LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[Pete7]

Originally Posted by john61ct

Not saying he does, just helping lay out the parameters of different choices, his evolving plans have always included future upgrades.

I've been using solar since the early 80's, have been a rabid anti- carbon tree hugger before Al first got elected in TN.

But on a boat with limited space, and especially where windage and appearance are priorities, I don't indulge in fan-boy blanket recommendations for any one piece of the puzzle.

If the fan-boy comment is directed at me, then I object and look forward to your apology.

I am not sure that in this instance it fits the parameters or is even close. So, perhaps you would like to put some numbers to your suggestion. What will it cost for a small LFP bank, the 250A alternator you mentioned above and the magic boxes to control it all? Ball park figures will do for the purpose of this discussion. Will the crank end shaft loads be okay with that size of alternator? Don't forget the cost of a new smaller propeller now that the engine is having to drive a huge great alternator, which takes how much hp?

Pete

[nebster]

Originally Posted by OceanSeaSpray

Which part of "unable to recharge past 70% SOC" is so subtle??

The way you wrote it, I thought you were describing a pack where you stopped at exactly 70% in a lab-like setting on purpose, but that you had a sure-fire way to charge further. (In fact, it does sound like the latter is true, or at least you hope it may be.)

It just happens that not everybody is fortunate enough to operate in conditions where charging can be carried out whenever desired. This bank was cycled daily and was recharged up to whatever energy availability allowed each day. For several months each year, it meant that it almost never even reached absorption stage.

I see, this is a real-life battery on a boat, not a controlled experiment. And in real life your pack underwent mostly partial charges.

When it was possible to recharge "fully", then the bank was charged to 14.0V and absorbed until the current dropped to a conservative 0.05C. Too conservative in hindsight... but this has not been an uncommon approach.

So, how often were you able to achieve this? Once a year? Once a month?

And here we go again... completely wrong. There is NO RELATION WHATSOEVER between the end of charge voltage and the SOC achieved. This is undissociable from the charge rate and the absorption time.

I was simplifying with an assumption that you were doing this on a bench with a controlled test, but of course that's true. SOC achieved is a function of both rate and taper time. I was imagining that you were holding the CV taper to a constant, in your lab on a bench.

Now, in this instance the current tapered off to the termination point after 70% of the nominal capacity had been absorbed by the battery. However, the immediately following stabilised OCV in discharge also confirmed a SOC of about 70%. In other words, the electrochemical state of the cells in terms of the distribution of the charge carriers also indicates that there is plenty of capacity left over and above the termination point. It is a matter of going there to get it.

So the gap between the two voltages was much higher than a normal cell would exhibit.

If I had done it, I imagine that I would have said so.

Again, I misunderstood the circumstances of your "experiment." It sounded like you were suggesting that the rehabilitation was already underway and achieving at least some degree of success.

I never decided on any "value". I always attempted to recharge the battery properly, by using 3.5/V cell at termination and charge absorption when this was feasible. It often wasn't, because there was not enough solar energy available.

Right, so how often would you say it wasn't making it through the full charge?

And, separately, what do you think the distribution of SOC was on average each night after the solar was done? I mean, was this pack getting to 90% half the time and 30% two nights a week, e.g.?

When the cells were new and fresh, they easily recharged close to 100% SOC whenever they got a chance and the absorption times were short. After 4.5 years of cycling randomly taking advantage of the energy available when it was available and never overcharging, the battery now won't recharge beyond 70% SOC when using a normal charging method. And unless I somehow sort it out now, this figure will be lower next year, because this is what happens. So this pack is screwed up and will stay so until I somehow recover it.

How much lower do you think it will be? I ask because a lot of curves with this chemistry exhibit a pretty strong asymptote... they don't continue to fall off forever. I guess if it were linear you'd expect 65% in a year?

What I already know is that charging to the normal "fully absorbed" termination point fails to push that upper limit back. Getting the missing capacity back is going to require more absorption to get into the upper 30%. It may become easier once I "break" through that 70% limit or not. I don't know.

That is the next part of the experiment, but I have more urgent things to do right now and the problem is not going anywhere. The bank is still perfectly operable.

I understand now. Yes, it will be interesting to learn whether you can "break through" again. It doesn't sound like something you can do easily with just solar charging, based on how infrequently you describe achieving a full charge in the first place.

On the other hand, you sound rather nonchalant about the whole thing. Maybe 70% is in fact "good enough" for your needs? Maybe in some way this problem is self-limiting: someone with greater energy needs would get fed up with partial charges after a while and either resize the array or, more likely, spin up the generator often enough that this memory effect doesn't grow into such an obstacle.

As I said earlier, I'm quite interested to learn how it goes for you.

[Delfin]

FWIW, I asked the engineers at Lithionics how infrequently the bank could be brought to shunting voltage of 3.65v for optimal battery health, and they pulled their beards for a moment and said "once a month or so". I did not specifically reference avoiding any memory effect since I was unaware that it existed in this chemistry, and I suppose that if you accept the usual definition of memory effect as a permanent degradation, then perhaps it is fair to say that it doesn't exist with this chemistry if lost capacity can be recovered. In any case, their recommended daily charge profile when using "inverter/chargers" is 3.65v with a 30 minute absorption time.

It has been noted by others with practical experience that SoC is affected by a number of factors. Without understanding the "whys", I can certainly attest to that. Depending on which charge sources I am using (no solar or wind), I can charge my 600 Ah 24v bank at 50 amps, 100, 150, 175, 240 or 300 amps to an acceptance rate of 3% and the resting voltage after 30 minutes or so will range from 3.325 v to 3.4 v, so the SoC can't be the same. The critical variable appears to be the CV time. I go for the 30 minutes "periodically" and when convenient. As Nebster notes, if the bank lasts for 1500, 2000 or 5000 cycles it is somewhat immaterial since even full time cruising would take many, many years for me to wear the bank out.

[sparrowhawk1]

Not my opinion. This is Will Prowse a you tuber who shows how to make lithium banks. His advice. Seems low to me but also most of the advice I've found says staying away from the ends prolongs life. Now I hear it causes memory. [emoji15] Attachment 194199

[CatNewBee]

Originally Posted by sparrowhawk1

Not my opinion. This is Will Prowse a you tuber who shows how to make lithium banks. His advice. Seems low to me but also most of the advice I've found says staying away from the ends prolongs life. Now I hear it causes memory. [emoji15] Attachment 194199

LFP have a memory effect on PSOC charge, especially in automotive applications with power recuperation. But it can be healed by a few cycles. What happens is, the cell voltage suggests a higher SOC, but the battery can still store a lot more energy. The battery seems degraded, in fact it is the surface load only.

[newhaul]

Originally Posted by sparrowhawk1

Not my opinion. This is Will Prowse a you tuber who shows how to make lithium banks. His advice. Seems low to me but also most of the advice I've found says staying away from the ends prolongs life. Now I hear it causes memory. [emoji15] Attachment 194199

full charge every month or so will eliminate the " memory " issue to a basically non issue .

[CatNewBee]

A non-issue on a boat with solar, the PSOC cycles are so different and DOD / end of charge varying, no measurable memory effect.

[Wood]

Originally Posted by john61ct

I've been using solar since the early 80's, have been a rabid anti- carbon tree hugger before Al first got elected in TN.

Careful John. You're exposing your creds!

[john61ct]

Originally Posted by sparrowhawk1

This is Will Prowse a you tuber who shows how to make lithium banks.

Will does not have much in depth tech knowledge, but great for 101-level noob orientation. True for most professional social media & "YouTubers"

[Q Xopa]

I havent seen him say too much glaringly wrong.

I hate break it to you but some of your previous 'in depth' knowledge you have been confidently advising others on has been at odds with many experienced and respected forum participents.

Being the prolific poster you are you are obviously passionate and well forum read on the subject.

However you have repeatedly shown to have gaps in what you believe and what actually is.

Id be pretty sure this is why you attract a fair bit of heat.

None of us know it all, especially me. However most of us are here to learn and realise, and freely admit what we dont know. Having the the sense to listen and learn instead of opening our mouths about stuff we dont have experience with and know about.

Others dont seem so restricted by such constraints.

[senormechanico]

Well said.

[newhaul]

Originally Posted by Q Xopa

I havent seen him say too much glaringly wrong.

I hate break it to you but some of your previous 'in depth' knowledge you have been confidently advising others on has been at odds with many experienced and respected forum participents.

Being the prolific poster you are you are obviously passionate and well forum read on the subject.

However you have repeatedly shown to have gaps in what you believe and what actually is.

Id be pretty sure this is why you attract a fair bit of heat.

None of us know it all, especially me. However most of us are here to learn and realise, and freely admit what we dont know. Having the the sense to listen and learn instead of opening our mouths about stuff we dont have experience with and know about.

Others dont seem so restricted by such constraints.

well stated

[OceanSeaSpray]

Originally Posted by Delfin

...I suppose that if you accept the usual definition of memory effect as a permanent degradation, then perhaps it is fair to say that it doesn't exist with this chemistry if lost capacity can be recovered.

Memory effect, by definition, is not permanent, but something that can be cleared by fully recharging.

[OceanSeaSpray]

Originally Posted by newhaul

full charge every month or so will eliminate the " memory " issue to a basically non issue .

Yes, that is also what I would expect. However, the longer you leave it, the tougher it gets to actually reach that full charge. So the risk is that you might think that you are reaching a full charge without quite succeeding and if this happens, then the issue starts compounding itself and capacity shrinks. Technically, I should have been "fully recharging" frequently each summer, but the verdict today is that it hasn't worked.

In my case, winter essentially prevents full recharging. This is now the 5th winter for this bank and it is still operating fine because there is still more than enough capacity for routine daily cycling, but reserve capacity is down. If I hadn't taken the trouble of looking into it and measuring it, the situation wouldn't look much different than when the cells were new.

To get a truly full charge when there have been a lot of partial cycles before, you need really good termination control and fairly solid charging parameters, not the kind of stuff some people talk about with low voltages and timed absorption.
Charging to 3.65V/cell and C/30 residual current as per the cells specs would no doubt get me a little further and it is likely to be the next step while data logging.

[masonc]

Originally Posted by newhaul

This should always be installed for maintenance disconnect purposes anyway.

For what purpose