LiFePO4 Voltage Chart?

MisterSandals

Participation Medalist
Click and read. ;)
Ya caught me, guilty as charged! But I REALLY like looking at data...

While I would have liked to have seen at least one scenario with the cells in a different order to act as a control, they do calculate each cells' capacity. And this capacity is the basis for determining the initial SOC.

I think I need to do more actual capacity testing on individual cells.

Good info, thanks for sharing. I learned something today!
 

Sojourner1

Itinerant
I love graphs and data but like to know how they got to the end result. Not that I don't trust info on the internet but like to separate the wheat from the chaf. o_O
 

LiFer

Hobbyist
You are fooling yourself. If you want to know the state of charge of a cell without letting it rest for a long time, you need to do Coulomb counting. Trying to gain information from cell voltage while charging and comparing that to a table of resting cell voltages is nonsensical. I'll leave you to your illusions and remain in the real world...
I'm new to battery charging, so I'm curious...
How much voltage drift happens with (example: 280Ah) LiFePO4 battery chemistry (at a given moment - when we want to test, at 1C)?
I will have to program my inverter and charge controllers to stop charging at some point. If this voltage changes depending on load then it seems that's something folks should consider right?
Should it be considered best practice for folks to take voltage drop measurements (load + no load) and configure their equipment accordingly?

Thanks!
 

jasonhc73

Cat herder, and dog toy tosser.
I'm new to battery charging, so I'm curious...
How much voltage drift happens with (example: 280Ah) LiFePO4 battery chemistry (at a given moment - when we want to test, at 1C)?
I will have to program my inverter and charge controllers to stop charging at some point. If this voltage changes depending on load then it seems that's something folks should consider right?
Should it be considered best practice for folks to take voltage drop measurements (load + no load) and configure their equipment accordingly?

Thanks!
New cells, for all practical sense, none.
Used cells, can drop in voltage considerably.

When you turn on a microwave, the surge will drop the volt reading, and as soon as the current levels, the voltage will rebound just slightly. After the microwave turns off, the voltage will return to pretty much where it was before the power usage.

I use the batteries as a system as if it was the grid. My house is connected and I don't care what the voltage is. If the voltage gets too low, then the inverter turns off the house. It's that simple. Do you care what the voltage is in the batteries in your remote for the TV? No, just use them and let the inverter/charger/BMS work. (It has taking me only 5 months to trust them) 🤣
 

Ampster

Renewable Energy Hobbyist
Were you deliberately being insulting or was this accidental? If it was deliberate, why?
I went back and read the entire thread and understand the context of what he was saying. I agree with his thought process that unless you let the battery settle you don't have a good starting point. While charging you can't use voltage because Coulomb counting is the best way to know how close you are to a full charge.
 

Systems Planet

New Member
I went back and read the entire thread and understand the context of what he was saying. I agree with his thought process that unless you let the battery settle you don't have a good starting point. While charging you can't use voltage because Coulomb counting is the best way to know how close you are to a full charge.

It would have been better to say: "We'll just have to agree to disagree".
Why insult the guy?
Claiming superiority is the last step before failure.
 

Ampster

Renewable Energy Hobbyist
It would have been better to say: "We'll just have to agree to disagree".
I think you have me confused with the guy who had already been accused of being insulting. I was merely agreeing with the concept that meauring voltage in the middle of a Lithium battery's charge curve was inaccurate.
 

Systems Planet

New Member
I think you have me confused with the guy who had already been accused of being insulting. I was merely agreeing with the concept that meauring voltage in the middle of a Lithium battery's charge curve was inaccurate.
You responded to someone questioning the guy being rude.
You said:
"I agree with his thought process"

Not sure, why would interject your agreement, except to support the guys rude behavior.

But, that's fine.
We'll just agree to disagree.
 

Ampster

Renewable Energy Hobbyist
Not sure, why would interject your agreement, except to support the guys rude behavior.
As I explained earlier I did not want the message to be lost about a mistaken battery measurement process. That was the subject of the thread and i wanted to try and get it back on track. I was very careful not to do anything to support his poor choice of words and further hijack the thread. We can agree that he did use a poor choice of words. However, I do take issue with your assumption that I supported his rude behavior.
:mad:
 
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travelbike

New Member
I've been tinkering too witha 12v 4s Headway USED battery setup and baffled by the charging voltage recommendations. I played around with charging it to 14.4 volts recommendations and let it sit to rest . It always falls down to 13.2 v. I have connected a battery balance monitor to see individual battery voltages as they charge. At 14.4V charging, the batteries go off balance. I presume it maybe because they are used batteries. A cell or 2 would go past 3.65V. Battery voltage difference bet highest and lowest would be close to 200mv. I slowly decreased charging voltages and monitor cell imbalances at lower charging voltages. The battery sweet spot for charging voltage seems to be bet 13.6 and 13.5. Battery imbalance is no more than 50 mv. How much of battery imbalance do you experience at full charge and at lower limit voltage ?

Also do you get more watt hours when you charge to 14.4v rather than a lower voltage say 13.5 ? I've tested my small setup. It seems whether I charge to 14.4 or 13.5, I would get the same watt hours till it hits 12v coming fr full charge. I think the theoretical WH for the 4s headway is about 100 WH. On my used battery setup, I get only 60WH. For a used batt setup, does it sound normal ?

I'll be keeping my charging voltage limit at 13.5V at this time. This is my first LifePo4 battery so any feedback will be helpful.
 

carlos1w

Lego Man
There is one thing bugging me... the typical values for cut-offs that "simple" BMS manufacturers set. Just got a message from a BMS Company and he said the cell cut offs were 2.2 V (low) and 3.75 V (high).... Those seem quite extreme! I've never seen anybody recommend (and this seems to be supported by most charge/discharge curves) going below 2.5 V or above 3.65 V (and some recommend even more conservative 2.9 and 3.5 V, which would likely still give you over 90-95% capacity).

I also asked about the low-T charge cut-off and he said it was set to -7 C. This is again WAY TOO LOW, isn't it?

I mean, the BMS is supposed to manage the batteries, but with those values it seems they'd be really pushing them to the brink of destruction...

I am planning to design my own cell monitoring system that would shut-off the large consumption part (i.e., inverter) but I really need the BMS to do its job keeping the charging reasonable and disconnecting things (for the low-current use devices) if the voltage of any cell goes below 2.5V... 2.2V? 3.65V? -7C? These numbers appear crazy, no?

Carlos
 

MisterSandals

Participation Medalist
These numbers appear crazy, no?
Yep, you're not missing anything. All these things you mention are why I'm in my 7th month looking for a BMS that I like.

and some recommend even more conservative 2.9 and 3.5 V, which would likely still give you over 90-95% capacity
I like 3.0 and 3.45 and am getting roughly 90% capacity. I need to be conservative too, because I am the BMS.
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
There is one thing bugging me... the typical values for cut-offs that "simple" BMS manufacturers set. Just got a message from a BMS Company and he said the cell cut offs were 2.2 V (low) and 3.75 V (high).... Those seem quite extreme! I've never seen anybody recommend (and this seems to be supported by most charge/discharge curves) going below 2.5 V or above 3.65 V (and some recommend even more conservative 2.9 and 3.5 V, which would likely still give you over 90-95% capacity).

I also asked about the low-T charge cut-off and he said it was set to -7 C. This is again WAY TOO LOW, isn't it?

I mean, the BMS is supposed to manage the batteries, but with those values it seems they'd be really pushing them to the brink of destruction...

I am planning to design my own cell monitoring system that would shut-off the large consumption part (i.e., inverter) but I really need the BMS to do its job keeping the charging reasonable and disconnecting things (for the low-current use devices) if the voltage of any cell goes below 2.5V... 2.2V? 3.65V? -7C? These numbers appear crazy, no?

Carlos

Quite correct, THAT is a BMS to AVOID. Please post a link to it so others know to avoid it.
During my BMS hunt, I was simply not satisfied with any of the "pre-configured" products out there and I certainly did not want to use 100% of capacity limits. I looked at many and decided on Chargery as that suits my use, needs & wants for my installation. There are several configurable/manageable BMS' like ANT, XioaXing and others that people use here with good success.
 

carlos1w

Lego Man
Yep, you're not missing anything. All these things you mention are why I'm in my 7th month looking for a BMS that I like.
I like 3.0 and 3.45 and am getting roughly 90% capacity. I need to be conservative too, because I am the BMS.
Thanks! I thought so. For the INVERTER I can easily create my own management system: read the voltages of each cell into an arduino and only enable the INVERTER through its "enable" port. For lower current loads (house lights, etc.) I could put a relay and control it the same way.

But I am concerned about charging. I guess I could have the charge circuit go through a relay too.

In all of these cases, I am not happy to have the parasitic current to drive the high current relays, though and solid-state relays are probably no-go at 12 V... I'd have to basically cook up a BMS of my own... Darn it.
 
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