Negative DC Loads while charging, positive while discharging (increases with load)

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hummingbear asked Sep 28, '22

Negative DC Loads while charging, positive while discharging (increases with load)

Before I start I want to say I've read through quite a few posts already where folks are describing negative DC loads, however I have not found anything that quite explains what's going on.

Some background on my setup: I live off-grid and run my home off our solar/battery setup. I recently switched over to a new 48V battery system I build, we have all new hardware, Multiplus II 48V, two victron charge controllers, smartshut...etc.

I see negative load on the DC side (indicating a charge coming from DC). This is odd as other than our charge controllers we have no other DC charging sources. However if we have a large AC draw (3Kw when running our well pump) DC load spikes to around 100-200W.

I'm perplexed because I never saw this on our previous system. What could be going on? Below is a schematic of our system:

During the evening (no solar input):

During the day (solar input/net positive load to battery)

I am using my own graphing system but my Venus device displays the same data as well.

SmartShunt dc system

1664384315266.png (106.1 KiB)

1664387473762.png (71.9 KiB)

1664387542761.png (69.6 KiB)

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knissone commented · Sep 28, 2022 at 06:01 PM

This is quite similar to several other posts I've seen recently, including my own post here. Not that I have any clue what I'm really talking about here, but I'm really starting to wonder if there's a firmware issue, as widespread as this problem seems to be.

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Modifications Space

Node Red Space

Answer 1 · 2022-09-28T17:25:36Z

Matthias Lange - DE answered · Sep 28, 2022 at 05:25 PM

The obvious question is to check if the shunt is installed in the correct orientation.

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hummingbear commented · Sep 28, 2022 at 05:46 PM

Yes...we have significant AC loads and they are reported correctly. But yes, system is on the system side, battery is on the battery side.

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Answer 2 · 2022-09-28T19:11:49Z

bathnm answered · Sep 28, 2022 at 07:11 PM

How is the shunt configured? Is it as a shunt or a dc load or dc source. This can change how data is presented and reported

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hummingbear commented · Sep 28, 2022 at 07:32 PM

Can you expand on what you mean a bit? Configured where, in VenusOS? It is a DC Shunt on the last negative before our battery, before any system loads.

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kevgermany ♦♦ hummingbear commented · Dec 31, 2022 at 09:56 PM

If you connect to the shunt using Victron connect, go into settings, you'll see the option. It can be set up as a battery monitor, or as a meter. As a meter, not attempt is made to calculate SOC etc. It just measures load, Voltage.

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Answer 3 · 2022-12-31T19:13:44Z

love2learn answered · Dec 31, 2022 at 07:13 PM

Did this ever get figured out?

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Answer 4 · 2022-12-31T22:55:55Z

Kevin Windrem answered · Dec 31, 2022 at 10:55 PM

If by "DC Loads" you refer to the value displayed in the DC System tile, then this value can be wildly wrong and vary greatly. It is calculated based on other DC system contributions and is basically what is "left over" when all other power is accounted for. The trouble is the samples for other powers are not synchronized so changing powers which causes an error in the value displayed. Errors add when AC loads and or battery charging current are high.

The DC System tile is intended for installations such as an RV or boat where DC loads exist. In a system that has only AC loads and only have charging sources such as Victron MPPT controllers, you can turn off "Has DC system" in the GX device's system setup and the DC System tile will be hidden.

If you do have DC loads or charging sources that do not report their values to the GX device (Orion or MPPTs other than Victron, alternator, wind generator), then you can insert additional DC shunts set up for DC meter of the appropriate sub type. If one of these is DC System, then the calculated value will be replaced by the value measured by the shunt and will be accurate and stable.

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mvader (Victron Energy) ♦♦ commented · Jan 01, 2023 at 08:23 AM

i think the issue is what @Kevin Windrem described, and also recommend disabling that tile.

There is more explanation in the Cerbo manual.

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