Battery voltage reduces the longer it is being MM voltage tested - Page 1
Forum section: electronics / beginners

author: DW1961
author_rank: Frequent Contributor
timestamp: June 26, 2023, 09:04:08 pm
content: I have the Uni-T 133A and the Amazon 90D120 (Which is a rebrand of a decent MM, but I can't remember which now).When measuring a dead battery with the normal no load voltage mode (edited - NON BATTERY TEST MODE):Uni-T MM: 1.38vAmazon MM: Starts off at .99 and the longer I leave it connected to the battery, the lower the volts go, down to about .63 where is stabilizes.

author: BeBuLamar
author_rank: Super Contributor
timestamp: June 26, 2023, 10:07:18 pm
content: Connect both meters at the same time and see what happens. I think one of your meter has lower impedance and it draws more current off the dead battery which will make its voltage drops.

author: DW1961
author_rank: Frequent Contributor
timestamp: June 26, 2023, 10:18:12 pm
content: Quote from: BeBuLamar on June 26, 2023, 10:07:18 pmI did and they both showed reduced voltage. Why do some MM have lower resistance than others when measuring voltage?

author: Shonky
author_rank: Frequent Contributor
timestamp: June 26, 2023, 10:21:45 pm
content: Quote from: DW1961 on June 26, 2023, 10:18:12 pmIt's just how they are. Why would they be identical? Cheaper designs are likely to have lower input impedances.Some meters have special ranges deliberately designed to have lower input impedances (although for ACV)

author: IanB
author_rank: Super Contributor
timestamp: June 26, 2023, 10:25:19 pm
content: Quote from: DW1961 on June 26, 2023, 09:04:08 pmAre you using the battery test mode for this measurement?

author: DW1961
author_rank: Frequent Contributor
timestamp: June 26, 2023, 10:31:44 pm
content: Quote from: Shonky on June 26, 2023, 10:21:45 pmI put the MM that drops voltage when testing a battery into 20 V DC and then measured resistance with the other meter that didn't drop voltage, and it measures .9 M ohms. Then I measured it the other way and it was 1.2M ohms. Would 200 Ohms make that big of a difference?

author: DW1961
author_rank: Frequent Contributor
timestamp: June 26, 2023, 10:32:28 pm
content: Quote from: IanB on June 26, 2023, 10:25:19 pmNo, just the plain old voltage mode. I already know what that is. But what is curious, to me, is that when tested with the battery test mode, it's reading is 0.0v. Using the voltage mode, it starts about .9 volts and drops to around .63v. The other meter measures the same battery in voltage only mode at 1.39v

author: wasedadoc
author_rank: Super Contributor
timestamp: June 26, 2023, 10:32:35 pm
content: Quote from: IanB on June 26, 2023, 10:25:19 pmThe OP has already been asking closely related questions. Photo in his post athttps://www.eevblog.com/forum/beginners/how-does-a-multimeter-meaure-voltage-and/msg4928545/#msg4928545shows his Amazon meter has battery test ranges for 1.5 and 9 Volt batteries and he knows those ranges have load resistors because he has measured their resistances.

author: DW1961
author_rank: Frequent Contributor
timestamp: June 26, 2023, 10:42:39 pm
content: Quote from: wasedadoc on June 26, 2023, 10:32:35 pmBig deal? I'm not measuring the resistance of the battery tester mode. What I noticed was that when using the normal voltage mode to measure AA battery voltage, one meter slowly dropped the voltage of the tested battery, while the other meter did not. I'm curious as to why?So before you post some obliquely negative connotative shit post about my post, why not read carefully what I am asking about instead of assuming I'm some sort of EEV blog conspiracy?

author: bdunham7
author_rank: Super Contributor
timestamp: June 26, 2023, 10:57:48 pm
content: Quote from: DW1961 on June 26, 2023, 10:18:12 pmYou didn't mention it (why?) but I'm sure you are using the 1.5V battery test on the 90DM120 meter and probably just the autorange voltage mode on the UNI-T. If you use the standard voltage range(s) on the 90DM120 you will likely get similar results.Different meters can have different input impedances on their regular DC inputs--I have meters that are 50,000 ohms-per-volt, 1M, 5M, 10M, 11M and virtually infinite--but these will all give similar results on a mostly dead battery. Your 90DM120 battery test range has a load resistor precisely because testing battery 'open-circuit' voltage, which is about what you get with normal DMM, doesn't do a very good job distinguishing between a somewhat OK and mostly dead battery. Since batteries drive a load, testing them loaded seems most sensible, wouldn't you agree?

author: DW1961
author_rank: Frequent Contributor
timestamp: June 26, 2023, 11:01:20 pm
content: Quote from: bdunham7 on June 26, 2023, 10:57:48 pmSpecifically, I said in the OP: "When measuring a dead battery with the normal no load voltage mode:" ("normal, no load voltage mode") I guess I should have said not the battery tester mode?As such, no I am not using the battery tester mode. I already have done that to see what the resistance was they were using for the test mode.So anyway, to reiterate, yes, this is measuring with the normal voltage side, not the battery test side. I mean it supposedly isn't suppose to load the battery, but it seems like it is, at lest slightly enough to incrementally lower voltage over a few seconds.

author: wasedadoc
author_rank: Super Contributor
timestamp: June 26, 2023, 11:07:55 pm
content: Quote from: DW1961 on June 26, 2023, 10:42:39 pmThe person posting sh*t is you. Show us where in my post I said or even implied that you were using the battery test mode of the Amazon meter. I gave background info to others who may not have been aware that your Amazon meter has such ranges and that you were aware of the load resistors used by them.

author: Shonky
author_rank: Frequent Contributor
timestamp: June 26, 2023, 11:11:26 pm
content: Any meter measuring voltage will place some load on the circuit. It may be so small it's irrelevant but it's still there.Some will be higher than others.They won't be as high as the battery test modes of your meter. That's the whole point of those modes. To load the battery to give a better indication of its state of charge. If loaded and the voltage drops then the battery should be considered flat.The value used as a load is somewhat arbitrary but understandable the 1.5v and 9v would be different as they are batteries with significantly different capacities and used with significantly different loads frequently.

author: DW1961
author_rank: Frequent Contributor
timestamp: June 26, 2023, 11:52:49 pm
content: Quote from: wasedadoc on June 26, 2023, 11:07:55 pmWell, I apologize then. I can see what you are saying and again I apologize. It sounded as if you were saying I was spamming or doing something negative. Again, my apology. Again, I reread your post and to whom you replied and I can see now that I was definitely in error. Damn, sorry.

author: DW1961
author_rank: Frequent Contributor
timestamp: June 26, 2023, 11:57:56 pm
content: Quote from: Shonky on June 26, 2023, 11:11:26 pmI guess that is the end of it then. In the UNI-T meter, as you state, the load is so small as to allow a dead battery to read 1.35V. In the Amazon meter, it gradually drops.However, and I don't know if this will give me a correct measure of resistance or not, but I measure, as stated above, the resistance of both meters in voltage mode, by setting one in voltage mode and the other in resistance/ohm mode and they both measured nearly the same at .9 M ohms and 1.2 M ohms. I'm assuming you can't measure resistance of the voltage mode by measuring that way?

author: bdunham7
author_rank: Super Contributor
timestamp: June 27, 2023, 12:05:31 am
content: Quote from: DW1961 on June 26, 2023, 11:01:20 pmThen I'd guess your 90DM120 meter is either broken or doesn't conform to normal standards. Are you using the range marked "2000mV"? Try to measure the resistance of that voltage range using your other meter--just set the UT133A for resistance, the 90DM120 for 2000mV and connect them together. The manual for your meter states that the input resistance is ">1M", but that's not very precise. Even so, at 1M, the current draw would be less than 1µA and it would be pretty surprising if that would draw your battery voltage down as you are observing.QuoteSure you can, as long as the resistance test voltage isn't exceeding the clamp voltage of the other meter. There's really no good explanation so far for what you observe other than some defect in the meter that allows excessive leakage current within the specfied voltage range. Which voltage ranges are you using? Try using the 20V range of the 90DM120 and see what you get.

author: DW1961
author_rank: Frequent Contributor
timestamp: June 27, 2023, 12:11:41 am
content: Quote from: bdunham7 on June 27, 2023, 12:05:31 amThat's exactly what I did to measure resistance of the Amazon meter.But, to be clear, the Amazon meter in normal voltage mode DOES NOT pull down the voltage of a good battery. It's only doing it on a very dead battery, one that measures 0.0v when tested in the dedicated battery mode.Both meters have around 1M resistance as measured like you say. But the Amazon still pulls the voltage don on a "dead" battery, whereas the UNI-T doesn't.I also did use the 20V setting on the 90D.I must have measured something wrong because I did it again and the 90D shows nearly 1 M ohm and the UNI-T on the 90D shows nothing "OL."

author: bdunham7
author_rank: Super Contributor
timestamp: June 27, 2023, 12:15:49 am
content: Quote from: DW1961 on June 27, 2023, 12:11:41 amWhich voltage range was it set to and what voltage did it read while you were measuring its resistance?QuoteAnd what did you observe? Did you also measure the resistance of this range?

author: DW1961
author_rank: Frequent Contributor
timestamp: June 27, 2023, 12:21:41 am
content: Quote from: bdunham7 on June 27, 2023, 12:15:49 amEdited post while you were posting, see images above.

author: bdunham7
author_rank: Super Contributor
timestamp: June 27, 2023, 12:56:37 am
content: Notice that the test voltage of the UNI-T is less than the voltage of your dead battery, so there's some possibility that its resistance drops with higher voltages. I think that's unlikely on the 20V range. If the 20V range also draws the dead battery voltage down in the same way, the logical conclusion seems to be that the very small current draw of the 1M resistance is enough to reduce the voltage but that of the UNI-T (which is probably 10M or so--your 90DM120 apparently doesn't go that high) is not. What exact battery is this? AFAIK it's not typical for dead batteries to recover their OCV that much, at least not quickly.

author: IanB
author_rank: Super Contributor
timestamp: June 27, 2023, 01:17:25 am
content: A very, very, very dead battery could behave in the way observed. Can we see a picture of the battery? The make and battery type would be interesting to know.

author: Shonky
author_rank: Frequent Contributor
timestamp: June 27, 2023, 02:57:27 am
content: Quote from: DW1961 on June 26, 2023, 11:57:56 pmTrying to measure the impedance of a voltmeter using the resistance mode of another meter is unlikely to produce consistent results. Resistance mode tries to apply a constant current through the "resistor" being tested and then measures the voltage across it. Reversing the connections tries to send the current the opposite way. The voltage input of a DMM is not a simple resistor so applying the current is will cause unexpected results.It does work on something like a battery test range since that will likely have a simple resistive impedance internally that will significantly dominate the impedance of the meter.

author: radiolistener
author_rank: Super Contributor
timestamp: June 27, 2023, 03:26:01 am
content: any DMM has some input resistance. It is required in order to measure voltage. Different DMM model and even different selected range may have different input resistance. Usually it is 1 MΩ, 10 MΩ or 100 MΩ. Most of DMM have standard 10 MΩ input. For a cheap Chinese DMM it may be random.Since it has resistance, it discharges battery, but DMM resistance is too high in comparison with a very low internal resistance of the good state battery, so voltage drop is very small and you even cannot see it.But when you're trying to measure old bad battery, it's internal resistance may be too high, so it's voltage drops down much faster when you connect it to DMM input and you will be able to notice voltage drop in time...

author: DW1961
author_rank: Frequent Contributor
timestamp: June 27, 2023, 03:34:38 am
content: Quote from: IanB on June 27, 2023, 01:17:25 amQuote from: bdunham7 on June 27, 2023, 12:56:37 amDuracell AA, a very dead Duracell AA.

author: wasedadoc
author_rank: Super Contributor
timestamp: June 27, 2023, 11:24:42 am
content: Quote from: DW1961 on June 26, 2023, 11:52:49 pmApology accepted. It is a rare person who doesn't get overheated occasionally.