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locke avatar image
locke asked

BMV 700 SOC percantage does not match to the used Ah

Hello together,

i have identified a problem at my BMV 700 regarding the shown percentage from SOC (State of Charge)

My current SOC shows 93,1 % with consumed Ah from 34,6 Ah.

From my calculation, 34,6 consumed Ah should be already 15 % of discharge.


Whats wrong in your opinion?

I am a litte bit confused about the datas which are shown.

Unfortunatley i become less trust in the percantage of SOC.



First some basic data from my system:


Battery: 230ah

Technology: AGM



Settings:

Battery Capacity: 20 hour capacity as recomended in the manual



Stauts:




Best regards Julian




BMV Battery Monitor
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8 Answers
simen avatar image
simen answered ·

This can be explained by the peukert exponent, I think.

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JohnC avatar image
JohnC answered ·

Hi @Locke

You probably have a system that can tell the time.. like a VRM connection.

And it's maybe showing what it's seen discharged since midnight, despite accepting charge in the meantime, then recommencing discharge.

Mine shows similar, and I don't look at it to check against SOC for that reason. Because it's not related directly. I'm not sure it has any use anyway on a daily basis.. in the History it's paired with Charged current, and has a use then..

But I'm guessing somewhat, happy to be corrected.

Edit/: Correcting myself, this is an incorrect 'maybe', more info in further comments.

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locke avatar image
locke answered ·

Hello @JohnC

you are right i have the VRM connection. That shows everythink but the time which is shown is corresponded to the SOC in Percent.


From my point of view the SOC percantage is not matching to the consumed Ah.



BR Julian

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JohnC avatar image JohnC ♦ commented ·

@Locke

My previous comment re 'midnight' will be wrong. I fooled a little with my own system today to try to nail it down.

I think now your issue is to do with 'Ah', which is not a true energy measurement. It is at a standard V, but not at varying V's, which are different depending especially on whether the batts are under charge or discharge.

The Ah will reset to zero when sync to 100% is applied. If you then manually reset to 90% SOC it will apply an Ah drawn based on the nominal 12V of your system and the battery total Ah you've entered. Then later you'll charge at (say) up to 14.4V and discharge at 12.2V. And I think the Ah uses the actual V then, and doesn't compensate. So the Ah will drift over time in relation to the SOC, as the SOC is calculating via the true Wh.

I'm not suggesting this is a 'fault', as to fix it would just add to firmware bloat. Understanding it is the key, and more frequent syncs should minimise it if you prefer to use Ah rather than Wh.

Re the Peukert, Victron recommend Pb users retain the default 1.25 setting, but have provided the formula for calculating it from battery specs if they're available. In the manual.. https://www.victronenergy.com/upload/documents/Manual-BMV-700-700H-702-712-EN-NL-FR-DE-ES-SE.pdf


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locke avatar image
locke answered ·

Hello @Simen,


thank you. Do you have an idea which Peukert exponent i need to set?

The Batter is an 230Ah Ective AGM.


Thanks.


Best regards.

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locke avatar image
locke answered ·

Hello,

I still have the problem, that the SOC does not match with the discharged Ah from the battery.

Please see the picture enclosed.


Battery: 230Ah Ective AGM

Discharged Ah: 44,6 Ah

SOC: 90

In my opinion in should be less than 90 SOC, around 80.

Someone an idea for which reason that the calculation is not precise ?


thanks a lot for helping.

Greetings and a good start into 2021.

BR Julian


@Margreet Leeftink (Victron Energy Staff) Can you help me further with this topic ?



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Alexandra avatar image
Alexandra answered ·

@Locke

Try a larger tail current, possibly start at 4%. Wait for the battery to fully charge not using the BMV SOC them manually set it to 100%.

If it is still out you can try checking the zero current calibration. Remove all loads and charger sources from the shunt and see what the bmv says is happening to the battery. If there is current being read yet all have been rmoved from the load charger side then see the linked post.

https://community.victronenergy.com/questions/20193/bmv712-how-to-do-a-zero-current-calibration.html

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locke avatar image locke commented ·

Hello Alexandra,

thank you very much! But in my opinion the Tail current is only a value which will calculate the reset on 100 % if Tail Current is reached and if the charged detection time is reached. Or do i misunderstand something ?


Thank you.

Best regards

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Alexandra avatar image Alexandra ♦ locke commented ·

@Locke

If you have not been fully charging then discharging calculations also deviate.

In my experience if one part is out it all goes off. With agm the voltage is also a factor in soc calculation as well as how long it has been in absorption.

If you are noticing the issue on discharge then you need to zero current calibrate the bmv.

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andre-pg avatar image
andre-pg answered ·

@Locke

I see that your battery capacity of the system is configured to 205 Ah and you actually have a battery capacity of 230 Ah.

This can make part of the difference that you are having.

Try to change on the BMW configuration to 230Ah the battery capacity.

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locke avatar image locke commented ·

Hello Andre,

thank you very much. I choose 205 because in the manual is written that the C20 value is required.

Do you think its wrong ?

Please see picture enclosed.


Thanks in advance.

BR Julian


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andre-pg avatar image andre-pg locke commented ·

It's correct 205 Ah at C20 and 25ºC. I thought that 230Ah was for C20.

Check the Peukert value and check also the temperature room/battery which influence the actual battery capacity.

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JohnC avatar image
JohnC answered ·

@Locke

@Andre PG

The 205 Ah is the C20 rating, and is the correct one to use (IMO) for a daily cycling system.

I ran the batt C-specs through the Victron calculator and came up with a Peukert of ~1.1 (this varies a little depending on which maker figures you apply).

Use this, and maybe a Charge Efficiency of 90%, and you may find the relationship between SOC and the truth/your-estimate a little closer. But it won't match as you might think, because it's not designed to.

I use 0.5% Tail. At 4% I'm still in Absorb. 3 minutes is enough. But YMMV with that. Make sure the batts are fully charged and synced to 100% before looking at a response to such changes.

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locke avatar image locke commented ·

Hello John,


thank you very much for clarification.

I will try some of theese options.

But i still do not understand why the SOC Value is not reliable.


Thanks in advance. Best regards Julian

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JohnC avatar image JohnC ♦ locke commented ·

@Locke

Ha, I didn't say that the SOC is unreliable (although it can be if not set up correctly). I'm saying that it won't match the Ah read directly and compared to batt Ah (another defined term).

An Ah isn't even a measure of Energy. It needs V to become that, so 1 Ah discharged at 12V isn't the same as 1 Ah charged at 13V. Of course this will be allowed for in the calcs towards SOC. Then what actually makes it into the batt is also affected by Charge Efficiency.

And discharged energy is affected by the rate of discharge. Mr Peukert comes into play then, and a slow discharge will leave more energy in the batts than the Ah-out would indicate.

I find this hard to explain, sorry. I wish Victron didn't even use Ah, a concession to consumerism I suppose. I'd prefer Wh, and at least we get that in the historical data.

In essence, Ah is power flow at the batt terminals, SOC the amount of energy left in the batt itself. They're different animals..

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