Overkill BMS - guidance to set SOC?

MrMatt

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I do not think that 20% of your capacity exists between 3.15V (40%) and 3.10V (20%).

Looking at Steves charts, it shows about 15% remaining at 3.15V.


Do you suppose it has something to do with his total battery capacity being 206 AH vs my 280?
I have to think about that....
 

MisterSandals

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Do you suppose it has something to do with his total battery capacity being 206 AH vs my 280?
I have to think about that....
His charts show % remaining and is independent of the cell/battery capacity.
There is little capacity above 3.40V and below 3.15V.

You can use whatever numbers you choose. But, I suspect that if you use the numbers you propose, that you will see the last "quarter of a tank" drop much faster than each of the other three quarters.
So when your BMS says 40% at 3.15V and you are expecting to get 80Ah left but only have 30Ah left its up to you to deal with it. If it becomes an issue you can always adjust these values to meet your use and tendancies.
 

MrMatt

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Yes, I see that, thank you
With my total battery reading 13.3V, dividing by my 4 cells = 3.325V per cell, a little off.
But in comparing with my own input values above, this is then above 60%; the icons dont indicate that.. they look below 50%
Steve's show the same values in that cells above 3.3 are above 60%
 

Bob B

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Here is what I have, and thanks for taking a look for me...
Perhaps my issue is I have not cycled my battery.. would not think it makes any difference but - her are my inputs. You can see at th bottom, the cycles are "0". Plan to cycle this weekend if time permits..

View attachment 58093View attachment 58094
It looks to me like it is calculating correctly based on the numbers you have in there..... assuming the battery was at or over 3.35 and has now dropped to and average of 3.325.

At 3.35 it would go to 80% and won't go to 60 % until the cell voltage drops to 3.3.

You can adjust the numbers in those parameters to try to get it more accurate .... but why worry about it ..... Just get the pack charged at or above 3.4 / cell and it should go to 100% and then track based on coulomb counting after that.

If that doesn't work, discharge it till it reaches the single cell cutoff of 3.0 volts and then fully recharge it.
 

MrMatt

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Hmmm thought I replied to you, but I do not see my post, .. thank you all for your help, I have a better understanding now..
 

MrMatt

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OH here it is.. thought posted it, - found in on one of my 50 tabs I have open.

........................................................

Yes you are right,

Bu there is one question that I did not yet receive an answer..

If the BMS sees the cells being at 85% charge, does the BMS ( through not replenishing used current), allow the cells to go to and maintain 50% SOC? - before allowing used current to be replenished?



For fun, I graphed the inputs of my BMS vs the one you displayed and added Steve's 12V battery pack graph for between 20 - 80% outside of this linearity goes off.

Your inputs are definitely more linear in the range stated and more closely reflect Steve's battery pack linearity
Quite a difference vs my inputs which are also graphed. I will be changing my inputs
Wonder if it is better to have inputs such as to create a more of a linearity

View attachment 58106
 

MrMatt

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here is the graph...
 

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MisterSandals

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If the BMS sees the cells being at 85% charge, does the BMS ( through not replenishing used current), allow the cells to go to and maintain 50% SOC? - before allowing used current to be replenished?
Think of the BMS as a safety switch. It cuts off at your max cell voltage or max overall battery voltage. It turns back on as well at the "release" values that are set.
There is not much thinking with a BMS, its pretty much about turning off on conditions and turning back on when release conditions are met.
 

MrMatt

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Think of the BMS as a safety switch. It cuts off at your max cell voltage or max overall battery voltage. It turns back on as well at the "release" values that are set.
There is not much thinking with a BMS, its pretty much about turning off on conditions and turning back on when release conditions are met.

thank you. I raised the question due to statements I heard about not keeping your battery more than 50% SOC for long periods of time.
Especially if you store them during the winter.

Just not sure how the BMS handles a battery that when connected is say at 90% of charge... does it keep it there or no

Thank you
 

wholybee

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thank you. I raised the question due to statements I heard about not keeping your battery more than 50% SOC for long periods of time.
Especially if you store them during the winter.

Just not sure how the BMS handles a battery that when connected is say at 90% of charge... does it keep it there or no

Thank you
I don't know what you mean by "does it keep it there." I am going to say no, because all the BMS does is stop charging if the voltages gets too high, or stop discharging if voltage gets too low. In a properly working system, neither of those things ever happen, the BMS is just an emergency cutoff switch. Any attempt to keep the charge at some specific point will need to happen with the charger settings. You could use something like a victron meter that has relay outputs to turn a charger on and off at specific points, for example.
 

MrMatt

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Thank you,
According to a chart on the previous page:
When SOC = 96%, each cell voltage is 3.36 volts, battery pack is a total of 13.40.
Overvoltage = 3.65, where the BMS would stop charging.

So after a month of so of normal ... say RV usage with having adequate solar system, would your batteries ever go to 50% SOC ( Cell is 3.263) and stay at this voltage?

I do not know if the BMS targets cells to be 50% SOC ( "this is the happiest state for the batteries to be in") or not.

Or with the above scenario, does the BMS keep the cell voltage at 96% during the month of usage...

Oh great....

Ok, as I am writing this I realized . The BMS just "Fences" the voltage between too low of a voltage, and too high of a voltage. It does not care if the batteries are at 96% for a long period of time being detrimental or not.. or 40% SOC.. the BMS allows the voltages to run the full gamet of being between too low, and too high. I was thinking the BMS was programmed to shoot for 50% of charge as much as possible.

Sadly, I am denser than I thought.

Thanks any way for helping me work through it tho.
 

MisterSandals

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I heard about not keeping your battery more than 50% SOC for long periods of time.
This is correct. I turn off solar array and let my RV batteries sit at about 13.2v and they drain slowly from the RV systems like the CO sensor, BMS and SCC which are all powered by the battery. When i seen 12.9V or around there I turn on the array for a day and then put back into storage mode. I think its about one charge day every 2-3 weeks.
Your setup will drain differently of course but its not hard to figure out the pattern.
 

wholybee

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This is correct. I turn off solar array and let my RV batteries sit at about 13.2v and they drain slowly from the RV systems like the CO sensor, BMS and SCC which are all powered by the battery. When i seen 12.9V or around there I turn on the array for a day and then put back into storage mode. I think its about one charge day every 2-3 weeks.
Your setup will drain differently of course but its not hard to figure out the pattern.
You could just set both the Absorption and Float voltages in your solar controller to 13.2, and it will hold the battery at that voltage. Because that is the flat part of the curve, the SOC will fluctuate some with small currents going in and out, but it will mostly stay close to 50%-60%
 
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