Over-discharge of LiFePO4 cells

[tanglewood]

Originally Posted by Delfin

Very, very similar to our system and usage, although we generally re-charge before hitting 12.8 volts, just because it fits a daily pattern, and I disconnect charging at 14 volts (28 for me).

One question I have, and this goes to Tanglewood's question about research showing that a shallower depth of charge increases cycle life, stems from the data Lithionics has posted here.

If you look at how they present the data, they have tested to 2,700 cycles at 80% discharge, then, it looks like they are just extending the rate of cycle increase to predict 35,000 cycles at 10% DoD - what they identify as "analog result".

However, if you look at this carefully, they are defining x% DoD in what looks like an odd way to me. 50% DoD is defined as ranging between 75% and 25% SoC, not 95% to 45%. This would suggest that they think that in addition to increasing cycle life by reducing DoD per cycle, you also would benefit from operating that cycle within the "middle" of the power curve, if you understand what I mean. Am I processing this correctly?

That is quite interesting, and makes me wonder whether I missed fine print elsewhere.


What they are really showing is a DOD cycle span, in all cases centered around 50% SOC.



So yes, their numbers reflect staying away from BOTH high and low SOC at progressively larger distances.


And who knows what an "analog result" is. Sounds like a guess best on who knows what.

[tanglewood]

Originally Posted by Delfin

The Papezova paper would indicate that (table 1 vs. table 2). One wonders whether the cycle life would not have been greater if the charge voltage was lower than the 15+ volts they used in the lab....

Agreed. And it indirectly suggests no hard from charging to full SOC.



But notable is that the test only ran up to 1400 cycles. Assuming loss of capacity continued without change, they arrive at the 4000-5000 cycle estimate. It's as good an assumption as any, but it sure would be nice to see a test run until they actually reach 80%.

[nebster]

Originally Posted by CatNewBee

Initialization of cells is usually done by the initialisation charge to 4.0V before first using them - this is what the manufacturer says and what you need to do to unleash the full potential of the LFP. This is a TOP balancing approach.

Yes, charging all the cells to the same top end voltage is how we prepare a pack for top-balanced operation. It doesn't "unleash" any special capabilities in the cells; that's more of the usual manufacturer hogwash.

There are "DIY"- BMS and circuit plans in the wild advocating bottom balancing, but there are no commercial builds for the obvious reason relying sole on bottom balancing. Most of the DIY circuits are semi-automatic (adding cell loads, that shut at a given voltage)

Please point me to a DIY managed BMS bottom-balancing circuit design. I've never seen one, but I can't wait to. If you can only find one, please let me know why you chose the words "most."

If you can't find any, please stop describing something that no one does.

or manual (opening the cell bridges, connecting the cells in parallel and discharging the pack to a given voltage, then rebuilding the battery for charging).

This sounds like good old hand re-balancing. That's not a "circuit," and it's not a BMS, unless you consider the human to be the BMS. Rebalancing is something people sometimes do, seemingly if they have old cells or if they have a circuit issue (like using a cellog8 to monitor their cell voltages).

The idea, however is not dead in commercial products. There are manufacturer, who do active balancing at all SOC stages - like the ABMS from REC. Their philosophy is to keep the cells always perfectly in balance and get most juice out of them even if one cell starts dying - until it gets replaced - and this with less possible power loss.

Yes, active rebalancing, that is very cool (well, it is actually HOT, and it is hard to do at reasonable cost at more than 4s). It is also not generally what we, in forums where people discuss installing lithium packs, are talking about when we say "bottom balancing a pack."

They do the balancing by DC-DC power converter, instead of burning capacity by resistors.

As a separate discussion, it's a neat idea. It seems like it holds the most value if you have cells that come out of balance regularly. (I would not want to deal with a pack composed of cells that behave that way.)

[tanglewood]

Originally Posted by CatNewBee

Initialization of cells is usually done by the initialisation charge to 4.0V before first using them - this is what the manufacturer says and what you need to do to unleash the full potential of the LFP. This is a TOP balancing approach.

Is this top balancing, or are you referring to the initial charge that creates the SEI layer that is fundamental the LFP? I could see some creative writer calling is "unleashing the potential...". But it's really the last step in creating the battery. I think it's important to distinguish the SEI formation charge from top balancing. Top balancing happens (if you choose) as part of synchronizing the SOC of a set of batteries.

[Delfin]

Originally Posted by tanglewood

That is quite interesting, and makes me wonder whether I missed fine print elsewhere.


What they are really showing is a DOD cycle span, in all cases centered around 50% SOC.



So yes, their numbers reflect staying away from BOTH high and low SOC at progressively larger distances.


And who knows what an "analog result" is. Sounds like a guess best on who knows what.

I assume it means a projection, but it is an obscure use of the word. And don't feel bad, I missed that fine print as well. However, the paper I referenced does show an increase in cycles from a starting point of a full battery, so as odd a presentation as Lithionics published on their web site, it does appear that reducing the depth of discharge will substantially increase longevity.

That said, and as Maine Sail has pointed out, it takes a really long time to burn through 2,500 cycles and when you do, you're still at 80% of the original capacity. I don't know of any data that shows whether it would take another 2,500 to get to 60% capacity but if so, LiFePO4 batteries should be included in our wills since they will likely outlast us, especially if you don't draw them down deeply with each cycle.

[newhaul]

Originally Posted by Delfin

That said, and as Maine Sail has pointed out, it takes a really long time to burn through 2,500 cycles and when you do, you're still at 80% of the original capacity. I don't know of any data that shows whether it would take another 2,500 to get to 60% capacity but if so, LiFePO4 batteries should be included in our wills since they will likely outlast us, especially if you don't draw them down deeply with each cycle.

now how about if you do draw them down with each cycle but it takes you 4 days to do it each time that is less than 100 cycles per year for a full time liveaboard.
And at 3000 cycles ( arbitrary number for illustrations)
that's over 30 years of daily use. Just to reach the 80% capacity point
(Sounds better and better all the time )

[john61ct]

Pretty sure Maine Sail has lost **zero** capacity over his testing period, due to avoiding the shoulders, charging below 3.5Vpc.

The 80% result was from a study using much higher charge voltages, pushing Absorb as well.

And low end DoD well below 3Vpc as well.

[evm1024]

There are many things that we take to be true, and indeed may be true. But thinking it is true and knowing it is true with research to prove it are often distant cousins.

Bringing in the research is part of the reason for my starting these research paper based threads.

Now, I have very little doubt (of the truth) in what you posted below -

Originally Posted by john61ct

Pretty sure Maine Sail has lost **zero** capacity over his testing period, due to avoiding the shoulders, charging below 3.5Vpc.

The 80% result was from a study using much higher charge voltages, pushing Absorb as well.

And low end DoD well below 3Vpc as well.

But without some research to prove it I myself would would write likely due to....

Of course that is just me.

[CatNewBee]

Originally Posted by nebster

Yes, charging all the cells to the same top end voltage is how we prepare a pack for top-balanced operation. It doesn't "unleash" any special capabilities in the cells; that's more of the usual manufacturer hogwash.



Please point me to a DIY managed BMS bottom-balancing circuit design. I've never seen one, but I can't wait to. If you can only find one, please let me know why you chose the words "most."

If you can't find any, please stop describing something that no one does.



This sounds like good old hand re-balancing. That's not a "circuit," and it's not a BMS, unless you consider the human to be the BMS. Rebalancing is something people sometimes do, seemingly if they have old cells or if they have a circuit issue (like using a cellog8 to monitor their cell voltages).




Yes, active rebalancing, that is very cool (well, it is actually HOT, and it is hard to do at reasonable cost at more than 4s). It is also not generally what we, in forums where people discuss installing lithium packs, are talking about when we say "bottom balancing a pack."



As a separate discussion, it's a neat idea. It seems like it holds the most value if you have cells that come out of balance regularly. (I would not want to deal with a pack composed of cells that behave that way.)

Hi nebster, DIY bottom balancing is discussed in several forums, use your favorite search engine.

Some use top balancer control boards, re-programmed to bleed the cells and use the LVC signal to stop balancing. There are various experiments to cycle them by relais through the cells, especially in high voltage storages to save costs instead to have a board per cell pack. I have read about various approaches to built discharge modules with a cut-off voltage.

So there are different projects, but no commercial BMS for that purpose. They are semi-automatic, balancing is started manualy once a year, but the cells then are equalized automaticaly. It is not rocket science.

I have soon lost interest in the concept, because for my battery there are in my opinion much better ready to use commercial BMS available. I do not buy in in the concept of occasionaly balancing, i prefer a battery, that is always balanced without user interferrence. So I stoped following the discussions there.

But if you want to try it or get some hints how to tweak the commercial boards to make them bottom balance, look at ev forums, electric car forums, off-grid power wall forums and the like. Some focus on LiIon cells, but the only difference is the voltage pre-sets. You'll find a lot of like minded enthisiasts making exactly what you are looking for.

[nebster]

Originally Posted by CatNewBee

Hi nebster, DIY bottom balancing is discussed in several forums, use your favorite search engine.

Some use top balancer control boards, re-programmed to bleed the cells and use the LVC signal to stop balancing. There are various experiments to cycle them by relais through the cells, especially in high voltage storages to save costs instead to have a board per cell pack. I have read about various approaches to built discharge modules with a cut-off voltage.

So there are different projects, but no commercial BMS for that purpose. They are semi-automatic, balancing is started manualy once a year, but the cells then are equalized automaticaly. It is not rocket science.

I have soon lost interest in the concept, because for my battery there are in my opinion much better ready to use commercial BMS available. I do not buy in in the concept of occasionaly balancing, i prefer a battery, that is always balanced without user interferrence. So I stoped following the discussions there.

But if you want to try it or get some hints how to tweak the commercial boards to make them bottom balance, look at ev forums, electric car forums, off-grid power wall forums and the like. Some focus on LiIon cells, but the only difference is the voltage pre-sets. You'll find a lot of like minded enthisiasts making exactly what you are looking for.

I'm only going to write this one more time.

Almost no one is "active bottom-balancing" with a BMS. It's just not done.

I'm glad you have moved on from it. So have the rest of us, years ago! Stop distracting the thread with weird one-off scenarios that only confuse people.

Please, when someone brings up "bottom-balancing," assume we mean "one time, initial setup, bottom-balance with no active rebalancing via BMS," because that is the only real world scenario where people with ESS-scale packs are using a bottom balance.

[CatNewBee]

Originally Posted by nebster

I'm only going to write this one more time.

Almost no one is "active bottom-balancing" with a BMS. It's just not done.

I'm glad you have moved on from it. So have the rest of us, years ago! Stop distracting the thread with weird one-off scenarios that only confuse people.

Please, when someone brings up "bottom-balancing," assume we mean "one time, initial setup, bottom-balance with no active rebalancing via BMS," because that is the only real world scenario where people with ESS-scale packs are using a bottom balance.

REC ABMS is a commercial system that does besides top balancing also active bottom balancing...

http://www.rec-bms.com/ABMS.html

[hellosailor]

Red flag:
A 1.1Ah 18650 cell, is like the 600mA NiCd and NiMh cells that are still being used in cordless telephones and other devices. They are intentionally built to be extremely low capacity, extremely robust construction, extremely resistant to heat and other abuse.

The leading 18650 cells right now are probably Panasonic, at 3300mAh, Ultrafire around 3000 (the real thing, not the many counterfeits), and then a host of commodity grade cells that are typically 2200-2400mAh.

I'd be very reluctant to extrapolate anything from an analysis of the most rugged commercial grade cylindrical cells, to cylindrical cells in general, or to prismatic cells at all. That would be like extrapolating the best food for a plow horse--based on what kept oxen happy. (Do oxen like oats? Carrots? Straw?)

[john61ct]

There are cylindrical cells that are in fact LFP chemistry. The behaviour / performance response depends on the chemistry more than the packaging, NiCd and NiMh have nothing in common with LFP in that regard.

Most 18650 format it is true are LCO like Tesla / Panasonic older ones, LMN, NMC, NMA.

[newhaul]

https://www.electronicdesign.com/pow...lways-standard