SOC% miscalculation

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Zoltán Vaczulin asked Dec 18, '23

SOC% miscalculation

Config: MultiRS Solar II + 10kWh LiFepo4, no BMS datalink

Situation: Saturday evening (no PV, after sunset) family is watching movies. No serious power consumption, more or less stable 500W. The battery is about 70%, the plan was running on battery until sunrise. The house consumes 200W...300W overnight, the fridge is switching on/off.

Suddenly we had blackout. Until now I don't understand the reason. The following found:

- I could connect to the inverter by VictronConnect app via Bluetooth

- the inverter switched to passthru from "on" state

- AC1IN didn't connected (but was in passthru! - strange)

- SOC% in inverter dropped from about 70% -> 41% (BMS was at 69% - no error code)

OK, I switched the AC1IN control off and switched to passthru - and it was working until morning. After sunrise I switched to ON state, because the PV doesn't charge in passthru mode (its another issue). It charged the battery full (up to 71% :) starting absorption at right voltage and then at float stage it was switching to SOC%=100%) within a second, I could see it.

This is not the first time to SOC% to fail, but I thought it needs time to calibrate the battery. Sometimes it gives real out of context numbers. LiFePO4 batteries has very similar chemistry all over the world and the calibrations are close to each other, eg:

How can it miscalculate SOC so seriously?

Why can't I set any parameter to help SOC calculation? (If it should work well...)

Can anyone give me a reason for a sudden blackout?

SOC Multi RS

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Alexandra ♦ commented · Dec 19, 2023 at 06:11 AM

@Zoltán Vaczulin

You are going to have to share your battery settings. If it is calculated I corrected the parameter entered are likely not quite right.

I have a multi rs and do not have this issue. The SOC matches my lifepo4 fairly close sometimes 1 or 2 % difference.

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Zoltán Vaczulin Alexandra ♦ commented · Dec 19, 2023 at 08:26 PM

I didn't change anything, unless trivial settings: LiFePO4, 200Ah. I reduced the max charge current to 40A, while the factory limit is 100A. Maybe it can extend the battery lifetime. I don't see any place to limit the discharge current. There is a 125A fuse inline. Factory grants 100A for constant discharge, the highest was about 80A...90A for a short time (microwave + teaboiler + toaster + washing machine + usual others :)

Do you see anything wrong in settings?

My problem is not the difference (30%) but the fact of a sudden drop. Can I go for holidays trusting the device or it needs continuous supervision?

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Answer 1 · 2023-12-19T12:09:34Z

Alex Pescaru answered · Dec 19, 2023 at 12:09 PM

You could try the following...

Charge the battery to 100%, battery BMS indication, to be sure the battery is full.

Use VictronConnect, connect with Bluetooth with the inverter, go to Battery settings, enable Expert Mode and go to the bottom, where you have Battery Monitor options and use the Synchronize SOC to 100%.

Maybe this will help...

Also you could check the voltage drop on the cables, because the inverter calculations are based on the voltage at the connection screws and it could miscalculate on bigger loads when the voltage drop on cable is greater. So a too low battery input voltage could trigger a shutdown, hence the blackout.

I believe AC1 IN relay is a different relay than the passthrough relay and is connecting the grid with the input of the inverter. Passthrough relay connects AC IN to AC OUT and AC IN relay connects AC IN to inverter.

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Answer 2 · 2023-12-19T21:37:17Z

Zoltán Vaczulin answered · Dec 19, 2023 at 09:37 PM

Well, it was charged to full, and it Synchronized SOC to 100%.automatically. I don't think if I press the button, it is better, stronger or whatever. But good to know about this feature, thank you!

I checked Voltagedrop on battery cables. I use 50mm² copper cables, half meter long, a bit bigger, then necessary. No remarkable drop @30A (~1500W) ~0.01V. There is a separate input on the inverter to connect the battery terminals for voltage check, and if any guru says that connecting this can improve the stability - I'll do it.

If I calculate the battery internal resistance (~0.015 Ohm) you may see that it cause 0.45V drop on battery terminals, which is much more than the drop on the cable.

OK, this feature in passthru mode that I can force the inverter to switch the AC1IN off, it is not logical, but if it is a feature, I can live with it. Another point to take care, but no problem :)

Anyway passthru is missing sometimes from the list of "on/off" functions. Is there a logic behind?

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