How much Cell Voltage drift is acceptable for LiFePO4?

[PNjunction]

Nochi - no problems. These are forums where there is no body language, tone, and even a difference in the way one expresses themselves in print rather than speaking. Too many forget this, ie I'm sure we'd all have a blast if we were on a cruise together!

Real quick - unless you have some sort of lifepo4 specific controller, we shoehorn those intended for lead acid. Ok, in this case be SURE to disable any sort of temperature compensation! That will easily put you well under or over charged if left operational. One trick if you can't disable it is to set any temp comp values to plus/minus 0.00 v in the menus... Heh sneaky.

FLOAT - we can fool the system by setting this abnormally low. What happens is that during a normal charge, say one that has gone through CC (bulk), entered the CV (voltage limited) stage, and is doing it's thing and eventually riggers to the much lower float voltage (because say you can't disable it).

If you watch your current, you'll see that there is NO current flowing at the float voltage it switched to. So in the case of your typical 12v lifepo4, bring the float voltage value UNDER 3.45v per cell.

This is the one thing that we can remember - 3.45v per cell *under charge* is the *minimum* needed to achieve a full charge given enough absorb time for LFP. So if you set your float voltage well under this, when the controller kicks into float, you'll either have zero current, or very little - so little and by virtue of being lower than 3.45v per cell, it will never over charge. The battery has already satisfied the SOC so high with normal charging, that float (SET TO LOW VOLTAGES) has very little effect.

Advantage of a very low float setting? Ah, we can shoe-horn this lead-acid feature as kind of a battery safety feature - that is - what if you have some sort of tiny short, or parasitic load like an alarm, or some other gadgetry that doesn't immediately seem to be visible?

In one controller I had for my 12v lfp, and I couldn't disable float, I set that value to about 12.8v. Low enough for no current to actually flow after it switched to it from a decent charge in the first place.

The low-voltage float setting will ALSO act as an emergency catch to help keep your battery from totally discharging if the battery DOES get low for some reason. NOW there will be current flowing, and keep them charged enough to prevent a total discharge.

So here's the deal where it gets all confusing - one *can* utilize controllers designed for lead-acid, provide we disable temp comp, and keep the float setting low - albeit float is not necessary in the truest sense of the word with LFP.

Shoehorning the features of a lead-acid controller into LFP use is what we're doing, but it takes a bit of knowledge to know if you can "get away with it." Heh, real-world vs the lab.

[PNjunction]

TIME TIME TIME for LFP

Even the guys over at marine Cruisersforum who have been running LFP for years are starting to see signs of "memory" and reduced capacity when doing repetetive small discharges, or only charging up to 80% and the like over and over for years.

Yet when they do a full capacity discharge test, and recharge to what is assumed to be full, they only get about 70-80% of the rated capacity.

Yes, there is the usual voltage-obsession. Some are knowledgeable about "taper current" C values being important too. But many forget to juggle TIME.

But the biggest juggling factor even if you know nearly everything about this is the time factor! More time spent near full-charge, means more oxidation of the plates - basically the non-organic junk clogging the pores. Making it harder and harder for ions to intercalate. Analogy - take a paper towel, dip the corners in some gravy, and try to wipe up a spill. That towel has less spill-soaking capacity because of the pore-clogging gravy. And nothing will get it back - not even "proper" charging techniques, or wringing the towel out. The gravy is in there!

That's why the usual recommendation is to charge up fast and get it over with. Don't spend too much TIME at or near full charge. And this is NOT a voltage issue, but an SOC issue.

THIS is why a true float, even at say 13.8v under charge is the killer. It is not because the cells are being held to a conservative 3.45v per cell under charge conditions, BUT they are spending all their time near a true full charge once that laboriously long CV (absorb) at 13.8v finishes.

AHA! So, unless one is doing daily cycling, thinking you are saving your cells by using "converative" CV voltage values like 13.8v / 14.0v or so is not a universal panacea. It is the *TIME* spent oxidizing / making gravy that is hurting the cells. Pulling your batt under the cv phase early just stops the clock sooner.

So basically that is why when I first started out with lifepo4, I purposedly *derated* the battery makers capacity claims, because in the real world, I knew that TIME was going to oxidize my cells no matter how good I thought I would be.

Time's a bitch!

[PNjunction]

SOLAR problems

So what does this mean in the real world to us solar guys, usually with very low / inconsistent charging rates?

You design your system by derating the capacity of the cells by 30% to support your application. Hmm .. while I won't try to revive any lead-acid vs LFP lifecycle costs, that is something to consider.. "I figured that a 70ah cell was what I needed, and now you're telling me I need to purchase a 100ah instead?" Yep. Time is your mistress no matter how good your bms is.

Or you totally over-purchase capacity so that you waste a LOT of bucks. Like charging cells to well UNDER 3.45v. WHOA - waaay too pricey, what's the point? Zheesh, just replacing lead-acid over and over is FAR cheaper.

And this is all assuming your install is lab-spec perfect. DErating the cells seemed to be the easiest way to deal with this mistress of time...

[PNjunction]

The big aha! Moment.

Sorry to be so bloggy - but had to share my battery nerd comment ...

Sulfation - right - we need to deal with that with lead-acid by trying to achieve full charge as often as possible. TIME left undercharged produces sulfation and reduced capacity.

Oxidation - right - we don't want to leave the LFP cells in a full or nearly full environment because of TIME oxidizing the plates and reducing the capacity.

Interesting - when designing a solar system, we typically derate by 30%, so I did the same for LFP. And it worked out.

Cycle-life claims for either LFP or lead-acid? Heh, same battery marketing department where they hammer out cycle claims without regard to time. Even if they test it, time is not taken into consideration. Hence why the online video by Prof Jeffrey Dahn of why lion batteries die was so instructive to me early on..

When I saw that the two ways of dealing with these batteries in an opposite manner - simply because it was either sulfation or oxidation I was compensating for - was a watershed moment.

[PNjunction]

Sorry guys - you got me on a roll - apologies in advance

PARALLEL problems - prismatics vs cylindricals

We go to great pains to try and avoid parallel connections to increase capacity, and get the properly sized battery from the start, or at the most two in parallel, with lead acid. If we can.

Still, for comparison the venerable 12v lead-acid battery only has 6 cells wired in series. You buy the capacity you need.

Not so with typical "3.2v" LFP prismatics. INSIDE, you'll find say 30 to 50 or so smaller cells wired in parallel. And perhaps snugly fitted inside a plastic case.

So the build-quality comes into play - taken individually, are each of those flat "pouches" of exactly the same capacity and internal resistance? How well are THEY balanced inside? In other words, after you get your prismatic and duly charge it, how can you guarantee that the manufacturer hasn't assembled it with pouch #14 and #37 well undercharged compared to the rest when first assembled? We KNOW that merely putting cells in parallel does NOT balance them - at least not enough to make a difference at the higher-voltage levels. Or that those two cells are really far out from designed capacity?

You used to see guys blowing the LFP banks all the time putting a random collection of cells in parallel, assuming they would self-balance, and when current was applied one goes nuts and the other just sits there. BMS individual cell charging aside, just a simple parallel can range from individual cells being within anywhere from 20 - 80 % SOC. Going by voltage is a recipe for disaster with LFP since the charge / voltage knee is so flat in regards to SOC capacity.

So, derating the overall capacity of the cell by about 30% helps in this real-world bad manufacturing aspect too. Oh sure, they may look good out of the gate, but over time, when those two weakling pouches have had enough, they may be putting a drag on the battery as well. But you wouldn't know it unless you cut the prismatic open, hence ruining it.

When Rolls or other major manufacturers dish out prismatic LFP to consumers, then maybe something like this would be an afterthought. But the prismatics available now to consumers? Yep, derating was my friend.

[PNjunction]

End of the rant - promise

The unpredictability of LFP (at least for larger scale projects) is why I am primarily back to being a lead-acid fan. I am so disappointed to have to say that - you have no idea how exciting it was early on. But it is not because LFP is bad tech - it is GREAT tech. It is the manufacturing and pricing structure of it in the real world.

But, DIY'ing LFP, in scales that matter to the average user, is just too much of a crap-shoot. I'd rather replace *quality* lead-acid from manufacturers I can count on. Sure, lead prices like always are rising, but LFP was supposed to counter that.

NOPE. LFP prices have NOT come down, and the typical battery salesmanship that doesn't take into account the real world, like poor manufacturing or pie-in-the-sky pure lab measurements have left me cold.

It's too bad. The only thing left are proprietary systems, or peddling one-off projects based on dismantling crashed cars.

At least with lead-acid, for our application, one still *can* DIY it with some sort of reliability and knowledge based on as usual - quality - components. LFP - cross your fingers.

[Mike90250]

End of the rant - promise.....

Thanks for laying that all out, and you don't have to have a hard termination to the rant, as TIME goes on, we may learn more stuff

[NochiLife]

Nochi - no problems. These are forums where there is no body language, tone, and even a difference in the way one expresses themselves in print rather than speaking. Too many forget this, ie I'm sure we'd all have a blast if we were on a cruise together!

Real quick - unless you have some sort of lifepo4 specific controller, we shoehorn those intended for lead acid. Ok, in this case be SURE to disable any sort of temperature compensation! That will easily put you well under or over charged if left operational. One trick if you can't disable it is to set any temp comp values to plus/minus 0.00 v in the menus... Heh sneaky.

That's exactly what I did (set temperature compensation to 0). Actually tried to see if there was some way I could make the voltage compensation "disable" charging if the battery bank was near freezing temperatures, by setting the voltage compensation to something ridiculous (with a system voltage cap)...but I couldn't figure it out. Ended up using a home-made controller to send the MODBUS "disconnect" command if the batteries were too cold.

FLOAT - we can fool the system by setting this abnormally low. What happens is that during a normal charge, say one that has gone through CC (bulk), entered the CV (voltage limited) stage, and is doing it's thing and eventually riggers to the much lower float voltage (because say you can't disable it).

If you watch your current, you'll see that there is NO current flowing at the float voltage it switched to. So in the case of your typical 12v lifepo4, bring the float voltage value UNDER 3.45v per cell.

Actually, the problem I encountered was NEGATIVE MPPT current (as seen in MSView software, and verified with a clamp meter) when it switched to "float" mode at 3.43vpc (55v "float", 56v "absorption"). It would draw a happy couple of amps, too. I contacted Morningstar tech support, but didn't get a solution.

This is the one thing that we can remember - 3.45v per cell *under charge* is the *minimum* needed to achieve a full charge given enough absorb time for LFP. So if you set your float voltage well under this, when the controller kicks into float, you'll either have zero current, or very little - so little and by virtue of being lower than 3.45v per cell, it will never over charge. The battery has already satisfied the SOC so high with normal charging, that float (SET TO LOW VOLTAGES) has very little effect.

Advantage of a very low float setting? Ah, we can shoe-horn this lead-acid feature as kind of a battery safety feature - that is - what if you have some sort of tiny short, or parasitic load like an alarm, or some other gadgetry that doesn't immediately seem to be visible?

In one controller I had for my 12v lfp, and I couldn't disable float, I set that value to about 12.8v. Low enough for no current to actually flow after it switched to it from a decent charge in the first place.

The low-voltage float setting will ALSO act as an emergency catch to help keep your battery from totally discharging if the battery DOES get low for some reason. NOW there will be current flowing, and keep them charged enough to prevent a total discharge.

So here's the deal where it gets all confusing - one *can* utilize controllers designed for lead-acid, provide we disable temp comp, and keep the float setting low - albeit float is not necessary in the truest sense of the word with LFP.

Shoehorning the features of a lead-acid controller into LFP use is what we're doing, but it takes a bit of knowledge to know if you can "get away with it." Heh, real-world vs the lab.

I noticed further down in your posts that you referenced "multiple small cells"; if only for the record, I got a good deal on overstock bulk 32650 Tenergy LiFePo4 cells on eBay, and assembled over 700 of them into my bank. (Plus a set of home-made 3 amp balancers; Lithium-based batteries do NOT self-balance like lead acid.) So far so good.

[Jman]

Thanks for all replies, juicy info I hope. Will read later tonight. Cheers