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martyfarkle avatar image
martyfarkle asked

Differing voltage between SmartShunt and voltmeter

I recently bought and hooked up a Victron SmartShunt. It works like a dream on BlueTooth, and I thought this would be an accurate view of my battery state/usage.


For storage I have two Valence, 40ah RT Series 12v batteries hooked up in parallel. There is a substantial discrepancy in the voltage between my Klein voltmeter (MM300) and the Victron SmartShunt (500a) as follows:


Load (amps)
 SmartShunt
 Voltmeter
 Difference
0 amps
 13.15 volts
 13.24 volts
 .09 volts
4 amps
 13.09 volts
 13.19 volts
 .10 volts


I have measured for voltage drops across my various connections and there are none. I have calibrated the SmartShunt to 0.0 amps under zero load conditions. But these should not be implicated in any of the voltage discrepancies. I see it as a battle between the Voltmeter and the SmartShunt.


The only remote caveat might be the Epever MPPT connection. The MPPT battery minus is connected to the 'system minus' side of the shunt, and the battery plus goes to the 150a circuit breaker, which goes to the battery. The circuit breaker is closed. Yet, I do not see that as influencing the differing voltage between the two devices.


Has anybody else come across this issue? Does anyone have some wisdom on this matter?

SmartShunt
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martyfarkle avatar image martyfarkle commented ·
Below is a simple diagram of how things are hooked up:

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The SmartShunt datasheet claims that it is accurate to ±0.3%. On a value of 13.15v then it could vary between a ±0.03945v, say ±0.04v to round it up. That would mean this reading of 13.15 should be between 13.11v and 13.19v. My meter says it is 13.24v: that is outside the specs of 0.3%, according to my (accurate?) Klein meter; more than double.

Being as finicky as I am about precision: the two devices should be much closer in agreement than they are!

Is there any way I can check the accuracy of my Klein meter? Stick it in the wall socket? lol. That might be my problem. Or should I exchange the SmartShunt until I get a more accurate one? That would be frustrating for me and Victron. Do the more expensive meters boast more accuracy, as well as features? If I buy the cheapest yardstick, will it be 1/64 inch longer or shorter than the most expensive one? LOL

Sorry for ranting on, but accuracy affects us all in this science.

Thanks for any help/advice.

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Matthias Lange - DE avatar image
Matthias Lange - DE answered ·

In my opinion this is not an issue. Both the BMV and your voltmeter have a accuracy range.

screenshot-20210117-171414-drive.jpg

screenshot-20210117-171603-drive.jpg


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martyfarkle avatar image martyfarkle commented ·

The SmartShunt datasheet claims that it is accurate to ±0.3%. On a value of 13.15v then it could vary between a ±0.03945v, say ±0.04v to round it up. That would mean my reading of 13.15 should be between 13.11v and 13.19v. My meter says it is 13.24v: that is outside the specs of ±0.3%, according to my (accurate?) Klein meter; more than double.

I am getting a voltage verifier/calibrator to double check the accuracy of my Klein. I may get a 'better' meter, one that has a clamp for amp on it. That will be useful.

Thanks for your input.

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raymiller avatar image
raymiller answered ·

I may be able to add a solution or at least something to check. My new SmartShunt 500A had a similar problem. What I found was a voltage difference on 0.1-0.2V between the +ve battery terminal which the SmartShunt fuse connects and the termination on the SmartShunt. Long story short I found two issues;

-my fuse measured 5 ohms! compared to a replacement at 0.2 ohms. (fixed the problem) The self-consumption current of a couple of mA was sufficient to cause a voltage measurement error with the high resistance fuse. I've never come across such an issue before..

- the suppled fuse holder is rather crude, relying on spring contacts with the fuse end and when measuring the resistance of the assembled inline fuse holder varies with any mechanical movement. As any change in resistance can affect the system voltage measurement any instability is not desirable for long-term reliability.

I dislike the use of an old-style cheap fuse and holder, while they work, is not in keeping with the quality and reliability needed for such a precise high tech device like the SmartShunt. My solution is to use a self-healing polyswitch to protect the cable, no spare fuse required. The manufacture Little Fuse RXEF series has suitable devices.

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