Super long time (hours) to settle on microamp value (UT61E) - Page 1
Forum section: electronics / testgear

author: seanspotatobusiness
author_rank: Regular Contributor
timestamp: March 22, 2020, 12:04:24 pm
content: I am sorting through my collection of NiMH AA cells to see what needs to be discarded (some of the fun you can have while in lockdown/quarantine!) and trying to get a measurement of their self-discharge (my battery charger measures capacity so that's taken care of). It's a bit clunky but I think the quickest way to estimate self-discharge with what I have, is to connect the cell to my power supply set to 1.2 V with my multimeter in series to measure the current. I figure that the current should tend towards a mimimum value which will be the self-discharge current. This should be good enough to distinguish between good and bad cells, right? At first I was doing this using my multimeter set to the mA range and after an hour or so, the current seemed to be going below 0.400 mA. I switched to the uA range and the value started at something like 40 uA and kept rising. It's been over an hour and the value is at 113.38 uA and still rising slowly. Is this normal? I realise that the UT61E isn't a great meter but it seems to settle fast enough for measurements I normally take. I think this is the first time I've tried to measure microamps in the few years that I've owned it.The true current value should be going down but the displayed measurement is still going up (now 113.88 uA). Could it be because the true value is not static that the meter is taking a long time to settle or is this what the UT61E is like for the microamp range? Or maybe just my unit?The cell being tested has a measured capacity of 1900 mAh (marketed as 2700 mAh when new but I never tested it back then) and is from the time before low-self-discharge NiMH were ubiquitous. According to my reading, 10-15% per month is a reasonable rate of self-discharge for this cell type which I think equates to 260-400 uA.

author: myf
author_rank: Regular Contributor
timestamp: March 22, 2020, 02:54:01 pm
content: Hello,Many thanks for your experience description ! Maybe replace the battery by an about 100kohm resistor. With it you'll be sure what current remains stable. Chemical reacts in accumulators are complexe and I don't know how batteries react.I also have a ut61e, and the last MOhm caliber measures will too sensible to my position (if I am near from the multimeter, even if I don't touch it).From 10kOhm caliber I notice that you must not touch the two alligator clips if you won't change resistor measures.Have a nice day !

author: joeqsmith
author_rank: Super Contributor
timestamp: March 22, 2020, 03:18:04 pm
content: The burden voltage on the 61E is pretty high. Are you sure that the voltage on the batteries isn't actually recovering with the 61E's higher resistance when set to the uA ranges? If you only have the one meter, replace the UT61E with an equivalent shunt resistor, repeat the test and measure the voltage across the cell. Try it again with a equivalent resistor for the mA range.

author: J-R
author_rank: Super Contributor
timestamp: March 22, 2020, 11:25:59 pm
content: Always a good idea to know/document the resistance of your DMMs in various modes. After all, you are modifying your circuit. 10A might be 0.01 Ohm, uA might be 1k Ohm.

author: myf
author_rank: Regular Contributor
timestamp: March 23, 2020, 04:43:21 pm
content: On my ut61E, with other multimeter 1 found :Caliber 10A - 35mOhm+/-2 on the main DMM-48mOhm+/-2 at the other end of the home made wires : 2x silicon awg14 1m and 4mm-32A Chinese banana socket.Caliber 20mA and caliber 200mA : 10.28Ohm +/- 0.02Caliber 2000uA(=2mA) and 200uA : 997Ohm.JR you are very close from these values !

author: J-R
author_rank: Super Contributor
timestamp: March 23, 2020, 08:16:15 pm
content: 0.01 ohm is a typical value for an 11A DMM fuse so I was throwing that out as maybe an ideal minimum. Most of the DMMs I tested are around 0.03 ohm for 10A, 1.8 ohms for mA and 100 ohms for uA. Some other approximate random values I ran into were 3 ohms, 11 ohms and 1k ohms.This also points out the value of having a DMM that can provide mA resolution & accuracy in the 10A range as well as uA via the mA range.

author: myf
author_rank: Regular Contributor
timestamp: March 23, 2020, 09:37:40 pm
content: Might we continue to exchange together ?(1) So there are 2 ranges with the same shunt in mA and in uA. Is it usual or a cheap tips in order to reduce price ?It's obviously impossible with analog device, it seems there is a similar way on rigol digital scopes.(2) Resolution of ut61e may be fine, but I'm surprised of these 2 values : not 10.000 ohm nor 1000 ohm.Do you think ii's an error from my two multimeters or is it usual ?(3) From 10.28 to 997 coefficient is 100 with a 3% error : (10.28/9.97)-1 = 3%. Do you think it's the error of this couple of multimeters ammeter / ohmmeter, or it does not matter ?(4) An other (and last) question : with very low counts (0-10) my multimeter often display 0, and from 100 to 20000 counts the variability / fuzzy measures grows, maybe 2counts before 4000 and 10 around 20000counts. Does this feature describe the A/D hardware ? Do you know how are others multimeters ?So accuracy from this 20000count multimeter is maybe about 2000count !Thank you very much for your interesting posts.F. from France.

author: J-R
author_rank: Super Contributor
timestamp: March 23, 2020, 10:42:18 pm
content: Who knows exactly what goes on in the heads of others, but I would say it comes down to accuracy and resolution, the size and shape of the DMM, locations and number of plugs, cost...To measure current, the specific value of the shunt resistor doesn't matter, as long as you know what it is at all (most) times. You simply calibrate the meter based on the actual resistance and using Ohms law you measure the voltage drop and calculate the current with I = V/R.If you are trying to detect a 1uA current flow across a 0.03 Ohm resistor you would have: 0.000001A * 0.03 ohm = 0.00000003VTo detect 1uA accross a 1kohm resistor it would be: 0.000001A * 1000 ohm = 0.001VIn general the first scenario would be much harder and more expensive to implement.I am not 100% sure I understand what you are asking, but I believe the answer would mostly come down to quality of the DMM components and design, and perhaps calibration. I don't have the UT61E, so maybe someone else should chime in here on its specific behaviors.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 23, 2020, 11:09:54 pm
content: Odd the OP hasn't chimed back in. I suspect that was what they were seeing.Keep in mind that you can't read across the meter to determine the shunt values. You have fuses, traces, connectors....... To find the values for the shunts, it's best to look at the schematic, which for the UT61E is available on-line for free.If you are really bored, I made a video where went over this part of their design and demonstrated making some simple changes to try and improve it.

author: seanspotatobusiness
author_rank: Regular Contributor
timestamp: March 24, 2020, 04:49:06 pm
content: Thanks for everyone's responses. I left the multimeter in mA mode for well over 24 hours and it settled at 0.200 mA. With two cheap (~$4) multimeters simultaneously measuring the voltage across the cell and the UT61E, the current increased to 0.202 mA. The voltage across the cell is 1.198 V and the voltage across the UT61E is 3.7 mV.In the uA range, the UT61E current settled at 105.0 uA with 1.095 V and 106.3 mV measured across the cell and the UT61E respectively.In conclusion, you were definitely right about the different burden voltages and having to wait for the battery to discharge for the current to come up to match the self-discharge current. I did not realise that the different ranges used different shunts but now that I see you discussing it, it should have been obvious.I need to find a quicker way to do this, because waiting for the current to settle will be a waste of my multimeter battery (I wish it used rechargeable AAs or something).