Which Meter. Extech EX330 or Amprobe AM240 ?

My meters are reading correct at zero (leads shorted) and at 5.00 volts (as checked against my 5.0002V reference) … As such , I have assumed (rightly or wrongly) that they read correctly at 4.20V … One of my previous meters was slightly out (+10mV) against the reference , so I corrected the Li-Ion reading by a factor of 4.2 divided by 5.0 X 10mv … I subtracted 8.4mV from the reading … Since the meter only read to two places of decimals , I subtracted 0.01V … It was easy to do , but I gave the meter away as the others were reading exactly 5.00V on my reference.

I was hoping to get a cheap meter (under £50) that would read 5.000V rather than the 5.00V , but I might be spending money unnecessarily for use on Li-Ions.

I think that to get what I want at 4.200V , I would need a 5000 or 6000 count minimum … They are not available in my price bracket.

Looks as though I will be sticking with my 2000 count meters.
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If you are simply taking the DC voltage of 18650s I’d suggest you don’t bother to upgrade, I can’t really see a need to be that accurate.

Try finding a VICHY VC99 DMM, it has a 6000 display and is fairly cheap.

Check out the stuff at http://www.voltagestandard.com/ He sells some very nice voltage references and meter standards for cheap. If you have a 4000 count meter, he has a 3.0000V reference for 25 bucks.

I have the equivalent of a national standards quality calibration lab (8.5 digit meters, sub-microvolt accurate references and dividers, 1E-13 accurate frequency, etc). For 25 bucks, it’s hard to beat those little references.

This was originally on my short list till I looked at reviews on meters under $50 (on the internet) and it came in the bottom two … I would like to find somewhere in South Wales that sells them so I can try one against my 5.0002V reference … If this meter gave a reading of 5.000V from my standard , I would buy one … I’m not worried about the physical size as it wouldn’t be carried about … It would be in my desk drawer.

I did also consider the Amprobe PM55A as it is a 5000 count meter … It is very small but only uses a CR2032 battery and has a drain of 6mA on auto range … So I didn’t know how long the battery would last.

Decisions , decisions.
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My five volt reference is 5.0002V , so I don’t really need another reference … $25 is a good price for what you got though.
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I’m beginning to think that way … For mains use , I use my Avo 8 … I’ve had it since 1975 and it is still working OK … Good quality leads on it for any mains voltage work too.

I’m wondering if I really do need to go to the third decimal place when reading voltages on my Li-Ions … I’ve managed for a few years now with my 2000 count meters (on the 20V range) … There would be no advantage in going for a 4000 count meter (once the voltage goes over 4.00V).

Decisions , decisions.
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I would not call it a bad DMM, compared to my 6½ digit Fluke it is only 3 mV wrong at 5 volt, i.e. it show 5.003 when the other meter shows 5.00010. The 5 volt is from a DMMCheck.

Today i used my colleges 287 rather than my 187 for some very basic mains checks, it damn well annoyed me having to wait for the silly little start up, heck id of been happy with my almost retired 70 series.

The 287 is a great DMM for some stuff, but the slow start up, the low contrast on the display and the user interface with soft buttons is annoying.

Is it user-adjustable ? … Can I use my 5.0002V as a reference and adjust the VC99 to read exactly 5.000V ?

This would be the ideal situation if there was just one “pot” to adjust to give the perfect reading … It would be a matter of which pot should I tweak ? … Unfortunately it would invalidate the warranty.
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It looks like it is adjustable, but I have not checked what ranges the pots affects:

If you have a 4000 count meter and want to check the accuracy on the lower voltage scales you DO need another reference. 5V will cause the meter to switch to the next higher range.

My meters are only 2000 counts.

I have been thinking of another way to get more accurate readings … My reference is known to be 5.0002V … If I connect my reference positive to my battery positive and measure between the reference negative and the battery negative with a high impedance meter , the reading with a fully charged 18650 would be about 0.800V … If I subtracted the 0.800V (or whatever) from the 5.000V then that should give me a more accurate reading than I get at the moment … I realise that this is a bit long-winded , but it would cost nothing extra to do.

I might not bother … I think that two places of decimals will be OK for checking my Li-Ion voltages.

Maybe one day I will justify a better meter.
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I recommend the HP3458A. And for a better reference a Fluke 732 or 734. Add a Fluke 720A Kelvin-Varley divider to it to get any voltage you want (< 10V).

But then, that brings up the question of how do you know it is all working properly? A man with one meter never knows what volt it is. A man with two meters can know something is wrong. Better get three of each… I did.

Unless this is necessary for some other enterprise, a super-accurate DMM to measure battery voltage is entirely pointless. The Vout is variable depending on load, so any crapo-meter with a “battery” mode (like the $3 harborfreight ones, which btw can be calibrated inside) are going to be more “accurate” than a fluke.

It’s also why getting thick leads are pointless for measuring current, too. The thin wire spring inside a tailswitch is going to offer far more resistance than any DMM probe.

Nope, the thin wire spring is very short and has a low resistance compared to long cheap meter leads (which are almost always made of some thin stranded non-copper mystery-metal wire). But the real killer in measuring current is usually the current shunt resistor in the meter.

The leads on the $3 meter have too low resistance to meaningfully measure with a meter. What’s the shunt resistor value on DMMs?

On a LiIon you uses the unloaded voltage, that makes a DMM perfect for the job.

For everyday use I do agree that a 4.xx reading is enough, but the tolerance on that reading is fairly high on some DMM's.

Not all tail springs are thing and long, on the more powerful lights they are usual with thicker wire.

Yes, it requires another method, you can see one in my DMM guide.

That varies with range, usual you can expect between 0.3 and 1 volt burden voltage when the current is maximum for that range. Unfused cheap DMM's can be than the more expensive DMM's.

With auto ranging DMM, you might get a lover voltage drop in the low uA and low mA range, because they uses the same resistor as the high uA and mA range.

I.e. the high uA/mA has 0.3 to 1 volt burden, the low uA/mA has 1/10 of that.