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billsulltx avatar image
billsulltx asked

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-IIVE.Bus Smart Dongle
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allie avatar image allie commented ·
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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kevgermany answered ·

Faulty unit?

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billsulltx avatar image billsulltx commented ·

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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bluztoo answered ·

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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bluztoo avatar image
bluztoo answered ·

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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Alexandra avatar image
Alexandra answered ·

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 avatar image billsulltx commented ·

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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bluztoo avatar image
bluztoo answered ·

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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bluztoo avatar image
bluztoo answered ·

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 avatar image kevgermany ♦♦ commented ·
Guessing it's instrument error. Question is which right, more likely which is closest to right. I
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briantax answered ·

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 avatar image billsulltx commented ·
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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nathan-hemming answered ·

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 avatar image Alexandra ♦ commented ·
@Nathan Hemming

Get the smart dongle changed.

See here.

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