question

topper avatar image
topper asked

Voltage difference between battery terminals and internal Bluetooth via VictronConnect

Hello,

I bought a new Victron lithium battery 25.6V/200Ah Smart-a. When I connect to the lithium battery via VictronConnect with Bluetooth, it shows a lower voltage than the actual voltage on the battery terminals. The voltage difference is around 0.3V which I think is a too big difference.

See next photo;
p1150937.jpg


I was wondering if the voltage measurements from the Bluetooth part in the battery are also used for the BMS that has yet to be connected, because then the VE.BMS will react to the wrong internal voltage measurements.

Thanks in advance

VictronConnectvoltagesmart lithium
p1150937.jpg (1.3 MiB)
2 comments
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Trevor Bird avatar image Trevor Bird commented ·
Hello @Topper ....May I ask is the battery being charged while thsi measurement is being made? Perhaps the 0.3 volt is the voltage drop from the terminals to the cells if the current were large enough?
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topper avatar image topper Trevor Bird commented ·

@Trevor Bird, the measurements were made without any charging, the cables to the battery terminals were not connected to the Multiplus

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3 Answers
Alexandra avatar image
Alexandra answered ·

@Topper

Check the calibration accuracy of your meter.

They are affected by storage conditions as well, for example heat.

But from experience, a difference of up to 3% in reading is not uncommon. If you check the battery charging datasheet/specs its design allows for a range that will cover the difference you are seeing there

See fluke's article on DMM accuracy. And how resolution affects readings.

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Trevor Bird avatar image
Trevor Bird answered ·

Ok, the answer may be as @Alexandra mentioned, the accuracy of the meter. The accuracy of the UNI-T 131A meter is stipulated as +- 0.5% + 3. The +- 0.5% is easy enough to understand and your reading out by more than that BUT the +3 in the specification relates to the error of the least significant digit. In this case the reading can be anyting between 26.8 + 0.5% = 26.9 + 3 least significant digits = 27.2 and still be within the manufacturer specifications. The precision of the meter cannot deliver the accuracy you are seeking to confirm the internal BMS of the battery is correct.

I hope this helps.

Regards,

trevor

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topper avatar image
topper answered ·

@Alexandra and @Trevor Bird thanks for the answers, I also did a measurement with the Multiplus connected, at the same time I measure the following values;

The Bluetooth reading is: 26.41V
The Multiplus measurement is: 26.57V

Still a difference of 0.17V but this will be within the specs.

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Alexandra avatar image Alexandra ♦ commented ·
@Topper

Yeah, there is always a bit of a difference. So long as it is within spec you will be fine.

Also with Victron BMS it is controlled by that anyway.

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