Battery monitor multiplus: SOC jumping and not accurate

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heinzie asked Oct 29, '22

Battery monitor multiplus: SOC jumping and not accurate

Hi all

In my setups with a color control connected to a multiplus 12/2000, Victron AGM super cycle 230, MPPT 100/50, i noticed some werid changes in soc. While i only have inverter as load i use the multiplus build in battery monitor . No BMV or smart shunt connected because no need to.

Now and then my SOC seems to change.

While i can control the system by VRM sometimes i have to switch of the inverter to protect from battery drain. While it seems when i set the inverter to off i cant see voltage and amp-usage, i tebd to switch to "charger only" while above mentioned seems visible.

For example:

My battery SOC is 100 % while i charged by generator and solar panel. During midnight battery voltage is.12,9 volts and no amps drawn except multiplus standby amps, around 1 amp.

So SOC seems to be correct. I Leave multiplus on because i need power and slowly battery is draining because of ther is not too much sun and i use more power then solar can breng up.

Battery soc seems to be ok and calculations seem to be in order. After a few days battery SOC reacties 65 percent, Voltage around12,25. While i dont like to damage the batteries and i it is not completely nesacery i turn of inverter and switch to "charger only".

After one hour battery voltage has dropped a bit (also because solar power is less) to 12,15 and battery SOC jumped to 85 percents. 85 percents never matches 12,15 volts, even not without knowing amps drawing.

Build in battery monitor is setup correct with 230 Ah.

Above mentioned happens when i turn inverter off , or to charger only. It seems like build in battery monitor is confused after i switch off inverter?

Is there a solution to this problem?

Best regards and thanks in advance.

Multiplus-II SOC battery capacity discharge level

screenshot-20221029-191410-vrm.jpg (182.2 KiB)

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Alexandra ♦ commented · Oct 29, 2022 at 06:41 PM

@Heinzie

Solution is get BMV.

The multiplus has no shunt or way of measuring current down to the battery separate from pv produced power or power direct to loads.

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heinzie Alexandra ♦ commented · Oct 29, 2022 at 06:59 PM

Dear Alexandra

Thank you For your reply.

This sounds a bit akward to me, no offense:

I do not have loads connected to the battery other then the multiplus. The multiplus "sees" the current and voltage it "uses" and communicates this to the color control. That is why i can select "enable battery monitor" in VE configure for my understanding.

Yes, the mppt is charging the batteries indeed by solar panel. If u mean to say the color control doesnt communicate this back to the "battery monitor" and so whe have a miscalculation, then i understand you. Even that i think it is a weird issue the data is not used in calculation but only shown..

2nd:

As long is i leave the inverter on the SOC is accurate so it seems the battery monitor from the multiplus hen uses the info from the mppt my guess..? Or is it coincedence that the reading is accurate because the battery monitor from multiplus uses the voltage readings when it is turnen on..?

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Alexandra ♦ heinzie commented · Oct 30, 2022 at 06:57 PM

This is an often discussed topic on the forum.

Basically the inverter measures the AC load, it has that ability, so it uses that to calculate the battery load. But there is another DC source, the mppt, the terminals cannot differentiate between the two sources.

So SOC is usually voltage derived. So when your battery voltage drops at rest, but the inverter was not on, it messes with the calculations. (Again because it is not a measured thing).

12.25v for a set of agms at rest is quite in quite a discharged state by the way. They need to be kept at a much higher voltage.

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