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

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marekp asked Apr 24, '21

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

Multiplus-II BMS

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bms-tmin-error.jpg (58.6 KiB)

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marekp commented · Apr 25, 2021 at 06:32 AM

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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Answer 1 · 2021-04-24T14:23:47Z

Matthias Lange - DE answered · Apr 24, 2021 at 02:23 PM

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 commented · Apr 24, 2021 at 03:41 PM

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

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Matthias Lange - DE ♦ marekp commented · Apr 24, 2021 at 04:31 PM

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

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 Matthias Lange - DE ♦ commented · Apr 24, 2021 at 05:59 PM

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 marekp commented · Apr 25, 2021 at 08:23 AM

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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marekp Paul B commented · Apr 25, 2021 at 10:08 AM

Hi @Paul B,

Thank you for your interest and suggestions.

This problem will be gone all together with rising spring temperatures. :)

This mornings SOC was again reset to 100% and at the same time BMS shows SOC at 17%.
For 3 hours (from 5 to 8) MP-II was at Float probably because the "charge not allowed" signal from BMS was still active. The temperature of one cell was below 5C at that time.
At 8 MP-II switched to Bulk and at that time the SOC, reported by MP-II, was still 100%. :)
It looks like the SOC number is not used by MP-II to determine the method of charging. It could be that the battery voltage is.

OR install a BMV 712 external battery Monitor

Does it have to be this model?

Can I use Smart shunt?

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 would be the best solution, Do you have the "inside scoop" of the work being done to make it so?

I cannot understand why Victron is not cooperating with the maker of 123smartBMS. They are from the same country. :(

As for your last suggestion, I see a problem with the temperature again, but this time Tmax.

If the Float voltage is equal to Absorption, the passive balancing of the cells happen not only during Absorption but also during Float.

This BMS has a problem with long balancing periods because the temperature of the boards on each cell is getting higher than Tmax and both signals "charging not allowed" and "discharging not allowed" are activated and everything stops.

Regards,

Marek

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Paul B marekp commented · Apr 25, 2021 at 10:54 AM

yes we use the 123 smart as well. the newer G3 version dont seem to give that temp issue as much

what I do is try and balance the bulk to be just at the balancing point ie 3.50 volts to start balancing so that all cells are just at 3.50 total 56 volts as a example. this then causes cells to drop in and out high ones balancing low ones and thus are NOT balancing when in float and if all at at 3.5 then they also stop balancing.

there is NO point in having all cells balancing as then they are all being discharged at the same rate and thus nothing is happening. some have to be green and others orange for balancing to be efective. well thats my thoughts on them.

yes a smartshunt would do (same as a BMV but no screen)

re the 123 interface to cerbo etc we are getting it done ourselves and paying for the interface software, it will be so much better to be able to see the cell voltages remotely in a live feed, (no VRM graphs though), this we hope will also have a very easy install interface. contact me directly if you want more info service@octopusasia.com

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marekp Paul B commented · Apr 25, 2021 at 01:20 PM

Hi @Paul B,

Thank you again.

I have G3 version.

Can you help me with setting this BMS properly?

BMS setting I have are:

My VEconfigure settings are:

Inverter:

DC input shut-down 47.0V

DC input low restart 50.0V

DC input low pre-alarm 49.6V

Charger:

Absorption 56.5V (should I set this to 56V?)

Float 54V

The battery is Winston 48V/200Ah

Best Regards,

Marek

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