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Jamie Greenaway avatar image
Jamie Greenaway asked

Negative value on the DC system

After upgrading my system to 48v with a new Multiplus II, I have noticed my BMV 600 is often reporting a negative (charging) value on the DC system side. There is currently nothing being used on the DC side, there is only the batteries to inverter, and an additional smartshunt for a solar controller which isn't fitted yet, and an AC charger which isn't turned on. This is a home installation so no other external sources of charge etc. I note that when there is 0 loads it's almost as if the DC system is showing the power consumption of the multi (the multi shows 0w drawing and the DC system shows a small value consistently)

I suppose one way around this is to fit another smart shunt for the AC charger and then turn off DC system so there's no calculated figures but I didn't have this much of a issue with it before.


screenshot-2023-08-26-08-21-45-297-nlvictronenergy.jpg


Multiplus-IIBMV Battery MonitorSmartShunt
screenshot-2023-08-26-08-21-45-297-nlvictronenergy.jpg (310.6 KiB)
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kevgermany avatar image kevgermany ♦♦ commented ·

Your bigger multi has a higher standby power than the older unit.

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Jamie Greenaway avatar image Jamie Greenaway commented ·

I don't think that's the case, the new multi II consumes less than my older multi. But either way inverter consumption has never shown under DC system. And the negative symbol on the DC side would suggest that the the DC system is giving the battery power, not taking it.

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4 Answers
Kienan avatar image
Kienan answered ·

This is common. The DC System or DC Power value is a calculated value, not a measured value. What I mean is that the system knows that the charge controller is reporting that it's delivering a certain amount of power to the system, the inverter is reporting that it's drawing a certain amount of power from the system, and the BMV is reporting that a certain amount is going in/out of the battery, so the system takes the difference and calls that the DC System... so any inaccuracies in the inverter or the charge controller will all pile up on the DC System.


There is a really good work around... Assuming that you're running a relatively updated firmware version of Venus OS on your Color Control, you can get a SmartShunt specifically for monitoring the DC System. You need to go into the programming of the SmartShunt, and select settings/Misc/Monitor mode, and then select "DC Energy meter." Once you do that, you need to change the "DC meter type" to "DC System." Once you do that, the GX device will know that the DC system shunt is the actual consumption or production from the DC System, and it will display correctly on the screen and on VRM.

This would also work with a BMV, but the SmartShunt is cheaper.

This would be better than disabling the DC System because you'd still see the AC Charger on the screen and on VRM more seamlessly.


If you are going to use a Victron MPPT charge controller, then you don't need the extra SmartShunt for the solar, so you could just repurpose that one for the DC System.


Thanks,

Kienan

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Jamie Greenaway avatar image
Jamie Greenaway answered ·

For instance here in this photo, the BMV believes that the battery is delivering less current than the AC loads are using due to this phantom power coming from the DC system.img-20230826-204527.jpg


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seb71 avatar image
seb71 answered ·

There is currently nothing being used on the DC side

Then disable "Has DC system" in Cerbo GX.

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Kienan avatar image Kienan commented ·

He was saying that there was "currently" nothing being used, but he has an auxiliary AC charger. The DC System option is good for monitoring that auxiliary AC Charger.

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seb71 avatar image seb71 Kienan commented ·
I read that. But you can't monitor devices not yet installed.
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Kienan avatar image Kienan seb71 commented ·
It doesn't say it's not installed, it says it's not turned on.
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seb71 avatar image seb71 Kienan commented ·

There is currently nothing being used on the DC side, there is only the batteries to inverter, and an additional smartshunt for a solar controller which isn't fitted yet, and an AC charger which isn't turned on.

One not fitted yet, the other not turned on (so also not used yet).

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seb71 avatar image
seb71 answered ·

So you see my issue of the bmv underreporting inverter/AC consumption due to a random "charge" coming from the DC system where there is nothing connected/turned on.

You got the workings wrong.

The most reliable value (the most accurate) is the one measured by the shunt/BMV.

The BMV does not "report" AC consumption at all. It only knows what enters or exits in/from the battery.

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Jamie Greenaway avatar image Jamie Greenaway commented ·
Yes I haven't said that in the best way, what I really mean is, the inverter is consuming let's say 100w of power, but the BMV is only reporting as 90w of DC power coming out of the battery, due to the bmv thinking there is an additional 20w being fed to the battery from the DC system.. where there is no chargers turned on providing this.
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seb71 avatar image seb71 Jamie Greenaway commented ·

due to the bmv thinking there is an additional 20w being fed to the battery from the DC system

You still don't get it.

BMV only measures the current into the battery (or out from the battery) and the battery voltage.

BMV does not "think" what a DC system might be consuming or producing.

BMV does not "think" what the inverter is consuming.

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