LFP - Decline in charge acceptance rate with battery age

[Jammer]

The high charge acceptance rate of lithium batteries is held up as a great thing here on CF on a twice daily basis, istm, often in threads that have nothing to do with lithium batteries.


The charge acceptance rate of lithium batteries declines with use, that is, with cycles. As the batteries progress into their cycle life, the internal resistance increases, and they become thermally limited sooner in the charge cycle.



When designing an electrical system and charging strategy around the charge acceptance rate, this might be something to keep in mind.


While this is also true of FLAs, the usual specified charging rate (C/5 or C/4) is conservative enough that they will accept it throughout their useful life.

[john61ct]

So with thermal protection, going way over 1C is no problem.

How much decline do they suffer?

So let's say it "declines" to .8C (which I doubt, link with details please?)

Note it holds that CAR most of the way up to Full, as opposed to even the best lead only for a very short while before rapidly trailing down

So a complete recharge in about an hour, that would still be much higher currents than most boats would ever be able to push anyway.

Seems a red herring to me.

I agree that some are too fanboy pushing LFP for too many use cases, but then it seems many are the same way about lead, and overstating the case in either direction "forces" pushback against inaccurate or irrelevant claims.

[Jammer]

Originally Posted by john61ct

So with thermal protection, going way over 1C is no problem.

How much decline do they suffer?

So let's say it "declines" to .8C (which I doubt, link with details please?)

Note it holds that CAR most of the way up to Full, as opposed to even the best lead only for a very short while before rapidly trailing down

So a complete recharge in about an hour, that would still be much higher currents than most boats would ever be able to push anyway.

Seems a red herring to me.

I agree that some are too fanboy pushing LFP for too many use cases, but then it seems many are the same way about lead, and overstating the case in either direction "forces" pushback against inaccurate or irrelevant claims.

It's the portion of the CAR curve as the batteries approach full charge that flattens out the most with age (cycle age). I saw an article the other day that had some graphs. I'll post a link if I can find it again.


Actual real-world sustainable CAR is going to vary depending on the installation because it will depend on cooling.

[john61ct]

Originally Posted by Jammer

It's the portion of the CAR curve as the batteries approach full charge that flattens out the most with age (cycle age)

Users striving for longevity should not be getting anywhere near the shoulders anyway, top or bottom.

But in any case as you say few owners will be getting to such high C-rates in the first place, so I can't see how this factor is relevant IRL, at least for House bank usage.

Maybe for EP use cases when very high rate fast chargers are deployed on buoys up and down the coast?

[a64pilot]

Originally Posted by john61ct

Users striving for longevity should not be getting anywhere near the shoulders anyway, top or bottom.

If or when I go LFP, it will be likely I’ll try to stay between 40 and 80% SOC, thereby staying way away from either “shoulder”.
Some of the EV car manufacturers are as conservative, but some seem to push the pack pretty hard, I think Nissan pushes theirs.
Time will tell, but it would seem if your not pushing a Lithium pack that Calendar life is maybe what will get them, they will just age out, not so much wear out.

[tanglewood]

Originally Posted by Jammer

It's the portion of the CAR curve as the batteries approach full charge that flattens out the most with age (cycle age). I saw an article the other day that had some graphs. I'll post a link if I can find it again.


Actual real-world sustainable CAR is going to vary depending on the installation because it will depend on cooling.

What kind of rates are we talking about? Higher that C? Lower than C?

[CatNewBee]

Originally Posted by tanglewood

What kind of rates are we talking about? Higher that C? Lower than C?

1C permanent, 3C short time. No cooling nor heating.

[tanglewood]

Originally Posted by CatNewBee

1C permanent, 3C short time. No cooling nor heating.

I was hoping to hear from Jammer as the OP saying that acceptance rate drops with age. Drops from what to what? If it drops from 3C to 2.5C as the battery ages, then I doubt any marine application will care. I expect it's rare for a marine system to be capable of 1C charging, let along 2-3C charging.


Now if as LFP batteries age they start to heat up at 1C where they don't when new, then it's possible a consideration. The marine systems I've seen are not mechanically cooled, so depend of charge rates that don't heat the batteries in any significant way. If that assumption changes over age, then it might matter.


It would also be good to see the source study/report that shows this phenomenon. I haven't heard about it before, which only means I haven't heard about it before. But it's good to validate key assertions.

[tanglewood]

Originally Posted by CatNewBee

1C permanent, 3C short time. No cooling nor heating.

BTW, all the battery spec sheets that I've ready say C/4 to C/3 for "normal" charging, and 1C for fast charging. Yet you are saying 1C as a normal charge rate, and up to 3C on special occasions. I'm curious where that comes from?

[john61ct]

You are likely looking at the limitations of a given BMS, link to the products.

The cells themselves can go much higher than catnewbee's numbers.

Those are likely just his own, or a conservative POV.

In reality very few setups as House banks can even get close to 1C anyway, which is why I reckon this "problem" is a non-issue IRL.

[hellosailor]

Funny how the cruising bloggers and other early adopters of big "lithium" banks have gone off the radar, more or less. There should be folks with five-year results of daily use on heavy marine installations by now. And IIRC MaineSail was going to do some intensive cycling on a Balqon 100? 400? pack that he eventually got hold of some ?three?four? years ago. Intensive, like one or two deep cycles per day, to find out if the claims of 2000-5000 cycles over a usable lifetime were realistic or not.

SOMEONE out there should be able to post long-term results by now?

[john61ct]

He just did, pretty sure it was in this forum

[a64pilot]

I read on another Thread where Main Sail said he’s still cycling them, and lost no capacity, however his weather, water and therefor bank is cooler than most so that may account for something.

[sailorboy1]

So are people installing these super batteries really also installing a 1C charging source to support them? Or is it a 'they charge real fast", but they are still only using the stock 90-100 alternator and battery charger they had before?

[john61ct]

Because amps acceptance remains high as SoC climbs, the same charge gear will be much faster with LFP than lead.

But alts can easily get burnt out by the higher demand.

All this is thoroughly discussed in a half dozen very recent threads, maybe read a bit then ask more specific Q's.