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MultiPlus II Inverter cutoff NOT at the low-shutdown voltage

I have started setting up a 48V LiFeP04 battery system with a MultiPlus II 48/5000/70-50. Before the installation, I am now doing various tests to understand all the features and parameters of setting up the MP-II. When I did the low-shutdown voltage test, I found that the MP-II shutdown the inverter before the set voltage.

I have voltage sense wire connected to the +ve and -ve ends at the battery pack. I set the low-shutdown voltage to 44v in VictronConnect. I do not use ESS Assistant. When the load is around 70A, the voltage at the battery connection terminals of MP-II is around 0.65V lower than that at the battery +ve/-ve terminals. The current drawn was kept stable at around 70A. When the voltage shown in VictroConnect dropped to 44.38V, the MM-II shut down the inverter. Before the shutdown of the inverter, the voltage was gradually/slowly dropping so the 44.38V was not caused by a sudden voltage drop. At the time of the inverter shutdown, the voltage at the MP-II battery connection terminals were 43.73V.


Can anyone help and tell me why MP-II shut down the inverter before the set voltage in my case?


Thank you.

Multiplus-II
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klim8skeptic avatar image klim8skeptic ♦ commented ·
Have you connected the V-Sense wires from the multi directly to the battery terminals??
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elel answered ·

Can anyone help? Anyone who is now using voltage sense has the MP-II cut off the discharge at the exact expected voltage?

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elel answered ·

In fact, it is not related to voltage sense or whether/how it is being used. I am in fact talking about the cutoff voltage set in VictronConnect (in this case, 44V) and the voltage shown in VictronConnect main status screen (showed 44.38V when the discharge was cut off). Are there any settings that may cause such behaviour of Multiplus-II?

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seb71 answered ·

What LiFePO4 battery do you have? Does it have a BMS?

What else it is in your system? How it is configured?

Maybe post screenshots with all your settings.


44V is too low for a 16 cell LiFePO4 battery (if yours is 16 cell). No cell should drop under 3V.

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elel answered ·

The battery consists of 16x 277Ah LiFePO4. Yes, it is connected to a BMS. I am doing various tests of the BMS and the MP-II and getting used to all the parameters / settings. The temp. has been stable at around 16-20C. The testing setup is simple, battery+BMS+MP-II. I will not let the battery discharge below 44v when it is put into duty but I am now doing various testings to check the capacity of the cells, the low/high voltage cell/pack cutoff of the BMS, and the low/high cutoff of MP-II.

The main question here is in fact that when VictronConnect main status screen showed 44.38v, the MP-II cutoff the discharge with the low cutoff set at 44v. Why did MP-II cutoff the discharge earlier?

So far, the BMS did its job exactly as expected, but not the MP-II yet.




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seb71 avatar image seb71 commented ·
Not enough data.

Unlike the inverter low voltage cut-off (which is based only on battery voltage), a BMS should be able to initiate a cut-off based on cell voltage, too. So the battery voltage might be above the battery voltage low limit, but a single cell might drop under the cell voltage limit and the BMS should trigger an inverter shutdown or battery disconnect.

-------

Even for "testing", you should not drop to 44V. You risk damaging the cells. But it's your money.

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elel answered ·

Just did another test at higher voltage. I set the MP-II inverter cutoff voltage at 51.90v. The MP-II stopped the inverter when VictronConnect showed 52.16v. If no one have similar experience, I wonder whether my new MP-II is faulty.

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seb71 answered ·

I think that the voltage sense measurement only matters when using the inverter to charge the battery.

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elel answered ·

I contacted the seller. They couldn't give me an explanation of what I found so they asked me to send it back for inspection. Wait and see.

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elel answered ·

I returned the MP-II to the seller and the seller sent it to Victron for checking/repair. After 3 weeks, the seller told me that it was still being checked by Victron and the cause of the problem was still unknown. I had waited too long for the repair, and I requested for a full refund. The seller agreed to that.

I then ordered the same MP-II from another seller. I did the test again once I got it. I was very disappointed that the same problem occurred in this 2nd MP-II. I set the cutoff voltage at 51.70v and the MP-II stopped the discharge when the VictronConnect showed that the voltage was 51.86v. Again, like the previous test I did, the voltage was dropping very slowly and there was no sudden drop of voltage when the MP-II stopped the discharge.

The 1st MP-II cutoff the discharge 0.26v earlier than the setting and the 2nd MP-II did it 0.16v earlier. Both cutoff the discharge earlier. I wonder whether it is in fact caused by something else instead of them being faulty. My BMS (which is only 1/10 of the price of the MP-II) can do the cutoff at the exact pre-set voltage (after a pre-set delay). I am expecting a much more expensive/sophisticated MP-II can easily do the cutoff also at the exact pre-set voltage.

Can anyone share your experiences in the MP-II cutoff voltage? What is your view? Does the early cutoff also happen in your MP-II? Am I just too fussy about the small early cutoff?

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