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Chriull

Multimeter accuracy needed for charger/battery measurements

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Unfortionately I stated in a couple of posts that any cheap multimeter is fine to check charger/battery voltages.

I just discovered that some of the cheap offers have +/-1.5% accuracy! Thats +/- 1.26V at 84V! Such equipment is not usable for calibration/measurements!

But there are many cheap +/-0.5% available - this are still +/-0.4V. Enough for a rough check and almost about enough for calibration?! One should be consious about this inaccuracy and pay attention, so the chargers voltage cannot be to high.

Really recommendable multimeters (+/-0.1% ~~ 0.1V at 84V) start to get expensive...

... But i did just a quick google search for some models - maybe there are great price/accuracy/quality deals available?

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8 hours ago, Chriull said:

But there are many cheap +/-0.5% available - this are still +/-0.4V. Enough for a rough check and almost about enough for calibration?! One should be consious about this inaccuracy and pay attention, so the chargers voltage cannot be to high.

It's slightly more complicated. First, it's important to determine the measurement range and measurement resolution. Why? Because multimeter can have accuracy of 0,1% but measurement range of 0 to 400 V with 0,1 V resolution (four digits). What does it mean? It's accuracy will be one digit. What does it mean for cell measurement? It will give us 100 mV of measurement resolution and accuracy. This equals to... 2,3 % !

Let's take a Brymen BM231 multimeter as an example. Lower shelf multimeter of generally proven brand - price tag of about 70 EUR. It's specified to have an accuracy of 0,6 % + 3 digits. Display is defined as 3 and 5/6, what effectively means that voltage of 4,2 V in ideal conditions will be displayed as 4,200 V.  Taking all this into account we can assume that when measuring cell voltage of 4,2 V it may display values from 4,167 V to 4,228 V. Not that bad :) Trust me, BMS circuitry doesn't get that accuracy of measurements ;-)

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Posted (edited)

Getting the real accuracy out of specifications is almost an own "sience".

11 hours ago, Seba said:

Let's take a Brymen BM231 multimeter as an example. Lower shelf multimeter of generally proven brand - price tag of about 70 EUR. It's specified to have an accuracy of 0,6 % + 3 digits. Display is defined as 3 and 5/6, what effectively means that voltage of 4,2 V in ideal conditions will be displayed as 4,200 V.  Taking all this into account we can assume that when measuring cell voltage of 4,2 V it may display values from 4,167 V to 4,228 V. Not that bad :)

That sounds very good!

But from the specs i found at a link with a datasheet for BM231,233,235 specifying dc voltage range 6.000 even with +/-.3% +2d leading even to +/- (0.0126+0.002)=+/-0.0146V!

For 84V at the 600.0V range with +/-(0.3%+2d) would be +/-(0.252+0.2)=+/-0.452V

Seems to be as good as it can get in this price range?

Just not with the Surpeer av4? :D currently not available but specced at incredible 200mV/2V/20V/200V ±(0.05%+5d) and ?4 1/2? digits for ~30-40US$. Input protection circuits shall be absolutely not state of the art but "usable good accuracy". But hard to tell if one got a good or bad one without an reliable multimeter to compare...

Here the 084.0V at the 200V setting would make +/-(0.042+.5)V=+/-0.542V :( Or do the 4 1/2 digits mean that at the 200.0V range 84V are displayed as 84.00V and the accuracy is +/-(0.042+0.05)=+/-0.092V?

.... The +/- digit are a crude way to "hide" inaccuracy in leaflets... And "hit" especially hard if one needs to measure this 84V... :(

11 hours ago, Seba said:

cell voltage of 4,2 V it may display values from 4,167 V to 4,228 V. Not that bad :) Trust me, BMS circuitry doesn't get that accuracy of measurements ;-)

From one curcuit i saw once of "common chinese" BMS they use comperators with fixed build in reference voltages for something like 4.27V as upper voltage threshold and they have +/-50mV specified. So, yes, they have almost twice the inaccuracy.

Afair there was also comment that many comperators/voltage references like this used have "horrible" temperature drift...

 

Edited by Chriull
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1 hour ago, Chriull said:

For 84V at the 600.0V range with +/-(0.3%+2d) would be +/-(0.252+0.2)=+/-0.452V

Seems to be as good as it can get in this price range?

It's good result. For example LG MJ1 cell charging voltage is specified as 4,2 +/- 50 mV. For 20S string it's 84 V +/- 1 V. So full-charge voltage difference as in datasheet is over twice of multimeter overall accuracy. Over 100 % of safety margin. Of course you can get much better multimeter for 150 EUR - Brymen BM857s has 0,03% + 2 digits of accuracy at 5V measurement range. It's less than 1,5 mV ! :)

1 hour ago, Chriull said:

The +/- digit are a crude way to "hide" inaccuracy in leaflets... And "hit" especially hard if one needs to measure this 84V... :(

It's normal. This is because analog-to-digital converters ane not ideally linear and have non-zero quantization error. So they have offset error specified in bits LSB.

1 hour ago, Chriull said:

From one curcuit i saw once of "common chinese" BMS they use comperators with fixed build in reference voltages for something like 4.27V as upper voltage threshold and they have +/-50mV specified. So, yes, they have almost twice the inaccuracy.

Afair there was also comment that many comperators/voltage references like this used have "horrible" temperature drift...

Yes, sometimes temperature drift may be well over declared reference range :( And we cannot forget about voltage divider that is built with two resistors. They also have limited precision and have temperature drift...

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27 minutes ago, Seba said:

It's good result. For example LG MJ1 cell charging voltage is specified as 4,2 +/- 50 mV. For 20S string it's 84 V +/- 1 V. So full-charge voltage difference as in datasheet is over twice of multimeter overall accuracy. Over 100 % of safety margin. Of course you can get much better multimeter for 150 EUR - Brymen BM857s has 0,03% + 2 digits of accuracy at 5V measurement range. It's less than 1,5 mV ! :)

You are right! If i want for example adjust the charger, so that the batteries are at maximum 84V i'd have to adjust it so that (84-0,452)V=83,548V are measured at the batteries. In reality the voltage could then be between 84V and  (83,548-0,452)V=83,096V leading to 4,155V for one cell in average up to 4,2V.

Or if one wants to stay within the 4.2V+/-50mV spec (85V max) one could adjust it to 85V-0.452V and has as lower limit of (85-0,452*2)V making 4.2V per cell for using the max capacity.

That sounds very acceptable!

 

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Posted (edited)
On 5/14/2019 at 10:26 AM, Chriull said:

Getting the real accuracy out of specifications is almost an own "sience".

That sounds very good!

But from the specs i found at a link with a datasheet for BM231,233,235 specifying dc voltage range 6.000 even with +/-.3% +2d leading even to +/- (0.0126+0.002)=+/-0.0146V!

For 84V at the 600.0V range with +/-(0.3%+2d) would be +/-(0.252+0.2)=+/-0.452V

Seems to be as good as it can get in this price range?

Just not with the Surpeer av4? :D currently not available but specced at incredible 200mV/2V/20V/200V ±(0.05%+5d) and ?4 1/2? digits for ~30-40US$. Input protection circuits shall be absolutely not state of the art but "usable good accuracy". But hard to tell if one got a good or bad one without an reliable multimeter to compare...

I was looking at Brymen BM859s  https://brymen.eu/shop/bm859s/   a couple of years back, seemed pretty good for its price, but then found a second-hand HP34401A (6½ digits bench meter) for 240€ with shipping. I then faced the same problem, how to tell how good it actually is (ie. how much it has drifted from last calibration, which was several years ago)...

What I ended up doing was to buy precision resistors & voltage references, make boards for them with measurement points and power the references from a battery (to prevent any "noise" from a linear or, even worse, SMPS power supply) and see if the meter reading was within the tolerance of the part. I got down to 0.05% in voltages and 0.01% in resistors with no problem (using good probes, they're just as important as the meter itself, for example:  https://www.tme.eu/de/details/pp-bm10a/messleitungen-komplettsatze/brymen/pp-bm-10a/#  , I got horrible jumpings in the readings with cheapo Chinese probes ), beyond that, no idea. Good enough for me  ;)

You can get 0.01% 5PPM/C (SMD) resistors in singles for around 1€ per piece for "common" values (I simply got 10, 100, 1k, 10k, 100k, 1M, 10M), don't remember what the 0.05% voltage references were, a couple of € maybe. After that, it starts to get expensive, 0.005% resistors are something like 20€ per piece. 

Edited by esaj
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