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ezwryder avatar image
ezwryder asked

Multiplus-II still autorestarted after failed attempts

I have a Multiplus II 120/3K in an off grid cabin. Over the weekend I forgot to turn off the heat pump when I left and the load ran the battery pack down overnight. The inverter shut down around 5 AM and after a couple of failed restarts, it remained offline all day.

As I understand it, at this point the inverter would require a manual reset to restart. But that is not what happened. I had a friend go to the cabin and pull the AC disconnect on the box outside above the heat pump compressor. Magically, the inverter restarted on its own.

Looking through the documentation I only saw the reference to manually restarting the inverter after 3 failed attempts. So I was curious to know how this unit knew that the heat pump circuit was disconnected and restarted on its own. I have no problem with that, I’m glad it did. Thank you.

Multiplus-II
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nickdb avatar image
nickdb answered ·

It depends why it is shutting off. Overloads and relay test failures will need manual intervention.

I can only think that it was a voltage related battery shutdown and the heatpump kept drawing it down. Without the heatpump it was able to get above the battery restart limits.

Its a mystery:)

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ezwryder answered ·

It was definitely because the heat pump was pulling down the battery pack, but when my friend pulled the AC disconnect, the pack was fully recharged and nearing absorption. So it long ago passed the threshold set for autorestarts. Nothing was actually running, the inverter was still shut down, then after pulling the disconnect, it restarted on its own.

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ezwryder avatar image
ezwryder answered ·

This is what it looked like from the data. Off around 5:30 AM with two attempted restarts around 8-8:30. At that point the inverter shut down for the remainder of the day.

During this time, the battery pack completely recharged. After 6 PM when my friend pulled the AC disconnect. He did not manually reset the inverter. So there has to be something in this scenario that the inverter was able to sense.


screenshot-2024-03-12-at-84137-am.png


screenshot-2024-03-12-at-84137-am.png (129.0 KiB)
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nickdb avatar image nickdb ♦♦ commented ·
Your AC disconnect only disconnects loads at AC OUT?

Send a copy of the vebus state widget and the alarms and warnings widget.

If the inverter is off, the AC OUT relay is open and it has no way of sensing anything.

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Al avatar image Al commented ·

What are the cell voltages during shutdown and at 100% SOC?

Is the BMS properly reporting SOC (Synced at true 100%)?

What is the BMS / Battery, how is it reporting to the Multi / VenusOS?

What do the alarm logs show?

What was the VE.Bus State at the time?

What are the voltage thresholds for Multi shutdown and restart set to in VE.Config?

*Edit to add: Do you have undersize DC cables or poor connections causing a voltage drop when a high load like the heat pump is on?


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ezwryder avatar image ezwryder Al commented ·
I don't have remote access to data on the battery pack but I do know that there is no issue with the pack. I go by the Victron shunt for SOC and it is accurate. The battery BMSs do not report to the Victron components. Cables are properly sized and I have run the heat pump often. I just forgot to turn it off, it drained the pack, and the inverter did something interesting. I am just trying to understand why it did.


I need to confirm and revise the DC input low shutdown and restart values. I did not change them at installation so if the default settings were still 9.3 shutdown and 10.9 restart, that is what they are currently.

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Al avatar image Al ezwryder commented ·

"I don't have remote access to data on the battery pack but I do know that there is no issue with the pack"

How, if you have no way of checking it?

From the screengrabs you posted below, it looks to me like you're running the battery down to 2.6V per cell and most likely the BMS is shutting off, then coming back on at the reset voltage around 2.9V

So maybe not Multiplus related.


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snoobler avatar image snoobler Al commented ·
Gotta say, you're either not reading the OP, not putting things together, or you're deliberately raising a red herring.


I can see from VRM the battery voltage is over 13V for about 8 hours before the inverter restarts./


This is not a BMS issue.



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ezwryder avatar image ezwryder snoobler commented ·
Right, thank you. I have posted to a few forums and most people think I am trying to solve a problem. I don't have a problem, I just had an interesting thing happen and wanted to know why. I did find that pulling the VE.Bus status, as one person suggested, was useful. It tells me that in this case the inverter wasn't really "off" off, it was in a fault state for the day. So not running, but it was also apparently monitoring the fault state. I guess when my friend pulled the AC disconnect on the heat pump, the inverter must've interpreted that as "fault removed" and restarted. I was just wondering how that detection occurred and why it restarted when the manual says you have to flip a switch.
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Al avatar image Al snoobler commented ·

Ignoring Snooblers petty comment.

I looked at the possible pre-curser cause of this, feel free to dismiss that if you're only interested in looking at the effect.

The BMS charge mosfets could have stayed closed after a LVD, but the discharge mosfets could have been kept open, then the battery voltage sync to 100% SOC closed them again and allow the inverter to come back on. Coincidentally around the same time the heat pump was also disconnected.

This suggestion could be wrong, but I'm not ignorant enough dismiss other possibilities.

With little information given on the BMS settings or how the system is wired it's hard to tell much, apart from suggest possible causes.

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ezwryder avatar image
ezwryder answered ·

Here are some of the VE.Bus charts for 3/11.

screenshot-2024-03-16-at-14538-pm.png



screenshot-2024-03-16-at-13406-pm.png


screenshot-2024-03-16-at-14521-pm.png



screenshot-2024-03-16-at-14538-pm.png (70.4 KiB)
screenshot-2024-03-16-at-13406-pm.png (105.9 KiB)
screenshot-2024-03-16-at-14521-pm.png (48.7 KiB)
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Matthias Lange - DE avatar image Matthias Lange - DE ♦ commented ·

Check the setting for restarting after low battery voltage.

There is a jump of battery voltage over 14V at around 18:00.

1710616506655.png

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1710616506655.png (58.5 KiB)
snoobler avatar image snoobler Matthias Lange - DE ♦ commented ·
That's very common for lower current LFP charging - extremely short absorption period. It may also be indicative of BMS overvoltage protection, but the spike is usually higher.



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ezwryder avatar image ezwryder snoobler commented ·
Yes, this was the end of the charge curve. It is just a coincidence that the pack was reaching capacity at the time my friend pulled the AC disconnect on the heat pump. Without that intervention, the inverter would've remained off and the pack voltage would have peaked at around 14.6 and entered absorption for a while. That was interrupted when the inverter restarted.
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