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Endre Kiss avatar image
Endre Kiss asked

BMV wrong SoC

Dear All,

I have an off-grid system with the following setup. EasySolar 24/1600/40 with BMV-712 and CERBO Gx all this connected to a battery with 465 Ah capacity. All software's are up to date.

I have a very annoying behavior (I guess is from the BMV) of the estimated time to go and the SoC. In the BMV the capacity is set correct to 465Ah and the synchronization was done correctly when the battery was fully charged in absorption mode).(GX takes the V/A readings from the BMV)

The actual state -> consumed so far -225Ah (out of 465) but the SoC it still shows me 74% (of course the battery voltage tells me that is unrealistic and not so long ago I lost the communications when it was at 50%) !

1643644728202.png

I have an average consumption of around 40-47W constantly and in two days 48 hours, yields 2.16kWh energy. The battery capacity (theoretical) is 11kW, so that should be ~20% consumed energy. However the graph shows me a drop of only -11% (from 85% down to 74%). No solar charging at all yesterday and today!

1643645142541.png

Where this unrealistic "optimism" comes from?

Thank you, Endre

BMV Battery Monitor
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Matthias Lange - DE avatar image Matthias Lange - DE ♦ commented ·
What batteries do you have exactly?

What are your BMV settings?

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

Hello Mathias,

Lead Acid tubular plate construction. The settings are:

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As you can see voltage is almost 23V and is still telling me 71%.

Thank you, Endre

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

What batteries do you have exactly?
How many? What size are each? How are they connected?

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4 Answers
JohnC avatar image
JohnC answered ·

Hi @Endre Kiss

That "optimism" is coming from the Peukert Exponent setting, and your low loads. The 1.25 setting is Victron's default for all loads, but may not suit such low loads as yours. Effectively you're getting a credit for operating at a discharge rate where your batts have a larger rated capacity. Grab the C20 and C100 (or similar) ratings of your batts and run them through the Peukert calculator (Victron website). Then try that.

But even Peukert isn't really capable of handling low temperatures accurately. The BMV has a (capacity) temp comp capability if you have a sensor and not using the aux port for something else, but it's fairly minor.

Bear in mind too that batts are rated down to some unholy V, like 10.5V. They're useless then of course, and even at 50% SOC can be borderline holding up enough V to run loads. Low temps worsen that too, as they're less reactive.

I'm not saying to derate your capacity settings, but awareness of where you stand is the key.

If you do change Peukert, you may need to tweak Charge Efficiency to balance things. Good luck working it out.


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Endre Kiss avatar image
Endre Kiss answered ·

Hi John,

Thanks for your answer, very helpful. I do not have the correct data for C100 discharge ration but I have plenty of historical data. on the BMV and the VRM portal (the system is already running about 2 years but only this winter became very annoying having the fact of heavy snowfalls). So, what I see that the deepest discharge was -533Ah, so I would take this as C200. That tunes the Peukert exponent down to 1.06 far lower than 1.25. I'm aware about when loaded with larger current there will be an error, but that is only when we are onsite and anyway the solar panels are cleaned from snow, is far more important to know in long term where we are.

About charge's efficiency, this looks like this in my history>

1643707343362.png

This figures are already computed values from the charged eff and Peukert's coeff.? Or these are raw kWh? If I compute a general efficiency from above is around 94%. Is that correct?

Thank you, Endre


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JohnC avatar image JohnC ♦ commented ·
@Endre Kiss

You could use that 1.06. It may be too low from the purist's point of view, but it shouldn't overestimate your SOC with low loads. Maybe it will show a little lower than the real, but that'll keep you on your toes, hey.

That 94% RTE is stunningly good. Mine is 81%. I've always considered those absolute measurements, raw data. My batts are only cheap fla's, so expected. But I'm not privy to the Victron code, can anyone from Victron confirm?

A side comment.. I think your 5% Tail is too high, and risks premature sync. I use 0.6% at a similar V. You may not want that low, but maybe you could take a closer look at how much current your batts can accept when they're fully charged.

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kevgermany avatar image kevgermany ♦♦ JohnC ♦ commented ·
To add. It's winter where I am. Solar gives good voltage but miserable current. Net result is that the tail current setting is very difficult to get right. Often the solar charge in bulk is lower than the tail current. As a result, my SOC is setting 100% too early. However if I lower it like @JohnC has done, when charging on AC, the tail current doesn't get low enough to synch 100%.

It's a tech thing and you may have to understand what's happening and correct mentally. But most of the time a reasonable figure comes out.


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Endre Kiss avatar image
Endre Kiss answered ·

Many thank for your answers, having the fact that I have always a load on my system is hard to measure the tail current but I'll have to :-). So yet, I'll decrease to 2.5% and closely follow when charged from genset.

After changing the Peukert's coefficient now shows me 44% (at -273Ah far more realistic) but soon will stop as the MultiPlus is set to stop when the supply voltage is bellow 22V (just save the battery's life).

There remains only one mystery related to the VRM, when charging from genset it shows me as a sum of consumed energy and charged energy from genset but not sure that this thread is correct.

Thanks, Endre


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JohnC avatar image JohnC ♦ commented ·
@Endre Kiss

Charging with a genset all the way down to a Float Tail isn't something sane people do. Too wasteful if no loads on at the same time. In practice you'd terminate earlier than that, say 5% Tail. And that's at Absorb V for me, so I'd raise the Charged V higher too. Nice if you know how close you are to fully charged at that point, for me it's ~98.5%. And you can only sync to 100%, unless you do it manually, so allowances need making here.

Your VRM 'mystery' is probably because it doesn't count 'Generator' batt charging, only direct consumption, and this fouls up any balance you might seek. If you rename it (in Cerbo) to 'Grid' then the issue goes away, but you lose data continuity between the input types. I really wish they'd fix this..

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Endre Kiss avatar image
Endre Kiss answered ·

Yes, you're right it is not exactly the way I would do. Usually, in the evening I kick in the generator and let it run for about 4-5 hour that is far less than I would need for fully charge (usual ~35A charging current ~150Ah). I assume next day will be charged from sun and I will wait until my battery is fully charged and the MPPT charger goes into absorption mode. After turns back to float I disconnect all my loads and I will force the MPPT to absorption mode. I assume, hat should be an enough accurate reading of the tail current.

I have identified several threads and request with the same topic on the VRM community portal, so I concluded that is not a question of settings, which possibly I did wrong, it should be a feature which will be implemented at a later date.

Many thanks, Endre

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