Why does VE.Bus Smart Dongle read battery voltage that is 0.7V too high?

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billsulltx asked Nov 7, '21

Why does VE.Bus Smart Dongle read battery voltage that is 0.7V too high?

My system has Multiplus-II, BMV 712 Smart and VE.Bus Smart Dongle. In the Victron Connect app, the battery voltage reads 0.7 V higher than is reported by the BMV, and about 0.6 V higher than the voltage measured directly across the Multiplus-II DC terminals. The Multiplus-II indicates it is float charging, but the BMV shows net approximately 0.8 A flowing out of the batteries and the LiFePO battery voltage is about 13.25 V. When I remove the dongle, the batteries net current is positive a few amps that varies, and the voltage goes above 13.5 V.

It seems this 0.7 V offset is preventing the inverter from putting out the proper amount of current.

Do I need to directly sense the battery voltage on the Multiplus-II battery sense lines to override this erroneous voltage?

Multiplus-II VE.Bus Smart Dongle

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allie commented · May 27, 2022 at 02:27 PM

Same problerm. VE Bus reads .06 or .07 higher than all other instruments/boxes. I used to sync it up with other peices but it was dropping the bulk charging too soon with the faulty reading. If I disconnect it, then reconnect it (resetting it) then it will read correctly for a while. I have dropped it off the bluetooth network so it can’t share it’s bad info with the charger. The one thing it is supposed to do it rarely stays on point long enough to make the money spent on it worth while. Waste of $.

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Answer 1 · 2021-11-09T06:12:32Z

kevgermany answered · Nov 09, 2021 at 06:12 AM

Faulty unit?

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billsulltx commented · Nov 19, 2021 at 11:55 AM

I also thought it might be a faulty unit so I described the problem to the dealer. After some discussion, the dealer recommended that I first re-configure the Multiplus II since its configuration gets overwritten and set to default with the firmware update that was applied when I connected the VictronConnect app via the Smart Dongle (not very bright way for Victron to do that, but it's a topic for another thread). So I'm awaiting a VE Interface MK3-USB to be able to reset the configuration for my batteries before taking the next step to determine the cause of the errant reading.

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Answer 2 · 2021-11-10T14:38:38Z

bluztoo answered · Nov 10, 2021 at 02:38 PM

I’m having that same situation. My V.E. Bus is on the driver’s side with the relocated batteries so I get a direct temperature to my multiplus. It always registers either .59v or .6 higher than at the BMV-712 which has its shunt in the passenger’s side where the original batteries were located. There are no loads on the batteries prior to the shunt. The connections are good and I used 10 feet of 2awg to displace the batteries. What explains the difference? I have four lithium batteries in parallel that I balanced individually. I have a 300 amp inline fuse after the BMV’s shunt.

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Answer 3 · 2021-11-18T14:34:20Z

bluztoo answered · Nov 18, 2021 at 02:34 PM

Anyone out there have a sense of what is going on for kevgermany and me. I'm guessing having different voltage on the bmv-712 and the v.e. bus is not unusual? My setup stays off by .61v, higher at the bus.

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Answer 4 · 2021-11-19T12:17:27Z

Alexandra answered · Nov 19, 2021 at 12:17 PM

Is this happening during charge or when nothing is going on? Not loads on either side?

You get voltage drop over cables and it is directly proportional to the current passing over it.

Under heavy load (with not charging or solar assist) does the reading differ the other way?

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billsulltx commented · Nov 19, 2021 at 03:54 PM

That's a good question. When driving a heavy load (115A from 12 V) the VictronConnect app shows 0.8 V higher than the BMV-712 as opposed to 0.7 V higher under low load, and 0.6 V higher when bulk charging (about 45 A). The voltages are rounded to 0.1 V.

So the cable/connection IR drop is a small part of the problem. The rest of the problem lies somewhere between the inverter's 12 V terminals, the Smart Dongle, and the Victron Connect app.

My cables are relatively short (longest is 3' for 2 parallel 1/0 cables) so IR drop has little to do with it. The voltage drop as measured with DVM between the BMV-712 sense location on the batteries and the inverter 12 V terminals is never more than 0.1 V.

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Answer 5 · 2021-11-19T16:02:17Z

bluztoo answered · Nov 19, 2021 at 04:02 PM

For me it is ever present, charging or not. I understand about a drop. Maybe I'll bulk up the connection with jumper cables and see if the voltage disparity disappears. But I ran 2awg wire and based on all the calculators I've seen that should have covered it.

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Answer 6 · 2021-12-12T10:50:33Z

bluztoo answered · Dec 12, 2021 at 10:50 AM

Interesting - I doubled my cables, so two 2awg positive and two 2awg neutral (negative). It made no difference. The only difference between what is measured by the dongle and the BMV-712 is the resistance traveling across the shunt. I don’t feel bad about the extra effort though. With a potential demand of 400 watts across the four 200w batteries I’m guessing the overkill doesn’t hurt. I still can’t understand the difference in voltage of .6 between the dongle and the shunt.

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kevgermany ♦♦ commented · Dec 12, 2021 at 02:59 PM

Guessing it's instrument error. Question is which right, more likely which is closest to right. I

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Answer 7 · 2022-02-10T21:32:16Z

briantax answered · Feb 10, 2022 at 09:32 PM

I am having the same issue, it is lowering charging current and slowing charging. BMV, Multiplus, and two multimeters are all in sync. Dongle is reading high. I placed the leads at the battery and at the shunt, it is still wrong.

Did anyone have a resolution? I feel it is a bad dongle.

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billsulltx commented · Mar 31, 2022 at 04:14 PM

My dealer told me that the Multiplus-II does not use smart dongle voltage readings. I'm skeptical that this is true because the inverter behaves differently (charges to a higher voltage) with the smart dongle disconnected. I got a data logger and will post some battery voltage data to show this.

Has anybody else found a resolution to this 0.7V error?

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Answer 8 · 2022-04-25T20:31:27Z

nathan-hemming answered · Apr 25, 2022 at 08:31 PM

Did anyone ever find a solution to this problem besides disconnecting the smart dongle? I just had the same issue with my dongle (connected at multiplus power) reading about .6 volts too high and reporting to my MPPT the incorrect elevated voltage thus not allowing to charge my battery bank. I confirmed with my meter that all battery, bus, inverter and mppt voltages were in fact at 13.8 volts however multiplus was indicating 14.2 volts on the app and the cerbo display. I disconnected the smart dongle and all is now reading within 0.03-0.05 volts.

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Alexandra ♦ commented · May 29, 2022 at 05:08 AM

@Nathan Hemming

Get the smart dongle changed.

See here.

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billsulltx Alexandra ♦ commented · Jun 16, 2022 at 07:27 PM

Thanks for the link to the other thread. Nice to see some indication that Victron admits there is an issue. My dealer argued that it couldn't be a problem with the dongle. They wanted to blame inadequate wire size or some other fault of mine. The voltage error exists nearly independent of charge current so it's not an IR drop issue. They did eventually offer to have me return the dongle. I've bought a new one to see if it has the problem too. If it doesn't, I'll keep both since the original dongle that reads too high is the only way I can charge the batteries without the Multiplus-II's annoying charge hum (60 Hz plus harmonics that is only 2 dBA better with the new v497 firmware). If I need the batteries fully charged, I'll use the new one.

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