LiFePo voltage sagging

[Almot]

120 AH, , nominal 12.8V.

12.95V unloaded, 12.8 under 4-5 A load.  According to discharge curve 12.8 is 30% (they measured at much higher 0.5C rate discharge): Lynac 12.8V 120 AH .

Quite a bit of sag. Battery is 3 years old.  I did bring it to 20% SOC once or twice.

[StirCrazy]

Just out of curiosity what size wires do you have that battery wired up with and how long is its run? 

[pianotuna]

Thanks for the care of LI link!

[Almot]

I don't have access to BMS, sealed unit, don't want to open and wouldn't know what to do if I opened 🙂

The battery is mostly for back-up, there is grid power. I'm trying to find a middle ground btw the popular mantra "Don't keep for long in Absorb, don't float, don't keep it 100% all the time" on the one hand, and the need to keep it near-full in case of blackout on the other hand. Blackouts happen a few times a month.

I would like it to stay at 80% and cycle shallow. What I do now is letting it charge on solar + converter, and then shut converter off for the night to reduce floating time. 

Ideally, I would like to store it unloaded at 50% charge for a longer life but this is unrealistic and I doubt that my life will be much longer than 4000 battery cycles (6 months a year = 180 cycles a year = 22 years).

[StirCrazy]

haha ya thats what I meen, I was all worried about getting the most life out of mine, I believe it was an old habbit of the flooded cell days, treat them right and you will only have to buy new ones every 8 years, with the LFP I am finding after two years on my 280AH running at 100% to 10% I havent lost one bit of capacity.  well 100% to 10 are my cutoffs, it rarely sees under 50% , because of the extra capacity I built in, but always is charged back to 100%.  its kinda funny people are still afrade to go to 100% of capacity for the charge, if you look at all the people running big 48V battery back up systems on their homes they charge to 100% every day with a specified amount of adsorbtion time then they drop off to 13.4 to 13.6V for the float.  and these guys are cycling every day, and floating as long as the sun is up.  I honestly don't know where half the rhoumors and old wives tails came from in the rv world that we can only use 80% of the capacity, don't float, don't fully charge and so on.   most of them are totaly BS and prevent people from using there batteries to the fullest.  the one hard one is don't charge below 0C , and don't float at 14.6V if you have a float have it set for 13.4 to 13.6 depending on your school of thought.  idealy a 3 stage charger that you can customize bulk, adsorb and float voltage (and time in the case of adsorb) would be ideal.

thats not to say park your rv for the winter and leave it floating for 6 months, thats concidered storage and I take my battery to 90 to 100% and shut it off ( battery disconect that leaves no parasidic loads) and it sits like that till camping season is close.  even during camping I don't charge between trips, just charge it to 100% the day before I leave, then the solar handles the charging during the trip which takes it to 100% every day.

As of this morning I have taken my old battery (280AH) out of the camper and it is going to a friend for his camper and I am just working now on the replacment battery for my camper.  upgrading to newer larger cells and if they are good I am buying 16 more for the 5th wheel.  the biggest advantage, aside from larger capacity, is the ability to run at -35C (discharge only, and only 10 degrees colder than the old cells) and increased cycle life  (3500 to 4500. ) at 100% usage

[Almot]

There is some rationale behind charging to 80%, cycle life is longer.  Having max available energy is more important for me so  I'm going to ignore this.  Besides, charging to 100% takes care of cells balancing.

OTH in storage, say unused for more than a week or two, I don't see why not discharge it down to 40-60% as per recommendations. Even if it saves me 5% of capacity in several years. Hot storage is the worst: https://www.uschemicalstorage.com/lithium-ion-battery-storage-requirements/ 

[StirCrazy]

because the link you provieded doesnt say which chemistry of Li it is referencing, we don't know which kind of Li they are refering to.  for example battle born and dragon fly recomed charging to 100% and turning the battery off.  eve who makes the cells I use says 3+ months is concidered long term storage saying to charge the to a minimum of 50% and up to 100% remove the battery and store it in a cool dark place 0C to 25C.  there are people in vegas who have been playing with cheep 100ah batteries and leaving them in the sun outside cycling every day and havent been able to kill them.. what does kill them is leaving them at 0% for long periods of time. 

I would be more concerned about the bottom 20% than the top 20%, unless you are going for huge cycle life.  the setting youer bottom at 20% and your top to 90% will get you upwards of 7000 cycles.  if you use yours from 100 to 20 you still have 100ah as they built in an extra 20, and by not using that bottom end you get 6000.  100% use will probably drop you to 3500-4000 but unless we know the brand of cells we are only guessing. 

so many of the myths and rhumors are exactly becasue of that link you posted which PT is now going to post everywhere inorder to bring about the rise of overpriced special agm batteries again 😁

[pianotuna]

We are not interested in 'opinions', which is what S has offered. Here is a page that is specific about LiFePo4.

https://www.evlithium.com/Blog/lifepo4-battery-care-guide.html

[Grit_dog]

wow. Lol

[StirCrazy]

I don't know how you can say it is all my opinion when it comes from a comunity that uses the cells for much larger and more powerfull builds than we ever do, and especial when you don't even use LiFePo4 batteries or cells.  my opinions are based on the comunity that has years and years more experiance than us and from the manufactures of the cells own data sheets, not from compasnies that only assemble them and sell them and are more worried about protecting their bottom line by minimizing warenty returns and may or may not use quality parts, which is what your "opinion" is bassed off.  how many cells have you bought, and how many build have you done?  are you going to tell me if you use 100% of the capacity in each cycle your not going to get the rated cycle life that the manufacture of the cells rates them for?  are you going to tell me that when using eve cells and they concider long term storage as greater than 3 months with no discharge or charging that they are not telling us the truth?  untill you have actual experiance with moderen cells should you have an opinion, you have been bashing LiFePO4 for years saying they are not as good as your SiO2 which you don't own either, but rather rely on sales sheets put out from people who sell them.  I would love for you to start reading studys and littiture from the actual makers of the cells so you can be more knolagable in this area and not continualy pump out misleading info about other chemistries of Li and try convince people that it applies to LiFePo4 also.  

things are changing all the time with new generations of LiFePO4 and other Li chemistries that are new, for instance the paper on my cells now say I can charge at -10C, am I going to, proably not, 0C is fine for me as my cells are in the conditiond space so there is no need.  for discharging they now say down to -30C, again the chances of me needing to discharge that cold is next to nill as the wife won't go camping when it is that cold, my self I would go camping to -15C if I had to and have a couple times.  for the most part I agree with the last link you posted, but they don't say which cells they are using or what generation of them they are.  this can make a big difference in some of the peramiters they are stating..  for long term storage I believe I stated between 0 and 25C , but I said between 50 and 100% which is what a lot of the recomendations are now.  what I don't understand is them stating to preven freezing to keep them above -20C if mine are rated for discharge to -30C and a few new versions to -35C then how does -20C cause it to freeze?  could be a older version would be my guess and more of an infrence from the older versions having a discharge operation range of -20 to 60C.  at any rate thats not the recomedations for my cells from eve, and that page isn't from eve either it is a reseler's recomedations.  I sent you the data sheet for my cells when you asked me to last year, guess you haven't read it yet.  

I used my old 280ah battery for almost 3 years in the camper and realy I started out using it from 90% to 10% then later I change to 100% to 10% .  I did shut the charge down on my last day of camping to drain it just off 100% so it would be 90% or a little lower when I got home, didn't charge it at home at all, but would charge it to 100% the day befor leaving to go camping.  then while I was camping I would leave the solar on charging to 100% every day.  in the winter when I wasn't using it all I did was tune the battery disconect on and let it sit all winter (only about 4 months here and only about 3 weeks of real cold other wise the normal high is around 0C and lows to -8C (that 3 weeks it does get down to -30 and this winter we had a week of -35)  we did a capacity test the day buddy came and picked it up and I had lost 0.003% of my capacity in almost 3 years...  so a loss of less than 1AH and yes I agree the worst thing you can do is have it at 0% capacity and cold, but that goes for any battery doesnt it?  

every year it seams a new version of LifePo4 is coming out which have many improvments and the bar is always changing, this is the reason I went with the newer cells as I do go camping in alberta in the winter, well I use the camper to visit people and you never know when your getting caught in a polar vortex now days haha

[pianotuna]

S by providing links that support your opinions. Which is what I do.

[pianotuna]

Hi Almot,

Best to bring the Li to 100% state of charge every 30 cycles. Do you have access to the bms on these units?

[StirCrazy]

 12.95V resting it is only starting at 25%.  what are you charging them up to? A flooded deep cycle would be drooping a lot more at that low state of charge if it even works.  The one problem with LFP batteries is getting an accurage state of charge from voltage due to the small change from 70 to 30% and every cell will be slightly different so charts will be close. 20% soc won't huirt a LFP battery and expecial in the way your thinking..  the cycle life is based off a 100 to 0 to 100 charge cycle, by limiting the top or bottom end all we are doing is increasing the cycle life.  so the battery you have says if you go from 100% to 20% to 100% all the time you will get 6000 cycles so they ignore the standard industry standard of 100-0-100% so you can't compare to other batteries.  also they kinda don't tell the truth in their littiture with this statment trying to increase sales. 

 "battery is truly rated for 12.8V 120Ah (1536Wh)
since roughly 20% of the rated power stored in all Lithium Iron Phosphate batteries is unusable. We
strive to give you more for less - change the game."

a LiFePo4 has 100% of its capacity available at the rated cycle life.  at the end of the cycle life you simply have a battery that only is 80% of the original batteries capacity but it can be used for who knows how much longer. 

I don't mind that they recomend from 100% to 20% capacity that will increase your capacit and is how they get up to 6000 cycles, and protects then from warenty claims. SO I guess their statment could be true if you look at it like you can only use 80% of the capacity of you want to get 6000 cycles, but even 3500 to 4000 cycles at 100% is a long long lasting battery, I would be tempted to use it as a 120AH battery myself haha.  I see it uses celindrical cells so you can pound the charge a bit harder than prismatic as they disapeat the heat a tiny bit better, I would like to see you charge it up to 100% and try it after it sat resting for 30 minuits to get a true 13.6V resting volatage.  in the mean time I will continue to look for my volatage chart for LFP under load, I changed laptops a couple months ago so I'm still trying to find a bunch of stuff....  with that chart I will know what a normal drop is if they have a measurment for a 5amp load.