reference Module with Case - - Amazon.com

AD584KH High Precision 4-Channel 2.5V/7.5V/5V/10V Voltage Reference Module with Case - - Amazon.com

www.amazon.com

Lets you calibrate all your cheap voltmeters.

I've had $3 Harbor Freight meters that maintain accuracy, and $75 to $200 units that don't.

At least with a reliable reference you know which is which.

 

[Hopeless Diamond]

I used DaveInDenver's system of checking a 9v battery and found my VOM (Power Probe) to be pretty accurate (at least for me), 2 Duracell batteries came in at 9.69v. That gives me confidence that what I read throughout the vehicle with it is accurate or at least "close enough". Next time I take my dash apart I'll be able to getting readings right at the device contacts and have a true reading. I can then figure out any offsets from the various readouts. Mostly inconsequential, but I did want to make sure things weren't wildly off.

In the telecommunications world and am very familiar with Fluke and their high quality equipment, have a lot of $$$ of it, most of it we are obligated to get calibrated yearly, again $$$. We just don't need to verify voltages down to the gnats ass. Thanks all, so much great technical info shared here, a lot of really sharp people