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Voltage consistency across devices, get an idea of multimeter calibration.


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It is regularly mentioned how multimeters or inbuilt voltmeters can give disparate numbers due to off calibration.

Is there a way to even out or check for consistency by comparing measures?

For example measuring à charger voltage with a multimeter, and the battery voltage with the inbuilt circuit will give different numbers because they are measured with different devices.

Would checking the battery by measuring on the charge port allow to have a second reading of the same voltage as the internal circuits read, or has it been lowered by the BMS stuff?

Is the voltage measured on the charger connector (no load) the same as enters the EUC?

The idea is that if it is possible to measure the same voltage (or correct a known margin) on several EUCs and compare internal and multimeter results, it would give an idea of the consistency of the multimeter, allowing to single out a measure that is more off than the others.

For example, my 16S charges to 67.5V according to internal reading. The charger show 66.5V on the multimeter. If I could check that the multimeter gives consistent measures between chargers, charge port and internal reading on several EUCs, it would allow to have a probability that the multimeter measures low or not.

Sorry for the tedious read and thanks for any input :)

edit: short version: Is there systematic margins to take in account when measuring battery charge port and charger output? The idea is to get to know your multimeter and compare it with internal reading.

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  • null changed the title to Voltage consistency across devices, get an idea of multimeter calibration.
2 hours ago, null said:

It is regularly mentioned how multimeters or inbuilt voltmeters can give disparate numbers due to off calibration.

You have to think about which accuracy your wishes mean and then look if this is worth the money. Plus yearly calibration service, etc...

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Is there a way to even out or check for consistency by comparing measures?

Just by measuring with something that's knowingly more accurate.

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Would checking the battery by measuring on the charge port allow to have a second reading of the same voltage as the internal circuits read, or has it been lowered by the BMS stuff?

Charge port voltage can only be measured while charging due to polarity protection circuitry (?if it's not a gotway?)

And one gets just another slightly different number, as different as the numbers shown from the devices before.

The main differences ones measures come from "offset and factor" differences. Unlinearities, temperature drift, ... will be about not noticable.

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Is the voltage measured on the charger connector (no load) the same as enters the EUC?

No (ks, inmotion, ninebot, ?veteran?, ?not gotway?), as the reverse protection curcuitry is high resitant while measuring and is thereby a voltage divider - ones voltmeter will just see a undefined fraction of the voltmeter.

Measuring is just minimal, but current flowing out of the charge port. If one does not charge at the same time.

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The idea is that if it is possible to measure the same voltage (or correct a known margin) on several EUCs and compare internal and multimeter results, it would give an idea of the consistency of the multimeter, allowing to single out a measure that is more off than the others.

Imho "normal" multimeters should be quite consistent over time  - maybe just o "extreme" temperature differences could be slightly noticable?

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For example, my 16S charges to 67.5V according to internal reading. The charger show 66.5V on the multimeter. If I could check that the multimeter gives consistent measures between chargers, charge port and internal reading on several EUCs, it would allow to have a probability that the multimeter measures low or not.

Imho not seriously possible. Way not enough measurements you'll make to get some statiscaly relevant data.

Btw - just to consider (67.5-66.5)/67.2~1.4%. So within a +/-0.7% range!

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Sorry for the tedious read and thanks for any input :)

edit: short version: Is there systematic margins to take in account when measuring battery charge port and charger output? The idea is to get to know your multimeter and compare it with internal reading.

Charge port and charger output should be about the same - just a cable and connector inbetween. 😁

Between charging input and discharge output of the battery should be some resistances (wiring, comtacts and mosfets) below or in the range of the internal resistances of the cells. So with no excessive currents measurements should not differ too much.

Best to borrow from somewhere or visit someone with an accurate voltmeter to get the calibration offset and factor for yours.

Just to break the brainz - the balancer and cell overvoltage comparator circuits seem to have accuracies of +/-0.05V at ~4.2V. That's an accuracy of ~1%. About +/-0.7V at 67.2V (all 16 together) - you remember this values from before ;)

So imho one would have to adjust the charger to the bms circuitry and life/get used to whatever the wheel shows. Could inflict a bit the usable range if the wheel shows less. Hopefully it's not the other way round and the wheel buckles as it believes the full battery has overvoltage...

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Thanks a lot for all the details @Chriull , much appreciated.
Yes of my EUCs only the Veteran shows voltage on the charging port. The schottky diode of the Kingsongs make it 0v.

Comparing my multimeter to a proper source would be nice, but I'd have to find someone trustworthy. I'll have a look around.

I'll try to lower the 16S charger a bit if I can get it open, as I don't like the high (EUC) internal reading, even if it could just be bad calibration.

Thanks again.

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