Uni-t multimeter erratic with DC charger comnected

[trinityracing]

Hi all. Can someone tell me if my new meter is faulty. It's a UT61E and when testing Dc voltage on 200ah Lifepo4 while a charger is connected pushing 20amps, the meter is all over the place. Once in a while it will show correctly. Sometimes it would hold a specific number but be greatly off. How could you trust this or know when the meter is having problems.

I had 4 meters and the ut61e was the only meter that was erratic. I swapped leads and same results. I installed a new battery as well. Oddly to me, if I left the meter connected using clamp leads, it would correct itself when connecting a 2nd meter simultaneously. In other words if two meters connected simultaneously it would correct itself and read perfectly fine and then back to erratic when 2nd meter is disconnected. It was like the 2nd meter cleaned up whatever was causing the ut61e to give erroneous readings

I assume the 20amp  charger running was causing interference somehow but I need a meter that can handle this. My $30 meter wouldn't act like this.

Should I get a replacement or skip this meter. I would like to use a ut61e. The price is right for me

[joeqsmith]

The UT61E is rated for 10A for 10 seconds and you're pushing 20A continuous though it?  I'm impressed that it didn't melt or blow the fuse.   

[trinityracing]

Im checking volts. Not amps..I have a clamp meter for that

[joeqsmith]

Makes more sense.   So, meter is looking at DC voltage across battery terminals (not the charger) and placing second meter in parallel with the UT61e makes it stable.  If you place even a 0.1uf ceramic in parallel with the UT61E, does it stabilize the readings?   

 

[floobydust]

I would say the multimeter is too sensitive to common-mode noise the battery charger is making.
It's just Uni-T they don't make a great product. Many different models of 61's out there, A-E and the + as well.

UT61E should have the two foil shields inside. Does yours have them? It might be missing or not properly grounded (spring makes poor connection) inside the multimeter.

To prove what I am saying, connect a 0.1uF cap between the charger (-) output, and earth ground. See if the multimeter behaves then.

[trinityracing]

Thanks for info. I don't have a cap that size but might would with some digging. What results should it show?

It does have the two shields. However the rear housing shield that makes contact with the long spring was not installed ideally to make contact. It was too far along the edge and the spring was soldered where it was leaning. I fixed it to have better contact.

I tested it with charger on and it held steady. I moved the leads, shut off/on, etc., trying to duplicate it being erratic but it held fine. You guys would know better than I do if the shield or lack of could have caused that issue.

That was almost certain that it was some type of interference with the charger for sure. What bothered me is the other meters especially one very cheap one had no issue. One of the biggest problems with any of these meters is QC. It's a hit or miss.

I will keep an eye on it and put it in the same situation before I decide it's junk or not. Yeah I agree just a budget meter. Having been a machinist I like having that extra digit especially calibrated and can duplicate accuracy. For hardcore electronics... I'd have to step up a couple notches.

[Kleinstein]

One should not use the DMM with the charger connected. This is a rather unsafe more of operation, as the meter part is not isolated from the battery and the chargers are usually not that well isolated on the low voltage side. Extra interference is an additional problem - here some chargers can be very bad.

The runtime with a battery should not be that bad, that one needs the charger connected.

Using the meter for too high a current for a longer time is a 2nd issue. This may cause damage even if it does not blow the fuse.

[joeqsmith]

What bothered me is the other meters especially one very cheap one had no issue.

The UT61E sacrifices robustness for being able to measure higher frequency signals.  When you look at a noisy source, it detects this noise.  Adding the cap may stomp on some of that.

[mariush]

I'd suggest using a small classic transformer to power your multimeter ... that meter will work fine with as little as 3-4v but will show low battery voltage at around 5.6v

Considering the meter uses a few mA to work, even a 1-2 VA transformer pcb mounted transformer would work

6-12v ac transformer > bridge rectifier > capacitor (something like 100uF 25/35v should be plenty) > a linear regulator (ex 7809 or 7805 or lm317) > capacitor

for example : https://www.tme.eu/ro/en/details/bv302s12006/pcb-transformers/zettler/   or https://www.tme.eu/ro/en/details/bv302s06006/pcb-transformers/zettler/

with the 6v ac transformer you could even forget the regulator, a bridge rectifier followed by a capacitor would be plenty.  6v ac gets rectified to around 6 x 1.414 - 1.4v = ~7v give or take 1-2v at low load, and a capacitor would just smooth out the ac output.

edit:  i read and assumed you meant a charger to power your meter for long duration measurements and that's why i recommended using classic transformer, to have the meter floating, isolated.

if meter runs on battery and has those issues, running on transformer most likely won't help.

[floobydust]

I say it's best to NEVER power a handheld multimeter from a bench/mains power supply. Input COM(-) is not BATT(-) and the lesson why they are different will cost you a blown DMM IC. There is no protection for that.

OP could also measure the noise. Measure ACV between earth-ground and the battery he's charging. I expect the charger noise is due to the Y-cap which easily causes a stray voltage 60-100VAC. Too much common-mode noise for some test equipment. Fluke ensured RF would not come in through it but not so with these low dollar meters.
The UT61E might be working now that the shield is attached inside.

[iMo]

I power my vintage Metex from the "Sinclair ZX" 12V DC PSU (~42y old now), via a diode and resistor I trickle charge the internal 9V NiMH "Westinghouse" battery.. 
When doing something special I disconnect the PSU (2mm jack), but it works fine connected.
Before that I had there a 9V switching wall adapter, and I blew up several $$ chips with that setup (poor isolation).

[floobydust]

I think that is what I am saying - that the mains PSU (transformer or Y-cap) can have enough capacitive coupling that causes spikes to the multimeter being powered, and the DMM IC easily gets zapped or it just crashes when connecting (-). That old Fluke recall was about common-mode interference too.

[joeqsmith]

Sounds like a few of you think he is actually attempting to power the meter from a battery charger rather than measuring the voltage drop across a battery they are charging.

[sleemanj]

One should not use the DMM with the charger connected. This is a rather unsafe more of operation,
(...)
Using the meter for too high a current for a longer time is a 2nd issue. This may cause damage even if it does not blow the fuse.

You have misunderstood the situation in two ways.

1. The user is not powering the meter from a charger, they are measuring a device which is being charged.

2. The user is not measuring current with the meter, only voltage.

[floobydust]

I think iMo mentioned running his Metex from the wall wart at times, so that is why I pointed it out.

OP sounds like the Uni-T foil shield was not connected properly inside and it was sensitive to CM noise more than the other multimeters.