GUIDE to properly Top-Balance and Charge a LFP Battery: Part 1

[DavidKelly]

You can always inspect the source code here at https://github.com/Sleeper85/esphom...lopment/packages/smart_bms_cutt-off_logic.cpp


Before I am in a position to say anything, I want to know how was this FCV figure determined ?

Standard charging model according to EVE documentation is:
CC 0.5 C CV 3.65 V with 0.05 C cutoff rate

Was this FCV determined after 30 minute rest at (0, 25, 45 C ) temps?

This is from the SOC OCV charge curve from EVE for the LF230. I shared it on this thread as well as an excel that helps with interpolation.

I’m not sure who is preventing you from being in a position to say anything. You describe in your method a FCV value for the LF230. I’m simply asking what it is and does it vary with temperature?

@upnorthandpersonal method of 3V-3,5V is perfect for someone just starting off as shown by the SOC OCV. I see you use the 3,37V value throughout all your posts.

An HV Bess system would have up to 257 CMUs. Manual is not an option. There are BMS out there with capable balancing algorithms.

 

[shvm]

This is from the SOC OCV charge curve from EVE for the LF230. I shared it on this thread as well as an excel that helps with interpolation.

I have already seen the PDF you sent. Testing methodology, description of procedures, anything ??
that will help me (or anybody else) verify this on their end.
Otherwise it is just blind data that I don't even know how to replicate on my system.

 

[DavidKelly]

I have already seen the PDF you sent. Testing methodology, description of procedures, anything ??
that will help me (or anybody else) verify this on their end.
Otherwise it is just blind data that I don't even know how to replicate on my system.

The document is from the manufacturer of the cell. It’s quite clear on procedure. Full charge as per datasheet and then discharge in increments, rest period, voltage test and log, Repeat at different temperatures. Done for both Charge and discharge - the same procedure you praised in an earlier post. The Rate is usually 0.1C which is not mentioned but I have seen on GFB datasheets, assumption is similar. I will ask EVE to clarify as we buy directly from them.

I do encourage you to trust the manufacturer with specifications of their own product. Surely that’s not the message you want to send to hobbyists.

You could easily replicate by changing the environmental temperature of your cells. You did mention that your battery operates in the heat. Your SOC OCV 100% value corresponds to a 0 degree set point on the chart.

Still awaiting your LF230 FCV? And confirmation if the method takes temperature into account?

I’m happy to engage in a productive discussion. If your method has merit it should stand up to criticism. At this point I don’t believe a solid case has been made.

 

[DavidKelly]

You can always inspect the source code here at https://github.com/Sleeper85/esphom...lopment/packages/smart_bms_cutt-off_logic.cpp


Before I am in a position to say anything, I want to know how was this FCV figure determined ?

Standard charging model according to EVE documentation is:
CC 0.5 C CV 3.65 V with 0.05 C cutoff rate

Was this FCV determined after 30 minute rest at (0, 25, 45 C ) temps?
View attachment 210763


I meant to say that I turn ON the BMS Balancer every once in a while to MANUALLY control when balancing takes place. It is off otherwise.
I am not aware of any BMS available currently implementing features that ensure convergent balancing.

May I ask, what do you mean by 'Convergent balancing'?

 

[shvm]

I’m interested in your thoughts. Perhaps I’m just not grasping your method. In my mind this will affect the gradient of the regression line and I’m concerned about setting a fixed value that should vary with temperature.

Using your example. Could you give us an example of the FCV you would calculate?

Here you admit you're prolly not grasping "my method" .

Further clarification, it is not something that I invented in the first place. The Nordkyn article referenced is a mandatory reading BTW.

Your SOC OCV 100% value corresponds to a 0 degree set point on the chart.

Then you go on and say this.

And confirmation if the method takes temperature into account?

And this.

I’m happy to engage in a productive discussion. If your method has merit it should stand up to criticism.

I too am happy to engage in such a discussion.
But when you haven't even physically implemented things to begin with, what criticism are you offering and on what grounds ?? based on a self-admittedly incomplete grasp of what is being said ?

At this point I don’t believe a solid case has been made.

This post was written last year. In the meantime, the implementation is already seeing stable, production usage in many user's personal solar systems, everyday, at their homes.

Here's the link to the thread for the same

JK-BMS-CAN with new Cut-Off Charging Logic (open-source)

ESPHome component to monitor a Jikong Battery Management System (JK-BMS) and communicate with inverters supporting CAN bus protocol compatible with Pylontech, GoodWe, Seplos, SMA or Victron. The project on GitHub Changelog Current version: 1.17.4 It's with pleasure that I announce the new...

diysolarforum.com

If you want to make a solid case, submit a PR, or a pseudo-code or any improvement as you see fit. This thread is already 12 pages long of me explaining things.

May I ask, what do you mean by 'Convergent balancing'?

It simply means that BMS prevents balancing when it would 'de-balance' the cells more.
Such as, when the there's current going a high resistance cell that is actually lower in charge (but higher in terminal voltage due to Internal Resistance).
This increases the work the BMS balancer has to do and is preventable.

 

[DavidKelly]

Here you admit you're prolly not grasping "my method" .

I’m being polite. Your method for determining FCV is not clearly explained, nor will you simply explain it. From my understanding it does not take temperature into account (Which makes it only work at that specific temperature and breaks at all other temperatures) and you refuse to clarify it. Your example of the LF230 is incomplete.

From the SOC OCV curve provided by the cell manufacturer your value of 3,37V only describes a FCV at 0 degrees.

I have reviewed the code. It does not explain the effect of temperature on the FCV.

Then you go on and say this.

Yes. I stand by it.

And this.

And this

I too am happy to engage in such a discussion.
But when you haven't even physically implemented things to begin with, what criticism are you offering and on what grounds ?? based on a self-admittedly incomplete grasp of what is being said ?

I feel you are deflecting and have made me less confident in your model. A simple example would suffice.

This post was written last year. In the meantime, the implementation is already seeing stable, production usage in many user's personal solar systems, everyday, at their homes.

We are all in agreement that this will not affect cell operation but only potentially protect cycle life. I have no doubt they are working without problems.
Nor is this a question of the code written by others of which you contributed to.

This relates to only using capacity to determine FCV (SOC OCV 100%) and treating all similar capacity cells equally across the board regardless of brand/ Internal resistance/ starting capacity or temperature. At the start of this thread you boldly said that everyone is wrong and your method is right. If your model was sound it would stand up to the scrutiny with easy answers to the above questions.

It simply means that BMS prevents balancing when it would 'de-balance' the cells more.
Such as, when the there's current going a high resistance cell that is actually lower in charge (but higher in terminal voltage due to Internal Resistance).
This increases the work the BMS balancer has to do and is preventable.

This adds to the above as a higher internal resistance cell will have a different FCV to the rest- not to mention dependancy on temperature.

Typically this is identified by the BMS as a weak cell fault. A pack that was well built with impedance matching should not have this issue. Better BMS such as Orion will also take internal resistance into account. But that’s a conversation for another thread.

A simple question:

At 25 degrees Celsius, using your model. What is the SOC OCV 100% Voltage (FCV) for an EVELF230 cell?

A) 3,37V
B) 3,469V
C) Something else
D) I don’t know
E) Phone EVE

 

[G00SE]

@hwy17 im curious to hear your thoughts on this thread as there seems to be some bullet points that tie into a thread you made about float/absorbtion.

 

[shvm]

Your method for determining FCV is not clearly explained

That may be a language barrier.

nor will you simply explain it.

Take a cell, and subject it to full charge (take note of the temperature & misc. Env conditions). Then let the cells rest until the terminal voltage stabilizes. Cell manufacturers usually standardize this time for new cells as 30 minutes. Plot terminal voltage vs. time. When it settles, that is the FCV.

From my understanding it does not take temperature into account (Which makes it only work at that specific temperature and breaks at all other temperatures) and you refuse to clarify it.

This adds to the above as a higher internal resistance cell will have a different FCV to the rest- not to mention dependancy on temperature.

This is where I have to call you wrong. Cell FCV (essentially a voltage) is a function of chemistry and temperature, but not Internal Resistance.
You can attach an external resistance in series with a cell and check for yourself that it still won't change its FCV.

Typically this is identified by the BMS as a weak cell fault. A pack that was well built with impedance matching should not have this issue.

A cell in a battery pack may see an increase in IR later down the road. I as an Engineer like to go beyond "should not have" and "might face this down the road during design life". A cell may have high IR but be nominal capacity.
It is our job to build systems that can tolerate simple imperfections like this. Convergent balancing is exactly that.

 

[G00SE]

At 25 degrees Celsius, using your model. What is the SOC OCV 100% Voltage (FCV) for an EVELF230 cell?

A) 3,37V
B) 3,469V
C) Something else
D) I don’t know
E) Phone EVE

I appreciate the thread and dialogue so far in it.
Much of it, I don’t have a solid grasp up. Am simply looking to treat my cells the most effective/reliable way.
I snipped this part of your other response to pose a question. (I don’t know the answer to your question btw)
Based on the data posted in this thread, if one has multiple packs of cells that are in a climate controlled room, (operating temps always between 50-80 Fahrenheit) with zero chance to exceed .5C discharge rate….
Is the jist of this that charging to 3.469v/cell both inconsequential/non detrimental, and also completely reasonable?

 

[hwy17]

@hwy17 im curious to hear your thoughts on this thread as there seems to be some bullet points that tie into a thread you made about float/absorbtion.

I'll look it over, I'm not sure I've ever read this one though.

My method is so reductive that I'm careful about getting into discussions about it directly against any of the other methods that it overturns. I need to prove it for myself a bit more before I'm ready to get more promotional about it.

But this is it: Bulk, boost, absorb, float, balance, dip, dive, duck, dodge, and dive at 3.437! Don't need active balancing, and just build then balance if your cells are fresh and from a premium vendor.

 

[G00SE]

I'll look it over, I'm not sure I've ever read this one though.

My method is so reductive that I'm careful about getting into discussions about it directly against any of the other methods that it overturns. I need to prove it for myself a bit more before I'm ready to get more promotional about it.

But this is it: Bulk, boost, absorb, float, balance, dip, dive, duck, dodge, and dive at 3.437! Don't need active balancing, and just build then balance if your cells are fresh and from a premium vendor.

The jist of what I got from the OP seems to support your other thread ideas with charge termination and single voltage set points. Hence why I thought to tag you lol

 

[shvm]

At 25 degrees Celsius, using your model. What is the SOC OCV 100% Voltage (FCV) for an EVELF230 cell?

Any LFP Cell with modern chemistry when fully rested after a full charge will have a FCV hovering just above 3.37 V.

Temperature do affect this but only slightly and that's the simple reason I am not taking that EVE data seriously determining FCV with just 5 minute rest.
At my place, summer temperatures are routinely around 35+ Degree C. And I still remember seeing cell voltages settling down in the ballpark of 3.4 V during my test after charge termination.

I guess EVE has much explaining to do.

 

[hwy17]

The jist of what I got from the OP seems to support your other thread ideas with charge termination and single voltage set points. Hence why I thought to tag you lol

Oh perfect, I will read then.

 

[shvm]

Oh perfect, I will read then.

Now that I am aware of your earlier work on charge termination
I would like to read up on them.

 

[hwy17]

Now that I am aware of your earlier work on charge termination
I would like to read up on them.

Here's where I started a mess. Also I knew more by the second half of this thread than when I started it. And I know more today than I knew at the second half as well.

Why is bulk/absorption voltage used?

I'm trying to get down to some charging basics here and interested in any resources or comments that explain the basis of 2 and 3 stage charging. Why would we want to use a higher voltage than our end goal saturation voltage? Why would we not just constant current / constant voltage all the...

diysolarforum.com

 

[shvm]

Here's where I started a mess.

I have a first hand experience in this well before even authoring this guide.

Also I knew more by the second half of this thread than when I started it. And I know more today than I knew at the second half as well.

What I have realised during the last four months or so is that Float value of LFP is even lower than my already very conservative values in this thread. I now use 53.4 V and still cycle what I like to assume 98% of my battery capacity.
The more your physically experiment, the more you know and understand.

 

[hwy17]

What I have realised during the last four months or so is that Float value of LFP is even lower than my already very conservative values in this thread. I now use 53.4 V and still cycle what I like to assume 98% of my battery capacity.
The more your physically experiment, the more you know and understand.

Yeah the true safe float voltage is probably in the lower 53's like that.

I'm working on a single voltage charging scheme theory for daily cycled solar systems, because I think a single voltage would greatly simplify things by eliminating concerns about rebulking cycles, eliminating the need for careful charge termination or any termination at all, and allowing balancing in float.

Right now, I'm starting with 3.437 because 55v is a round number, but the best single voltage could be elsewhere.

There are two criticisms my single voltage has to face:

1. Bulk/Absorbing at 3.437 won't be fast enough - This is the big one that I need to explore more in practice. After this summer I will be able to speak more confidently about it. I think that the difference at typical solar C rates is minimal.

2. Floating at 3.437 is too high - Yes, this is definitely true if it was a constant float, like a UPS application. But I think it's ok for two reasons, one that if we absorb at 3.437 in the morning of the solar cycle then we aren't really ramming the charge in, and that should leave a bit of vacancy for the float to keep trickling it in, and two that a solar cycle float is naturally limited to 1-8 hours, depending on how early you acheive the float in the day. The float will always end when the sun goes down, so the float duration is naturally limited.

 

[DavidKelly]

That may be a language barrier.

Take a cell, and subject it to full charge (take note of the temperature & misc. Env conditions). Then let the cells rest until the terminal voltage stabilizes. Cell manufacturers usually standardize this time for new cells as 30 minutes. Plot terminal voltage vs. time. When it settles, that is the FCV.

Full charge being as per datasheet to 3,65V and CV or CP. Then cell rests and lowers voltage to FCV.

I think everyone is in agreement with the above. This is the method for the SOC OCV plot supplied by the manufacturer. This test is done at a variety of temperature intervals to form the SOC OCV curves that characterise the cells.

I can assure you the Eve datasheet is correct and they hover at the higher values at those temperatures (after hours). Are you sure your cells aren’t B grade?

So to confirm:

• Does your Method calculate the FCV or do you use the SOC OCV curve from manufacturer?

• Does your method compensate for changes in temperature?

• Does your method calculate the FCV based on capacity alone?

• Do you still think everyone is doing it wrong?

This is where I have to call you wrong. Cell FCV (essentially a voltage) is a function of chemistry and temperature, but not Internal Resistance.
You can attach an external resistance in series with a cell and check for yourself that it still won't change its FCV.

How can it be a function of Chemistry and not have an effect on internal resistance. The FCV will decrease as the cell looses capacity. The increase in internal resistance will affect the FCV.

Are you implying that a resistor placed In series does not incur a voltage drop? The measured voltage will be lower. I suggest you use a resistor with a proportional resistance to that of a cell internal resistance increase.

A cell in a battery pack may see an increase in IR later down the road. I as an Engineer like to go beyond "should not have" and "might face this down the road during design life". A cell may have high IR but be nominal capacity.
It is our job to build systems that can tolerate simple imperfections like this. Convergent balancing is exactly that.

The literature is clear on this. Capacity and internal resistance are inversely proportional. A decrease in capacity will see a rise in internal resistance ( and vice Versa).

The majority of balancing algorithms will easily deal with this. The goal of an impedance matched pack is to have all cells age similarly under standard balance conditions. I appreciate your altruism as an engineer but I feel it is misplaced. I too am an engineer not that it’s important. We all want to design batteries to last.


I absolutely disagree that there is a general voltage for all LFP. The 3,37V is not standard across the board. I feel this is misleading. compare this to GFB SOC OCV that hovers at 3,4…It does seem we’ve reach agreement on the importance of temperature.

Your sample set doesn’t sound large enough to be drawing such strong conclusions. Especially if your cells are B grade, aged or not directly from Eve. I think EVE knows exactly what the are doing. I have seen the higher FCV on the regular but it will vary with age and cycle life.

 

[G00SE]

What defines a grade B cell

 

[DavidKelly]

I have a first hand experience in this well before even authoring this guide.

What I have realised during the last four months or so is that Float value of LFP is even lower than my already very conservative values in this thread. I now use 53.4 V and still cycle what I like to assume 98% of my battery capacity.
The more your physically experiment, the more you know and understand.

This is true on some of the cheap and nasty cell makes a float voltage of 51,8V- 52,5V is not uncommon for a 15S battery.