question

colindernst avatar image
colindernst asked

Why does state of charge drop while voltage is high?

I have 200Ah of lithium batteries monitored by a BMV712. From fully charged batteries the only draw on the system is a fridge (I have turned everything else off) which draws about 3Ah. Overnight the SOC has dropped to 17% with a voltage of 13.3v. This situation happens consistently when I have been using the system in the past. I have reviewed my settings but perhaps I am still missing something there. Do you know of a reason why the SOC drops so quickly with very little load on the system?

BMV Battery MonitorSOC
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3 Answers
wkirby avatar image
wkirby answered ·

What value do you have in the "Battery capacity" field within the Battery settings page?

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colindernst avatar image
colindernst answered ·

7 comments
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klim8skeptic avatar image klim8skeptic ♦ commented ·

What are your shunt settings?

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colindernst avatar image colindernst klim8skeptic ♦ commented ·
My shunt settings are shunt current 500A and shunt current 50mV
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colindernst avatar image colindernst colindernst commented ·
make that shunt current 500A and shunt voltage 50mV but you probably realized my error.
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wkirby avatar image wkirby ♦♦ commented ·

The next thing I can think of is the current reading. What does the actual current when just the fridge is on? Naturally, it should read quite low or ideally 0A when there is no load in the system at all. If the calibration is off then it might think that more current is flowing that what actually is.

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A zero current calibration can be performed to ensure that it knows what 0A is, but you need to be sure that 0A is indeed flowing through the shunt when you do this.

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colindernst avatar image colindernst wkirby ♦♦ commented ·

I have some solar coming into the battery so I see output current of +14.17A. When I add the fridge load in it drops to +10.42A. I guess to perform the calibration I will have to disconnect the solar input and turn the loads off. I know this morning when I was not getting any solar input (not disconnected just no solar gain) and there were no loads the output showed 0.18A. Is that close enough to zero or is 0.18 significant? Thanks,

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wkirby avatar image wkirby ♦♦ colindernst commented ·
That seems OK. 0.18A would not be significant enough either way.

Shunt settings have already been checked, so I'm running out of ideas now, very peculiar. I'll have to do some more thinking.

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colindernst avatar image colindernst wkirby ♦♦ commented ·
Thank you for your help. I appreciate your time and effort. The search will continue.
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askdba avatar image
askdba answered ·

I have the similar problem, do we have any updates on this?

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