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rh5555 avatar image
rh5555 asked

BMV-712 SOC during charging reads too high

I have been testing various converter setups to charge 2 6V AGM batteries connected in series. Total capacity is 225Ah, Peukert exponent = 1.12. I discharged the batteries to 20% state of charge, then connected the converter and monitored battery voltage, charge current and SOC as reported by the BMV-712. I also calculated the state of charge using the Coulomb counting method, assuming a charging efficiency of 95%. I would have expected my calculated SOC and the SOC reported by the BMV-712 to match, but they don't:

The BMV-712 SOC reads consistently high and reaches 100% while the converter is still in bulk charge mode. My calculated values seem much more reasonable and believable. Interestingly, if I set the charging efficiency in my calculations to 120% (crazy, I know) then my calculation of SOC matches the BMV-712 exactly.

Anyone any idea what is going on here?

SOC
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thomassolar avatar image
thomassolar answered ·

I can not see anywhere you mention the "charge efficiency setting" in the BMV..? (Only in your calculation "outside" the BMV you talk about that) You only mention adjusting the Peukert.

Maybe that efficiency setting is too high?

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rh5555 avatar image
rh5555 answered ·

The charge efficiency setting in the BMV is 95%, which is the default value. If I were to set that to 79%, I could fool the BMV into reporting the SOC that I measured, but that seems to be rather extreme. I'd really like to know how Victron are calculating SOC in the BMV-712.

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Gregory Cavanagh avatar image Gregory Cavanagh commented ·

Why do you think that is extreme and fooling it?

Thats what the setting is there for?

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rh5555 avatar image
rh5555 answered ·

Just a follow-up on this issue. I recently re-ran this test after upgrading the firmware in the BMV-712 to 4.07 and the issue has been resolved. We now agree 100% on the SOC during charging. I had tried to bring this issue to Victron's attention, but never seemed to gain any traction - I'd send information, they'd be quiet. I'd call, they wouldn't be available. Seems like someone was listening though.

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JohnC avatar image JohnC ♦ commented ·
Hi again @rh5555 Welcome back.

I went for a look through the firmware changelogs and can't see anything reported about algorithm changes. Nor in the Smartshunt's.

What you may be noticing is a seasonal change, like battery temperature and solar charge rates. I've learnt a little from my own system since I last posted here - notably, overall efficiency of pb's improves in cold weather. This might be a combo of chemistry and slower solar charge. Or other things too.

In the cold I have to raise set charge efficiency from about 91% to 94%, then if that's not enough I raise Peukert from my specced 1.16 to 1.20 (my loads are way less than C20).

Temperature is underrated as a factor, but Peukert is known to be subject to (unquantified) temp variations, and all chemical reactions slow in the cold.

So we have to learn to live with it. I was quite surprised recently after a two week bout of cold and foul solar weather, that when I managed to get back to full charge I was within 0.3% of reported SOC. I can live with that, but I expect to need to change settings as summer comes on. Now all I want is for Victron Connect to report SOC to 1 decimal place rather than single digits.

And oh, the poor rte readings I reported earlier weren't really that bad (a VRM thing). The shunt readings vary from ~81% summer to ~83% winter. Still not good, but at least something to reference as they age.

Thanks for reporting back though. I wish you well..



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