Lithium BMV712 inconsistent reporting of SOC and Volts

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Brian123

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The collective wisdom is much needed.....
My van has been parked up since since middle of December and the batteries were being managed at 80% charge via the setting on the voltronic main charger (e.g it has a charging program which automatically maintains a charging state of 50-80 % of the LiFePO4 battery, when the vehicle is stopped. This charging state also simultaneously supplies the 12 V consumer loads and systems, such as alarm systems, WLAN etc., as well as the vehicle's starter battery and the consumer loads in idle mode).
I have also removed the fuse from the votronic solar charger, so i thought that the batteries should not get a charge unless the battery SOC falls to<50% and the battery temp. was >0C. However during the recent cold weather i noticed that the BMV was reporting the batteries were charged at 100%, so I switched off the main charger and have brought the SOC on the BMV down to 80%, however the BMV is also now reporting the voltage is only 12.51v which according to the table below means the batteries are at 14% SOC not 80%!!?.
LithChartBattleBorn.jpg

My setup is.....
330ah (3x110ah Ecotree lithium's)
votronic solar charger via the [Broken Link Removed] 29
50amp votronic mains charger wired direct to the batteries
victron 30amp B2B
victron BMV712

I would really appreciate your thoughts on why the BMV would reporting only 12.5v's at 80% SOC
 
Need to know what your settings are on BMV, I suspect the voltage reading is correct but the percentage is wrong.
 
Hi Lenny these are my settings

DC7A2C8F-7DA6-4AE2-B2EC-365539D2467B.png
 
Brian123 said:
The collective wisdom is much needed.....
My van has been parked up since since middle of December and the batteries were being managed at 80% charge via the setting on the voltronic main charger (e.g it has a charging program which automatically maintains a charging state of 50-80 % of the LiFePO4 battery, when the vehicle is stopped. This charging state also simultaneously supplies the 12 V consumer loads and systems, such as alarm systems, WLAN etc., as well as the vehicle's starter battery and the consumer loads in idle mode).
I have also removed the fuse from the votronic solar charger, so i thought that the batteries should not get a charge unless the battery SOC falls to<50% and the battery temp. was >0C. However during the recent cold weather i noticed that the BMV was reporting the batteries were charged at 100%,so I switched off the main charger and have brought the SOC on the BMV down to 80%, however the BMV is also now reporting the voltage is only 12.51v which according to the table below means the batteries are at 14% SOC not 80%!!?.



I would really appreciate your thoughts on why the BMV would reporting only 12.5v's at 80% SOC
Click to expand...
Question for you. From your post, you sounded surprised that the BMV SOC was 100%. Is that correct?

Looking at the settings on your BMV, there is an option I don't recognise and never seen before - "Battery SOC on Reset" whiuch is set to clear. The option in that position is usually "Battery starts sychronized" with a toggle. What this means is the SOC is reset to 100% it the BMV is reset - which is the result if the BMV ever loses power even momentarily.
So if the BMV-712 DID lose power, the SOC resets to 100% and gives a false indication of the real state of charge.
Why may that have happened? Lithium Batteries have a habit when not on any load to go to sleep and that can cause the power to drop of course.
Maybe that is the reason, maybe it is not, but it is pretty possible.
That setting on yours ... I don't know if clear means resets to 100% or replaces the SOC with a "- -" display (which is what happens if you slide the toggle on the more usually seen setting and lose power). To know for sure, I would pull power to the BMV and put back and see if the display says "- -" or "100%". if "100%", I would change that setting from clear to the alternative (personally, I always set it so it does the "- -" as otherwise it can be very misleading.

PS. I would very wary on using a voltage table on a lithium battery to guage state of charge - it can be VERY misleading. I could show you a graph on my batteries where I have the same voltage on a battery at 90% and at 30% - the voltage varies far far more on the load than the state of charge.


These are the more usual setting options with the key setting highlighted:
1674831755485.png
 
I don't know what your chargers are set to but charged voltage at 14.2v might be a bit high and it's not synchronising also Peukert Exponent is a bit low 1.05 more normal for Lithium.

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Hoovie said:
Question for you. From your post, you sounded surprised that the BMV SOC was 100%. Is that correct?

Looking at the settings on your BMV, there is an option I don't recognise and never seen before - "Battery SOC on Reset" whiuch is set to clear. The option in that position is usually "Battery starts sychronized" with a toggle. What this means is the SOC is reset to 100% it the BMV is reset - which is the result if the BMV ever loses power even momentarily.
So if the BMV-712 DID lose power, the SOC resets to 100% and gives a false indication of the real state of charge.
Why may that have happened? Lithium Batteries have a habit when not on any load to go to sleep and that can cause the power to drop of course.
Maybe that is the reason, maybe it is not, but it is pretty possible.
That setting on yours ... I don't know if clear means resets to 100% or replaces the SOC with a "- -" display (which is what happens if you slide the toggle on the more usually seen setting and lose power). To know for sure, I would pull power to the BMV and put back and see if the display says "- -" or "100%". if "100%", I would change that setting from clear to the alternative (personally, I always set it so it does the "- -" as otherwise it can be very misleading.

PS. I would very wary on using a voltage table on a lithium battery to guage state of charge - it can be VERY misleading. I could show you a graph on my batteries where I have the same voltage on a battery at 90% and at 30% - the voltage varies far far more on the load than the state of charge.


These are the more usual setting options with the key setting highlighted:
View attachment 710639

Thanks Hoovie,
Click to expand...

Yes i was surprised to see the SOC at 100% as it had sitting at about 80% for a while.

I have just remove and replaced the small inline fuse for the BMV712, below are the current screenshots once the fuse was replaced

IMG_6525.PNG


also if i click on the 'Battery SOC on reset' setting i get the following options.
IMG_6526.PNG
 
That "- -" would suggest that the BMV never lost power before it went to 100%. Have to admit that is not what I expected to hear :)
The only other thing I can think of with regards to the BMV and SOC value is that you have a connection to the battery that is bypassing the BMV-712 Shunt. Is that a possibility? The battery -ve should have just one connection on it - and that is to the BAT- on the shunt (and the BAT- only connection is to the battery as well of course).


EDIT: I forgot about the auto-reset to 100% when the three factors of "tail current", voltage, and "current threshold" all co-incide. They can cause a premature jump to 100%. It is not what Victron recommend, but I often make the voltage value higher than the charger is set to. That prevents a premature jump, but it does also stop the BMV ever reaching 100% as well - but it it shows 99.9%, that is good enough if it stops an incorrect 100% setting.


Got to say I am very interested in what version of VE.Connect you are using to have those options of Clear, Set to 100% and Keep SOC. That is much handier than the Reset to 100% or clear the SOC. Been using BMVs for quite a few years now and never seen that before.
would you mind clicking on the little 3 line "hamburger", going to settings and posting the version number and also if you are using iPhone or Android? (I only use Android. maybe this is an iPhone thing??)
 
Last edited:
Lenny HB said:
I don't know what your chargers are set to but charged voltage at 14.2v might be a bit high and it's not synchronising also Peukert Exponent is a bit low 1.05 more normal for Lithium.
Click to expand...
Hi Lennie,

The Voltronic Pb 1250 battery charger only has one LiFePO4 setting which is U1=14.40v (0.3-1nrs) U2=13.80v (24hrs) U3=13.50v (Continuous)
The Voltronic MPP 250 is set at LiFePO4 14,4v
Ecotree state that the charging voltage should be between 14.4v - 14.6v
 
Hoovie said:
That "- -" would suggest that the BMV never lost power before it went to 100%. Have to admit that is not what I expected to hear :)
The only other thing I can think of with regards to the BMV and SOC value is that you have a connection to the battery that is bypassing the BMV-712 Shunt. Is that a possibility? The battery -ve should have just one connection on it - and that is to the BAT- on the shunt (and the BAT- only connection is to the battery as well of course).


Got to say I am very interested in what version of VE.Connect you are using to have those options of Clear, Set to 100% and Keep SOC. That is much handier than the Reset to 100% or clear the SOC. Been using BMVs for quite a few years now and never seen that before.
would you mind clicking on the little 3 line "hamburger", going to settings and posting the version number and also if you are using iPhone or Android? (I only use Android. maybe this is an iPhone thing??)
Click to expand...
No problems, here you go....by the way I am a iphone user

On the negative pole there is nothing connected apart the shunt and the parallel cable from the previous battery


IMG_6527.PNG
 
Peukert for LFP ist almost non existent, so the 1.01-1.03 will cover most. If you don’t exceed 0.3C to 0.4C discharge, then peukert setting at 1.01 it’s fine. Some ppl that don’t exceed 0.2C have peukert disabled. That’s not his problem, his settings look good, it’s a matter of knowing his charger absorb voltage and absorb time. It the charger is set to 14.2v then that’s good to, time to reset depends on his absorb time.

Just seen the post, the 14.2v it’s ok as it bulls to 14.4v plenty of time to reset. In fact it will reset prematurely if anything.

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thanks for that. Same version as mine but I don't have that option!
Now I don't really connect to my VE.Direct devices via bluetooth, but I use VRM via the web. So I went to the motorhome, connected using bluetooth to my BMV-712s and it wanted to do a bluetooth update. Did that and the new setting that you have appeared after a reconnect!
But on the BMV-700s I have, there is no bluetooth so it didn't update the settings. So now when I connect via VRM, on the BMV-712s I have the new setting, and on the BMV-700s I have the old setting despite not using Bluetooth on either. Victron need a new update :)


PS. I posted an additional "EDIT" with another bit of info that could explain the SOC jump to 100% that I forgot about.
 
Last edited:
Raul said:
Peukert for LFP ist almost non existent, so the 1.01-1.03 will cover most. If you don’t exceed 0.3C to 0.4C discharge, then peukert setting at 1.01 it’s fine. Some ppl that don’t exceed 0.2C have peukert disabled. That’s not his problem, his settings look good, it’s a matter of knowing his charger absorb voltage and absorb time. It the charger is set to 14.2v then that’s good to, time to reset depends on his absorb time.

Just seen the post, the 14.2v it’s ok as it bulls to 14.4v plenty of time to reset. In fact it will reset prematurely if anything.
Click to expand...
Sorry Raul....showing my ignorance....what does 0.3C to 0.4C means
 
Brian123 said:
Sorry Raul....showing my ignorance....what does 0.3C to 0.4C means
Click to expand...
Discharge rate, 0.3C is 30% of total capacity: 30a out of a 100ah battery discharge.
If you discharge at 0.5C it will be 50% as in 50a discharge out of a 100ah battery, and so on.
 
Raul said:
Discharge rate, 0.3C is 30% of total capacity: 30a out of a 100ah battery discharge.
If you discharge at 0.5C it will be 50% as in 50a discharge out of a 100ah battery, and so on.
Click to expand...
OK thanks got it.....I have never previously gone below 0.5C of the battery bank, typically 0.25C to 0.3C usage
 
Brian123 said:
OK thanks got it.....I have never previously gone below 0.5C of the battery bank, typically 0.25C to 0.3C usage
Click to expand...
Brian. I may be wrong but I think you may have misunderstood Rauls explanation? he is not talking about how much capacity in Ahs from a battery, but how much current in one go is being taken. 0.3C means you are drawing a current of 30A out a 100Ah battery, not you are taking 30Ah in total out of a 100Ah battery.
You get the high current draws typically when using an Inverter powering a Microwave or the like.
This is why you get Lenny HB banging on about how you need a big Battery Bank to use an inverter - in order to reduce the "C" usage of the battery bank and stress the battery less.

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Yes, as Hoovie said above.
What you referring to is DOD depth of discharge.
 
Hoovie said:
Brian. I may be wrong but I think you may have misunderstood Rauls explanation? he is not talking about how much capacity in Ahs from a battery, but how much current in one go is being taken. 0.3C means you are drawing a current of 30A out a 100Ah battery, not you are taking 30Ah in total out of a 100Ah battery.
You get the high current draws typically when using an Inverter powering a Microwave or the like.
This is why you get Lenny HB banging on about how you need a big Battery Bank to use an inverter - in order to reduce the "C" usage of the battery bank and stress the battery less.
Click to expand...
OK.....thank you.....just to make sure i have got this, a couple of example's

if i draw via the inverter a current of 30A from my battery bank (330Ah) I am using 0.09C (30A/330Ah=0.09)

if i draw via the inverter a current of 100A from the same 330Ah bank I am using 0.30C (100A/330Ah=0.30)
 
Brian123 said:
OK.....thank you.....just to make sure i have got this, a couple of example's

if i draw via the inverter a current of 30A from my battery bank (330Ah) I am using 0.09C (30A/330Ah=0.09)

if i draw via the inverter a current of 100A from the same 330Ah bank I am using 0.30C (100A/330Ah=0.30)
Click to expand...
sounds right.
Basically with Lead batteries, the greater the current draw the lower the batteries effective capacity for that duration.
A batteries capacity is quoted as such for a certain C rating - usually either C/100 or C/20 (so for a 100Ah battery, 1A or 5A) and as the draw load goes up, the capacity goes down. So a 110Ah battery at C/100 may be just 95Ah at C/20. The difference become a lot more extreme as the loads get higher and higher.
This is what the Peukert Component is about - https://en.wikipedia.org/wiki/Peukert's_law.
Lithium Batteries are affected a lot less by this, but still a little.
 
thanks Hoovie and Raul

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