Michelle Konzack avatar image
Michelle Konzack asked

SOPzS Batteries and SOC mismatch

Good day,

I am Off-the-Grid (24V) and have only a simple System running which work perfect, except for the SOC.

I use 4 battery sets of each 735Ah at a C100 and use 4 BMV702 plus 4 BatteryBalancer.

Currently I charge my Batteries (I sucked 1680Ah out of 2940) from a Generator but one thing pzzzle me since quiet a while.

If i have a nominal C100 capacity of 2940Ah, i can get only 2352Ah (80%) out at 20°C, hence in the summer it is perfect.

Now in the Winter I get only 60%, hence 1764Ah, out, and then my batteries are at 22,2V.

However, the SOC state, that I have still 45% left.

Do my four BMV702 work correctly or do I miss a setting?

I have this question, because the Grid Provuder Elektrilevi has installed a 75kVA Transformator near by and it is standing on our ground, hence I can dig a 200m trench myself, but an AXPK 4G25 in the ground with a connection box at the end and have only to pay 1500€ for connecting (plus my cable and digging) it instead of 36000€ letting them doing it.

And I can connect to the Grid to feed-in.

Also in Spring I will get my new batteries with 7329Ah (8 sets) with in toral 175kWh.

So, how do I then config (Dynamic) ESS for this odd SOC?

Note: On Tuesday 2923-11-21 we had energy prices whic i have not seen in the last 6 years in Estonia:

06 - 07: 21.722

07 - 08: 66.004

08 - 09: 58.559

09 - 10: 65.995

10 - 11: 47.75

11 - 12: 48.218

12 - 13: 58.556

13 - 14: 48.01

14 - 15: 39.598

15 - 16: 58.553

16 - 17: 93.262

7 - 18: 41.461

18 - 19: 24.002

Hence with my singel MultiPlus-II 24/5000 i could have feed-in 13h 3,8kW or 49kWh and earned 25,5€.

However, I plan to install three Fronius Symo 10-3 ACin coupled and a 3,5kW Windmill ACout cozpled (should deliver in my location at kwast 16MWh a year) in addition to my existing 1,86+4,10+5,5kWp Solarpanels and 5 windmills with 7,4kW in total.

Currently I have a trailer with 2175Ah of batteries with a Quatro 48/15000 here which emulate the Grid, but i can not get my system correctly wirking because of the SOC, the training videos on victron-professional did not help me with this problem.

Thankyou for your help and tips

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2 Answers
pwfarnell avatar image
pwfarnell answered ·

Answer corrected in the comment below

The BMV only uses a single value of battery capacity, it does not include a calculation of SOC at different temperature even if the BMV702 has a temperature sensor for charge voltage compensation.

the only thing that I can suggest is that you manually update the battery capacity setting during the year as the temperature varies, maybe 4 times a year so that you are always close to the actual temperature and any errors are small.

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pwfarnell avatar image pwfarnell commented ·

@Michelle Konzack I have to retract my answer above, I have just been reading the BMV-702 manual and Section 7.6.8 states that if you have a temperature sensor connected to the BMV-702 you can set a capacity compensation factor that reduces the effective capacity at lower temperatures. A not very common feature.

If you have a temperature sensor then look at this, otherwise you will have to adjust manually as noted above.

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michelg avatar image
michelg answered ·

Hi Michelle,

Usually, the capacity is based on C20 / C10. I also use OPsZ batteries, and had to change the Peukert factor to 1.14, charge efficiency factor to 80, and battery capacity at 1250Ah at C20, tail current 4,00%, charged detection time 45m, charged voltage 29,7V ( I am on a 24V system). Discharge floor 50%. Current threshold 0,20A

The Peukert was difficult to find as you can see different numbers on Internet, but for my setup, this one is the most accurate I could find.


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

You can calculate the Peukert for your battery.

You only need two of the C-ratings of your battery (printed on the battery or in the manual)

How to do the calculation is described in the BMV/SmartShunt manual.

Or you find a Peukert-calculator in the web.

1 Like 1 ·
Michelle Konzack avatar image Michelle Konzack Matthias Lange - DE ♦ commented ·
Thankyou @Matthias Lange - DE

I did not know this document.

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Matthias Lange - DE avatar image Matthias Lange - DE ♦ Michelle Konzack commented ·
Reading the manual sometimes helps :-)
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Michelle Konzack avatar image Michelle Konzack commented ·
Thankyou, I will try the same settings
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