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marekp avatar image
marekp asked

MP-II jumps SOC value from 20% to 100% when 2 signal BMS said "do not charge"

Hi,

My 3 phase system build on 6 x MP-II/3000 uses Lithium battery with BMS that connects to the system through 2 signals (4 wires). One signal is for preventing charging the other discharging.

ESS assistant has "2 signal BMS" selected.

Recently I had a situation when battery was discharged down to 20% and BMS sent the signal "charging not allowed". As a result MP-II set the SOC of the battery to 100% and charging mode to float. In reality, the battery was practically discharged.

The reason the signal was sent, was not that the battery was full (obviously), but temperature of one cell dropped below 5C.

Does anyone have a solution to this problem?

Regards,

Marek

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Multiplus-IIBMS
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bms-tmin-error.jpg (58.6 KiB)
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marekp avatar image marekp commented ·

Hi,

Do anyone have any suggestion how to deal with this problem?

This morning again MP-II set the SOC to 100% but my BMS reports SOC at 17%.

Again minimum battery temperature at 6AM fell below 5C.

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Matthias Lange - DE avatar image
Matthias Lange - DE answered ·

There is an option during the setup of the two-signal-BMS assistant. "Set SOC to 100% if cell is high" or something like that.

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marekp avatar image marekp commented ·

Hi @Matthias Lange - DE,

I do not remember seeing this during the ESS setup.

There is "force to Float when BMS signals battery full...."

This is not saying that the SOC will be set to 100% but maybe "Float" is synonymous to SOC=100%.

But this signal from BMS was not about battery being full but the temperature of the battery was below Tmin=5C.

When I turn of this "force to Float..." option, what will happen when battery will get to full charged state and BMS would send the signal "do not charge"?

Do you know of any other way to get battery's SOC, that would not react to the BMS's signals?

Regards,

Marek

l1-m-ess.jpg

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Matthias Lange - DE avatar image Matthias Lange - DE ♦ marekp commented ·

OK, this option don't appear in the ESS assistant only in the two-signal BMS assistant.
two-signal-bms.jpg

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Do you get a SOC signal from the battery or do you use the internal battery monitor of the Multi?

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marekp avatar image marekp Matthias Lange - DE ♦ commented ·

Hi @Matthias Lange - DE,

Thank you for your input.

If this option is not available in ESS assistant, than what I am to do with this problem?

ESS assistant does not allow the "2 signal BMS" assistant loaded.

My BMS has only two dry contacts.

One for "allow to charge" and second for "allow to discharge" and each of them has two reasons for happening.

First indicates that the battery is charged (one cell reached a highest voltage allowed) or temperature of the cell is below the T1min or above Tmax.

Second indicates that the battery is discharged (one cell reached the lowest voltage allowed) or the temperature of the cell is below the T2min or above Tmax.

(T1min is a minimal temperature for charging)

(T2min is a minimal temperature for discharging)

There is no SOC information send to MP-II from this BMS.

The SOC state is "controlled" by a MP-II.

I am willing to buy an external device (smart shunt maybe) to keep track of SOC of my battery but I need to be sure that it will not be just another additional expense and the problem will remain. (just like buying power meter for my PV inverter)

Regards,

Marek

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Paul B avatar image Paul B marekp commented ·

Hi Marek, Suspect that it maybe is a software bug in the Multiplus II internal Battery monitor, If it is then there is not much you can do except live with it for the time being.

OR install a BMV 712 external battery Monitor as this then would not be affected. but this would not fix the float issue only the soc reading

OR another option which may occur in a few weeks is that we will (not Victron that is) have a 123 smart battery monitor interface add on app for the cerbo and all the GX range etc,

That you should be able to select as your battery monitor. and thus use the 123 smart instead of the Multiplus II units. Again this may not solve the float issue

OR try this

Another way around it is to increase the float voltage to say the same as Absortion and then in the 123 BMS settings adjust the start to charge Percentage down to say 80% so that once they are full then charging is turned OFF fully until SOC drops to said figure. This will then basically do away with the float figure as a work around.

I dont know if this is of any help just a thought

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