LFP's Charging Voltage and State of Charge

[tanglewood]

Thanks for all the good summary info. That said, I think it's worth a step back to look at why one might want LFP batteries. There often seems to be an inordinate focus on battery life, in my opinion.


I want my batteries to serve me. I don't want to be slave to them so they last 5000 cycles instead of 3000 cycles. I would be very, very happy with 3000, or even 2000 cycles which corresponds to around 10-15 years of typical use. The typical battery datasheet quotes a 2000+ cycle lifetime, and there are tons of studies demonstrating that. And all of it is cycling through from full nameplate charge, to full nameplate discharge, and at 1C or more rates. In other words, conditions that most here would consider abusive.


I'm all in favor of lighter cycling to get longer life, but I don't want to become pre-occupied by it. I want to enjoy batteries that don't have an endless absorb cycle, and that can be realistically recharged in 2-3 hrs. And batteries where I don't need to fuss over whether they are fully recharged or not, because it really doesn't matter. To me, that's at least as important as the longer life expectancy of LFP.

[john61ct]

Well I for one am interested in knowing as much as possible about how to get optimum lifespan from LFP even while getting very high usage out of each bank.

I think well over 10,000 cycles is achievable, unless calendar life issues, like ambient temperature, start interfering with the cycling factors.

I may not actually want or be able in all use cases to implement the necessary requirements, but I at least want to know as much as possible, so I am doing so consciously, not out of ignorance.

As I have stated many times, if mfg spec'd lifetime cycles is sufficient for you, then charge to their spec 14.6V holding to .005C or whatever.

In which case, there is also not much point in your engaging in these discussions further just to say "y'all are fussing to much" because you aren't as interested.

Originally Posted by Cpt Pat

With apologies, I'm going to go down this slightly off topic tangent because I believe it brings up an interesting topic.

If it is of interest to you then it is on topic.

The whole purpose of this thread is to help me pick your brains on these longevity factors.

The mod who corrected my original title's use of the Ampersand character, also removed your handle which was originally cited there.

[rgleason]

Thanks Cpt Pat, good considerations, re alt charging & heat. What do you think of an ideal rated 170a alt charging at 130ah actual, to 200ah or perhaps a 300ah bank? Would have natural vents and tstat fan.

[john61ct]

Originally Posted by Cpt Pat

When charging at higher constant currents, such a 0.2C rate or higher, you can detect the 100% SoC point by a tapering off of the charging current. That detection cannot be performed with low and varying charging current from a solar controller.

I am not here arguing for argument's sake, if you infer disagreement, I am likely playing Devil's Advocate in order to draw you out so I can learn more. All my questions are sincere, I am actually looking for answers, not making sarcastic commentary (tone is so hard online)

Your statement quoted above implies a couple of assumptions.

First off, that the goal in setting your endCharge condition is to get to 100% SoC.

Second, what is your definition of 100% SoC in general?

say for the purpose of resetting your BMV, which I believe you know, must happen pretty frequently in order to maintain reasonable accuracy.

Next, what would your 100% Full definition be at a .2 - .3C charge rate?

How about .1C? .05C?

At what (steady) charge rate do you think it too low a current to use for endAmps purposes, ending the charge cycle?

Or is it just solar's variability, plus perhaps too much concurrent consumption from loads?

________
So far, I always have .2-.3C ICE or shore charging available when needed, so I usually define "my 100% Full point" for 4S as

hold Absorb (V=13.8V) until .05C endAmps.

Personally, in daily use cycling, I don't hold Absorb for any time at all.

I do hit it, maybe every 4-5 deep cycles, for the purpose of resetting a AH-counting BM if present, or more often if I suspect drift sooner.

(OT side note, hoping that need might go away with Balmar's recently released, maybe alpha stage? SG-200. Any responses on that new development, please post here http://www.cruisersforum.com/forums/...or-209056.html)

Do you think these choices are "too aggressive" for the sake of bank longevity? If so, why?

[newhaul]

Originally Posted by tanglewood

Thanks for all the good summary info. That said, I think it's worth a step back to look at why one might want LFP batteries. There often seems to be an inordinate focus on battery life, in my opinion.


I want my batteries to serve me. I don't want to be slave to them so they last 5000 cycles instead of 3000 cycles. I would be very, very happy with 3000, or even 2000 cycles which corresponds to around 10-15 years of typical use. The typical battery datasheet quotes a 2000+ cycle lifetime, and there are tons of studies demonstrating that. And all of it is cycling through from full nameplate charge, to full nameplate discharge, and at 1C or more rates. In other words, conditions that most here would consider abusive.


I'm all in favor of lighter cycling to get longer life, but I don't want to become pre-occupied by it. I want to enjoy batteries that don't have an endless absorb cycle, and that can be realistically recharged in 2-3 hrs. And batteries where I don't need to fuss over whether they are fully recharged or not, because it really doesn't matter. To me, that's at least as important as the longer life expectancy of LFP.

all good and well but what is a full cycle defined as for Lfp banks ie: for example a 400ah bank . You use 100ah per day do you recharge daily or do you just keep discharging for an additional 2 days , for a total discharge of 300 ah before recharging ergo approx 120 cycles per year ( full time cruiser) Which puts the 2000+ in the 20 year lifespan . I would be much more concerned about any shelf life not lifecycles

[john61ct]

Originally Posted by rgleason

the affect of rate of charging on longevity or cycles

You're right, your post is relevant to that specific issue, and that issue is important in designing / setting up charge sources.

I used to think current rates higher than .5 - 1C were fine for longevity, but in a long and contentious thread not so long ago (you were already around) Maine Sail very strongly slapped me down for saying so. At least that was my inference.

As far as your goal of sizing an alternator, you went to the trouble of creating a specific thread for exactly that purpose http://www.cruisersforum.com/forums/...ml#post2790242
Please continue that convo there, I believe anyone quoted there gets notified.

Sorry to be so nitpicky

Thanks

[tanglewood]

Originally Posted by newhaul

all good and well but what is a full cycle defined as for Lfp banks ie: for example a 400ah bank . You use 100ah per day do you recharge daily or do you just keep discharging for an additional 2 days , for a total discharge of 300 ah before recharging ergo approx 120 cycles per year ( full time cruiser) Which puts the 2000+ in the 20 year lifespan . I would be much more concerned about any shelf life not lifecycles

It's the same definition as for AGMs on a boat :-) The only hard data points we have are the lab studies and those are typically running a full name-plate cycle. Years of service on a boat are nearly meaningless since the usage pattern is unknown, and anecdotal at best. I doubt many, if any people, count lifetime Ah cycled through the banks, for example. But at least that would be hard data.



But as I said before, cycle life matters, but so do many other characteristics of LFP. We just shouldn't lose sight of the whole picture.

[newhaul]

Originally Posted by tanglewood

It's the same definition as for AGMs on a boat :-) The only hard data points we have are the lab studies and those are typically running a full name-plate cycle. Years of service on a boat are nearly meaningless since the usage pattern is unknown, and anecdotal at best. I doubt many, if any people, count lifetime Ah cycled through the banks, for example. But at least that would be hard data.



But as I said before, cycle life matters, but so do many other characteristics of LFP. We just shouldn't lose sight of the whole picture.

agreed
And just an fyi I have already put my Lfp bank in my will . I won't live long enough to ever see the life cycle maximum out of them . ( the specific way I choose to use and cycle)

edit: I actually am keeping track of lifetime ah for the bank. Via my solar controller . It has a cumulative ah count in and out. The only out that is not counted is my bilge pump.

[Maine Sail]

Originally Posted by newhaul

I would be much more concerned about any shelf life not lifecycles

In a few months, May to be exact, the cells in my bank will have a birthday. They'll be 10 years old... They are still producing in excess of the rated Ah capacity with well over 1000 cycles on them.

On each cycle I tried to get to 80% DOD before recharging though when opportunities existed, when below 60% DOD, we would put some in if it was necessary. They have also been to 0% SOC every 50 cycles for capacity testing/tracking.

That said these cells have not been treated as the manufacturers spec sheets suggest. The only reason we every brought them to 100% SOC was to reset the Ah counter and then we quickly began discharging again.. Those frequent trips to 100% SOC just ended with the new Balmar SG200.

[john61ct]

Originally Posted by newhaul

all good and well but what is a full cycle defined as for Lfp banks ie: for example a 400ah bank . You use 100ah per day do you recharge daily or do you just keep discharging for an additional 2 days , for a total discharge of 300 ah before recharging ergo approx 120 cycles per year ( full time cruiser) Which puts the 2000+ in the 20 year lifespan . I would be much more concerned about any shelf life not lifecycles

I also don't want this getting back to the "impact of DoD on lifetime" debate, pretty well covered here http://www.cruisersforum.com/forums/...th-206151.html.

Yes use the bank fully as needed, for longevity **storage**, lower is better (opposite of lead), calendar limits well treated lower temps, I suspect scale of decades but of course we just don't know yet.

During active cycling use, if it is practical, try to stay in the flatter middle region of the V / SoC curve.

The **real** answer to your question depends on whether or not "on demand" charge sources are conveniently available.

You are correct in that with LFP "counting cycles" becomes much more nebulous than with lead.

This is a good chart for LFP https://www.powerstream.com/lithium-...ge-voltage.htm

[john61ct]

Originally Posted by Cpt Pat

It pays to be conservative - though how much -- I cannot say.

This to me is really worth highlighting.

Too many people IMO use the non-scientific shibboleth, "if it hasn't been proven or can't be measured, the whole concept is not worthy of study".

[newhaul]

Originally Posted by Maine Sail

In a few months, May to be exact, the cells in my bank will have a birthday. They'll be 10 years old... They are still producing in excess of the rated Ah capacity with well over 1000 cycles on them.

On each cycle I tried to get to 80% DOD before recharging though when opportunities existed, when below 60% DOD, we would put some in if it was necessary. They have also been to 0% SOC every 50 cycles for capacity testing/tracking.

That said these cells have not been treated as the manufacturers spec sheets suggest. The only reason we every brought them to 100% SOC was to reset the Ah counter and then we quickly began discharging again.. Those frequent trips to 100% SOC just ended with the new Balmar SG200.

that is exactly my point . There are several rather lengthy threads on here about how to wrong the maximum number of cycles out of a Lfp bank. Your own data suggests few of us will live long enough to actually see the end of life cycles out of an Lfp bank. So why are so many seemingly stuck in the Fla slave mode with them? Personally that is the reason I switched . My batteries serve me not the other way around.
Btw I'm a bit jealous ( the sg200)

[senormechanico]

I like to treat my LiFePo4 banks like the soup in the story "Goldilocks and the 3 Bears".
Not too hot and not too cold, just right !
Same goes for charge / discharge percentage.

[newhaul]

Originally Posted by john61ct

I also don't want this getting back to the "impact of DoD on lifetime" debate, pretty well covered here http://www.cruisersforum.com/forums/...th-206151.html.

Yes use the bank fully as needed, for longevity **storage**, lower is better (opposite of lead), calendar limits well treated lower temps, I suspect scale of decades but of course we just don't know yet.

During active cycling use, if it is practical, try to stay in the flatter middle region of the V / SoC curve.

The **real** answer to your question depends on whether or not "on demand" charge sources are conveniently available.

You are correct in that with LFP "counting cycles" becomes much more nebulous than with lead.

This is a good chart for LFP https://www.powerstream.com/lithium-...ge-voltage.htm

John I didn't ask a question . Or if it appears I did it was rhetorical . Why be a slave to your Lfp like you had to be with Fla.
So I only get 3,000 cycles and you get 4,000 . For my usage on my boat that's still about 30 years . And I'm happy about that.

Didn't I post that chart last summer on one of these threads. It does look familiar .

[john61ct]

may have if so thanks

I plan to live long enough to get most of the way to the 10,000 I'm shooting for, but that's no-shore-power liveaboard cycling.

And my kids & grandkids range from toddlers to middle-aged, more likely to pass the bank on than any boat / vehicle / offgrid shack I now own.

I don't see "sweating the small stuff" as burdensome but part of the fun. After I'm au fait enough with the electrickery etc infrastructure, my OCD will be occupied elsewhere. . .