Overkill BMS - guidance to set SOC?

MrMatt

New Member
Ok, I am still declaring myself a newbie,
So will have to put up with me, and likely my more obtuse follow up questions:

Here is what I have:
4, 280 amp eve cells.
Top balanced them
Knocked their voltage down a bit to "normal, happy voltage - 3.2? don't remember
Overkill BMS, 12V, 120 amps
Android phone to connect

~ 2 weeks later, put them into my bus
4, 100 watt solar panels, all hooked up to Victron 100/50 mppt
Hooked up BMS etc.
This was a drawn out project

When I got around to it, noticed the BMS is showing the cells to be at about 50% charge, based on the icons,
which they do at this moment. ( I know the icons are qualitative, not too accurate).
The voltages of the cells right now are 3.333V +/- .001

As I never knocked the cells further down after top balancing them, are they really at approx 50% SOC, or more like 80%? -
Can the BMS tell somehow?
Is there something the Overkill BMS does to drive them down/up to 50%?
Is there something I have to do?

I will stop here for now for the questions..

Thanks you very much.
 

Bob B

Photon Sorcerer
The JBD BMS will guestimate the SOC based on voltage until you go thru a full charge cycle with it connected .... after that, it will track the SOC based on current in and current out .... and will reset to 100% at each full charge.

Writing a parameter change will revert back to the original guestimate until cycled again.
 

MrMatt

New Member
Response here is so fast most of the time, its kinda scary....

Thank you, I was reading more,
A full charge cycle means to drain the batteries to zero/ 10V(?) then fully charge them up to what? ( or discharge them to be at the BMS cutoff ~ 10V?)
I am guessing the BMS cannot be told to do this, ..wow, I think this was a stupid statement, but this is where I am at..
So, I would need to drain the batteries while they are in my bus via hairdryer or something, to get near to zero/ 10 volts then charge them up with a Lithium battery charger..( I think I have a 45 or 75 amp LI charger until the BMS cuts back in.

You get the idea of what level of advice I need....

Thanks
 

Bob B

Photon Sorcerer
Just getting them back to a full charge may be all that's necessary .... but I don't remember for sure.
 

MisterSandals

Participation Medalist
As I never knocked the cells further down after top balancing them, are they really at approx 50% SOC, or more like 80%? -
Two tidbits:
After you top balanced your cells, did you take note of where they discharge by themselves in an hour?
This is what I consider to be 100% Soc as there's dang little capacity above that voltage.

For MY cells, that settle voltage is 3.35V for all my cells.

Second one, and maybe a question for @Bob B too:
In the parameters, you can enter voltages that the user wants to represent a specific state of charge (80%, 60%, 40%, 20%). At what point does the BMSs calculated (from "going thru a full cycle") override the user entered parameters?
CE3DA8BE-C3F2-4AF2-B3A5-E1449DC720AE.png
 

MrMatt

New Member
Two tidbits:
After you top balanced your cells, did you take note of where they discharge by themselves in an hour?
This is what I consider to be 100% Soc as there's dang little capacity above that voltage.
i did not take note, but I do recall after top balancing them, they dropped a bit, and I brought them up again, and repeat. I understood this is what one does. Yes, very little capacity, that is for sure.


Second one, and maybe a question for @Bob B too:
In the parameters, you can enter voltages that the user wants to represent a specific state of charge (80%, 60%, 40%, 20%). At what point does the BMSs calculated (from "going thru a full cycle") override the user entered parameters?
View attachment 57778

So what I am gathering you are saying.. if you measured the capacity of your cells/battery, bring it up to near top balance voltage, and then use a hair dryer or similar, and measure the amps being used, you could stop at .. say,... 80% of what you measured of your total capacity, then enter 80% into the state of charge into the BMS. (?)


At the very least, I am wondering if it is possible that my batteries are at 80% SOC, but the BMS is reading 50%, .. what is the negative consequence if any...
Just getting them back to a full charge may be all that's necessary .... but I don't remember for sure.
I think you mean just to top balance them, or get near that voltage..



What is the procedure for performing a full cycle.. just curious
 

MisterSandals

Participation Medalist
So what I am gathering you are saying.. if you measured the capacity of your cells/battery, bring it up to near top balance voltage, and then use a hair dryer or similar, and measure the amps being used, you could stop at .. say,... 80% of what you measured of your total capacity, then enter 80% into the state of charge into the BMS. (?)
That would be the scientific way of doing it! I used @Steve_S state of charge tables because I liked the methodology and effort he put into these (and he's a smart guy with loads of experience):

LFP Voltage Chart.jpg
 

Bob B

Photon Sorcerer
Two tidbits:
After you top balanced your cells, did you take note of where they discharge by themselves in an hour?
This is what I consider to be 100% Soc as there's dang little capacity above that voltage.

For MY cells, that settle voltage is 3.35V for all my cells.

Second one, and maybe a question for @Bob B too:
In the parameters, you can enter voltages that the user wants to represent a specific state of charge (80%, 60%, 40%, 20%). At what point does the BMSs calculated (from "going thru a full cycle") override the user entered parameters?
View attachment 57778
It uses those parameters on initial startup .... or after a reset that happens when writing a parameter. And it will keep using those parameters if the cell isn't fully charged. Those parameters determine the guestimated SOC I was talking about

Once the pack is charged to the point one cell reaches the voltage set in the cell full voltage parameter, it should reset to 100% and do coulomb tracking after that.
It should also reset to zero % if a cell reaches the cell minimal voltage parameter setting.
 

MrMatt

New Member
Ok so this is what I am understanding,
The user has to put into the BMS the state of charge as best he can determine.
He can determine best by fully charging the battery set to a point near top balance, guidance also by the charts relayed by MisterSandals.
Once that is done the BMS tracks incoming and outgoing current (Coulomb Tracking) to provide near accurate ( YMMV) individual cell values.

Whether or not the BMS ever moves and tries to keep the cells' voltages at 50% of charge... say .....from solar panels I do not know.

IF you start at 80% does the BMS keep the battery voltages at that point or allow a drift down to 50%


Whenever the BMS sees one cell in the pack reach a value in the cell full voltage parameter ( I never noticed that) the State of charge is determined by the BMS to be 100%, and tracks from there. This also occurs at the other end of the voltage spectrum in accordance with minimal voltage parameter setting input by the user.






Cliffe notes: Put the battery near top balance enter into the BMS this is 95% SOC.
Option: top balance the cells, put in 100% SOC into the BMS
BMS will perform Coulomb counting with reasonable accuracy
Disconnecting the BMS, ( say for battery retrieval from vehicle in the winter) need to do this again when user reconnects

Does this sound correct?


Much appreciate all your help.
 

Bob B

Photon Sorcerer
It looks to me like you are saying to manually enter the SOC into the BMS .... there is no need or method for this.

At initial startup, the BMS will determine the SOC automatically based on the parameters mentioned by MrSandals.
Once the pack is charged to the level of the "cell full voltage" parameter ... it will automatically set the charge to 100% and begin tracking via coulomb counting.

The accuracy of the coulomb counting CAN drift out of accuracy over time. It is possible that small load currents won't be recognized.
To remedy that, it is a good idea to occasionally fully charge the pack and that will help keep it in sync.
 

MisterSandals

Participation Medalist
.... there is no need or method for this.
Did you see the pic in post #5? The thing we've been discussing as shown in the pic is setting the values for 80%, 60%, 40% and 20%.
This is what I was asking about as to when these entered values are used and when/if the BMS gains enough info from a full cycle or two to then use the counted coulombs.
Screen Shot 2021-07-27 at 9.46.56 PM.png
 

Bob B

Photon Sorcerer
Did you see the pic in post #5? The thing we've been discussing as shown in the pic is setting the values for 80%, 60%, 40% and 20%.
This is what I was asking about as to when these entered values are used and when/if the BMS gains enough info from a full cycle or two to then use the counted coulombs.
View attachment 57945
That's what I answered in post #8
 

MisterSandals

Participation Medalist
That's what I answered in post #8
I thought you were referring to that in post #8 until you said:
It looks to me like you are saying to manually enter the SOC into the BMS .... there is no need or method for this.
I am not sure what happens if you leave these items blank so cannot evaluate whether it is "needed", but there is certainly a method for entering it. I think the fields are populated when one reads the data from the BMS.

I think we're on the same page, i'm just a little confused by that statement. Maybe there is something i need to learn about those values?
 

Bob B

Photon Sorcerer
I'm really not sure what your question is ..... those parameters have a default value that is as good as anything else because they are only used during the initial period where the BMS is guestimating the Soc .... Those parameters can be changed, but that is not the same as changing the value of the SOC as MrMatt seemed to be asking.

To be 100% honest, I'm not sure I understand what he was asking.
 

Gazoo

Dumb Dumb
What is the procedure for performing a full cycle.. just curious
Discharge the battery until the BMS's low voltage disconnect kicks in. Then charge until the BMS's high voltage disconnect kicks in. This will be a full cycle and the BMS will report the SOC reasonably well, and the reported capacity will be quite accurate. This will also test the BMS to make sure it is working properly. The BMS default settings are fine. If you change a BMS parameter it will throw the capacity off and then you will need to cycle the battery again.
 

MrMatt

New Member
At initial startup, the BMS will determine the SOC automatically based on the parameters mentioned by MrSandals.
Once the pack is charged to the level of the "cell full voltage" parameter ... it will automatically set the charge to 100% and begin tracking via coulomb counting.
So these parameters being the total voltage of the battery (?) as provided by MisterSandals in post 8,? where according to the chart if your 12 volt battery is reading 13.30 volts, then the state of charge is 80%, if the 12 volt battery is at 13.05V then the state of charge is 50%.
Is this what the BMS looks at - the 12volt battery voltage for it's guesstimate? - when first hooking up the battery to the BMS?

To be 100% honest, I'm not sure I understand what he was asking.
Thank you so much for your patience!
To clarify, here is a scenario.


You put together a battery, balance the cells, leave them at 3.337 volts, (~85% charge) attach the BMS, ......

1) Does the BMS just default to 50% SOC, and not truly "know" the cells are at 85%?? - It seems the BMS would know they are at 85% if my above statement is true. *

2) If your battery usage is minimal in that your solar panels can more than easily replenish any used current used, are your batteries being held at 85% ( BMS thinks 50%) for a prolonged time? (which I hear is bad)

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


* So right now, my cells are reading 3.329 volts, which according to MisterSandels provided table means my cells are at above 80% charged! - although the cell icons through the BMS app ( I know qualitative, but still are indicative), show my batteries to be at ~40%
Battery voltage is 13.3 volts








Discharge the battery until the BMS's low voltage disconnect kicks in. Then charge until the BMS's high voltage disconnect kicks in. This will be a full cycle and the BMS will report the SOC reasonably well, and the reported capacity will be quite accurate.
Thank you Gazoo, very helpful.
 

Bob B

Photon Sorcerer
So these parameters being the total voltage of the battery (?) as provided by MisterSandals in post 8,? where according to the chart if your 12 volt battery is reading 13.30 volts, then the state of charge is 80%, if the 12 volt battery is at 13.05V then the state of charge is 50%.
Is this what the BMS looks at - the 12volt battery voltage for it's guesstimate? - when first hooking up the battery to the BMS?


Thank you so much for your patience!
To clarify, here is a scenario.


You put together a battery, balance the cells, leave them at 3.337 volts, (~85% charge) attach the BMS, ......

1) Does the BMS just default to 50% SOC, and not truly "know" the cells are at 85%?? - It seems the BMS would know they are at 85% if my above statement is true. *

2) If your battery usage is minimal in that your solar panels can more than easily replenish any used current used, are your batteries being held at 85% ( BMS thinks 50%) for a prolonged time? (which I hear is bad)

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


* So right now, my cells are reading 3.329 volts, which according to MisterSandels provided table means my cells are at above 80% charged! - although the cell icons through the BMS app ( I know qualitative, but still are indicative), show my batteries to be at ~40%
Battery voltage is 13.3 volts









Thank you Gazoo, very helpful.
If you haven't brought the pack to full charge..... or fully discharged .... the parameters MisterSandals posted .... should .... be what determines the SOC the BMS is showing.

Are your parameter values the same as those MisterSandals posted?

Can you please post a screen shot of those parameter values for your BMS?
 

Bob B

Photon Sorcerer
There's another thread going with similar problems ..... Maybe there are some versions starting to show up that don't work right?

 

MrMatt

New Member
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..

PERCENT CHARGE VOLTS.jpgFRONT SCREEN.jpg
 

MisterSandals

Participation Medalist
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.
 
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