Can I trust my BMV 712?

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mad asked Dec 18, '20

Can I trust my BMV 712?

Hi all,

I hope you can help me sorting out some questions I have regarding my setup, or better, the values shown on my BMV-712.

I built up a solar system using:
- One Q-Cells Q8 module with 355Wp
- One Victron smart MPPT 100/50
- One AGM-battery 12V 202Ah at C20 from offgridtec.com
- One Victron battery monitor BMV-712 including temperature sensor
- One battery protect BP-65 connected to the BMV-712
- One raspberry pi with venus Software connected to the MPPT and the BMV-712

Later on I added a second battery of the same kind and put it in parallel mode (ca. 2 weeks after initial setup), this means I have now 404Ah total capacity.

And now my questions, cause I am wondering if the values or the charge level shown on the BMW-712 is correct.

Now for example the battery monitor shows a state of 77% but got already -138,6Ah, last full charge was 4days 5 hours ago.
I normally have a continous discharge between -10 and -25 watts, sometimes when I need light I had around 150watts maximum consumption.

The temperatures of the batteries are between 0 and 8 degrees Celcius.

For example some days ago still in 1-battery-setup I had at a voltage from 12.2V a state of charge from 75% at a power consumption of 2A (24Watts).

Does this sound like my battery monitor is working correctly?
Its a lot more capacity as I thought I would have, I am getting a little nervous if more than half of the batterys capacity is consumed (202 of my 404Ah).

I don't want to brick my batteries of course :)

Beside the setting of cutting of the less important loads under 60% of SOC, can you recommend a low voltage cut-off-setting? Or is it not important?

Thanks in Advance

Martin

BMV Battery Monitor

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Answer 1 · 2020-12-19T11:42:09Z

hvdt answered · Dec 19, 2020 at 11:42 AM

Your bmv (and also batteries) require a full charge abt. 1 time per week. This will help fight sulfatation and calibrate your bmv back a 100% soc.

the bmv has a 500a shunt, with a precision of 0.4% (=2a), as your average current draw is around 1-2 a, you are operating inside the precision limits.

the combination of the above makes your six reading draw off from reality.

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JohnC ♦ commented · Dec 20, 2020 at 01:13 PM

Hi @HvdT

I read that 0.4% as the accuracy of the 'measurement', not as a % of the shunt capacity, and it's defined that way. That's pretty accurate, and for low readings, better than the displayed resolution.

I may be wrong, but I hope not.. :)

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Answer 2 · 2020-12-19T22:36:59Z

mad answered · Dec 19, 2020 at 10:36 PM

Thanks for the information.

Getting the batterys full is no problem as I am charging them using a power generator and the BMV is able to synchronize.

A bit strange is that the actual power consumption seems to be correct (the above mentioned -10 till -25 Watts) but the SOC seems not.

But thanks for the information about the shunt precision, I still will have a look at the voltage drop and try to avoid a fully discharge by not looking only on the SOC.

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Answer 3 · 2020-12-20T12:54:41Z

JohnC answered · Dec 20, 2020 at 12:54 PM

Hi @Mad

Using the displayed 'Consumed Ah' and comparing it directly to the rated C of the battery is a trap for the unwary. A bit technical, but bear with me..

Firstly, Ah needs Voltage to make it a reading of energy, so the V of discharge (and that varies) determines the actual Wh discharge. So, with a V estimate in hand, then you might do the sums, and still can't find a good match for the displayed Ah vs %.

The reason is the Peukert Exponent, which relates to the speed you're discharging your batts. Drawing out load slowly (as you are) may leave the batts in better condition than Ah alone will show.

I can't say what the Victron coders have done here, but I expect they've got it right (well I hope).

In the washup, don't trust the Ah, trust the %. By all means distrust the settings you've input to get the %, and it's up to you to get them right.

Have your BMV sync to 100% regularly. The Tail setting important here, it really can only be 100% if the batts can't accept much charge (It Tails off).

Familiarity with the manual is essential. and really, only understanding what's going on will instil true confidence. It's good you've asked, and you can keep asking until you're happy. Google can help too.

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