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lemonjelly avatar image
lemonjelly asked

BMV SoC and Battery Cell status not alligning?

2 x 150ah Victron SmartLithium batteries:

Battery 1

  • Cell 1: 3.05V
  • Cell 2: 3.04V
  • Cell 3: 3.05V
  • Cell 4: 3.06V

Battery 2

  • Cell 1: 3.08V
  • Cell 2: 3.09V
  • Cell 3: 3.00V
  • Cell 4: 2.97V

System is saying 71% state of charge. And the A-SL11 under voltage alarm is saying "battery full discharged, cell voltage below the allow to discharge cell voltage set in the app.

The manual says:

4.8.2. Cell under voltage pre-alarm thresholdWhen the cell voltage drops below this threshold, the pre-alarm signal is sent to the BMS. The purpose of the pre-alarm is to warnthe user that the system is about to shut down due to undervoltage. For more details see chapter: “System design”.The default value is 3.10 V and the range is 2.80 V to 3.15 V.

Is it correct that my system is at 71% SoC? To use the system below 70% I can just change this Cell voltage disconnect feature down to 2.80V? And are the Cell 3 and 4 readings anything to worry about?

BMV Battery MonitorLithium Battery
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2 Answers
Stefanie (Victron Energy Staff) avatar image
Stefanie (Victron Energy Staff) answered ·

Hi @lemonjelly,

although with Lithiums it's hard to tell SoC just from voltage, but in your case it's definitely not ok. SoC seems to be rather in the range of 10% or even lower. And it is time to charge and balance (because battery 2 seems to be slightly out of balance - see the manual for the right procedure) the batteries and stop discharging them.

There might be another issue with your battery monitor. If you have a BMV or SmartShunt as your battery monitor, what are the settings?

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lemonjelly avatar image lemonjelly commented ·

Hi @Stefanie,

Thanks, this is what I was worried about. Something doesn't seem to be calibrated quite right. I had the batteries set up by an electrician, who may have not realised they weren't at 100% on arrival. Tomorrow I will do a charge each battery individually, max charge current 75 A, for a 2 hour fixed absorption period, as per the manual. Then will report back.

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seb71 avatar image seb71 lemonjelly commented ·

You did not answer the question about how you are measuring the SOC.

Do you have a BMV or a SmartShunt?

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lemonjelly avatar image lemonjelly seb71 commented ·

Sorry - using a BMV-712, and the VictronConnect app.

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Stefanie (Victron Energy Staff) avatar image Stefanie (Victron Energy Staff) ♦♦ lemonjelly commented ·

So what are the battery settings in the BMV?

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lemonjelly avatar image lemonjelly Stefanie (Victron Energy Staff) ♦♦ commented ·

1609686736992.png


I am currently charging from shore power, the SoC is now 100% with both batteries reading ~13.5V. Although I still question if this is really 100% SoC, so I am leaving the shore power connected for the full 2 hours.

The only thing I didn't do is charge each battery individually.



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1609686736992.png (334.4 KiB)
Stefanie (Victron Energy Staff) avatar image Stefanie (Victron Energy Staff) ♦♦ lemonjelly commented ·

Settings looking good.

In parallel configuration you don't need to charge them individually. But if this is the first charge, then they may need time (which can be hours) to fully balance. So if your charger reducing voltage from 14.2V to 13.5V (absorption time too short) too early, they won't get fully balanced. But of course fully charged. Don't think you need to question SoC now that the charger did a full charge (but not balance) cycle.

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lemonjelly avatar image lemonjelly Stefanie (Victron Energy Staff) ♦♦ commented ·

Thanks.

Cells in Battery 1 are all 3.38V and all cells in Battery 2 are 3.39V now. It has been charging for 1h40mins.

Would I change the settings in the inverter to charge to 14.0V in an attempt to get more life out of the battery? Or even 14.2V? Or is this not possible?

I notice online now that there was a firmware update on the batteries v1.19 which corrected the potential for cells to become imbalanced. It's possible I only just did this update.


Another thing that I noticed when intensely using the batteries is that on the BMV screen the Time remaining was roughly half of what I expected based on Ah consumed and the size of my bank.

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Stefanie (Victron Energy Staff) avatar image Stefanie (Victron Energy Staff) ♦♦ lemonjelly commented ·

I think that Time remaining is based on the discharge floor setting.

Are you using a Victron MultiPlus Inverter/Charger? If so, you can choose the LiFePO4 Lithium setting. This defaults to 14.2V (which should get the cells to 3.55V +/-30mV each) absorption for 2 hours and 13.5V float once absorption time has finished. Depending on the usage you later can change absorption time to less (1hour or so) with low discharge current. Higher discharge current may require more often balancing, where 2hours of absorption is the minimum. All in all I think it's pretty well documented. See Manual for Lithium battery 12,8V & 25,6V Smart

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lemonjelly avatar image lemonjelly Stefanie (Victron Energy Staff) ♦♦ commented ·

Yes - MultiPlus inverter/charger. I had already set it up for Lithium batteries and the Charger settings do show Absorption voltage 14.2V and float voltage 13.5V.

But my cells are still only around 3.40V.

2 hours has now finished, had no change for the last hour or so.

So was my system actually at 71% originally, but also at low cell voltages?

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Stefanie (Victron Energy Staff) avatar image Stefanie (Victron Energy Staff) ♦♦ lemonjelly commented ·

No. As said before. Looking at all the individual cell voltage you posted, SoC was probably less than 15% or 10%.

If the batteries were new and never charged after installation, then they were roughly around 50% SoC.

Depending on the charge current, it will take its time to get to 100% and 14.2V respectively 3.55V/Cell. You said 75A charging. 2 hours makes 150A, which is only half of the capacity of your batteries.
If everything is properly wired and setup, the BMS will take care of the batteries and stop charging/resume charging when anything goes wrong.

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lemonjelly avatar image lemonjelly Stefanie (Victron Energy Staff) ♦♦ commented ·

Makes sense.

It's just that on the BMV info the cell voltages and battery voltages were no longer showing any change, for the last 1h of charging. So I figured it had done all the charging it was doing. The inverter was on Float.

Without changing a setting somewhere I'm not sure how to make it charge fully?

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Stefanie (Victron Energy Staff) avatar image Stefanie (Victron Energy Staff) ♦♦ lemonjelly commented ·

Understand. Probably the charger finished a full cycle then. That was not clear. Just to be sure you could start another cycle up to 14.2V.

And as @Matthias Lange - DE mentioned correctly, adjust Charged Voltage in the BMV battery setting to 14 or 14.1V. I missed that.

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seb71 avatar image seb71 lemonjelly commented ·

3.4V cell voltage is a very safe top voltage for LiFePO4 cells. But for cell balancing you might want to charge at higher cell voltage, such as 3.5V (but not higher than 3.6V).


"Another thing that I noticed when intensely using the batteries is that on the BMV screen the Time remaining was roughly half of what I expected based on Ah consumed and the size of my bank."


Your setting for "discharge floor" is 50%. "Time remaining" is estimated until that point.

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lemonjelly avatar image lemonjelly seb71 commented ·

Thanks, makes sense.

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Matthias Lange - DE avatar image Matthias Lange - DE ♦ lemonjelly commented ·

You should increase the "Charged voltage" to at least 14V or 14.1V to prevent the BMV from jumping to 100% SOC to fast.

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seb71 avatar image
seb71 answered ·

"To use the system below 70% I can just change this Cell voltage disconnect feature down to 2.80V?"

That's the "Cell under voltage pre-alarm threshold". And no, do not lower it to 2.80V. Leave it at 3.10V (or even increase it to 3.15V).

Also, leave the "Allowed-To-Discharge cell voltage" at maximum value (2.80V).


"And are the Cell 3 and 4 readings anything to worry about?"

Probably not. Cells are never exactly the same. In the case of your batteries, the cells are balanced at the top (when fully charged). There they are (or should be) equal (same voltage).

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