Lithium Charge Limits, 75%?

[bobwilcox]

I was reviewing some Lithium (LiFePO4 chemistry) charging recommendations, and came across a recommendation to only charge to 75% SOC. The article said that if you keep the battery between 75 and 25 SOC, you will get 10 times the cycles, compared to keeping the battery between 100 to 50% SOC.

I have been charging to 100% every day (if possible) thinking that if I get several cloudy days, I will still have battery capacity. If I only charge to 75%, I'll be more limited, and will have to run a generator sooner with a streak of bad weather.

How do you guys charge your batteries? Is anybody limiting your charge? How do you do it? Do you set your max charge voltage lower? I don't want to physically monitor the charge. I'm using all victron, with a bmv 712 monitor.

If we are only supposed to be using between 75% and maybe 10% SOC, this is kind of the same limitation that a lead acid or gell has, never taking them below 50%, to keep from cutting lifetime cycles. It kind of destroys the 'capacity argument' for lithium. Am I missing something?

Thoughts, please!!!

[HarryStone]

https://dakotalithium.com/2021/09/22...hoCnH8QAvD_BwE

Interesting read. While not dealing specifically with your question, the article does state that it is charge time, and not charge capacity that injures your Lithium batteries.

[RoadTrip2084]

LiFePO4 cells typically have between 3000 and 6000 cycles life expectancy, the differences between those are controlled by many variables, such as temperature (higher is worse), charge/discharge rates (very high is worse), wether the cells are compressed to manage expansion (unlikely), and yes, how long you keep them at a very high or very low state of charge. My understanding is that cycling them zero-100% is actually fine, but that ideally you wouldn't keep them at 100% (or 0%) for very long.

If you cycled from 100%-0%-100% everyday you're still looking at 8 years of use, and even then the cells will likely have 80%+ of their original capacity remaining.

It makes no sense to voluntarily restrict yourself to 75% of your capacity in the hope of extending your battery's lifespan to try to avoid potentially losing 20% of your battery's original capacity in 8 or 10 years. What's the point of having 100% capacity remaining if you've never used it?

So if it were me, I'd go ahead and use 100% of their capacity and only concern myself with trying to not leave them stored/idle at >90% and less than 20% (while rig is stored).

[gatorb8]

for everyday use I would just go to 100%, if you store your rv for say a few weeks or more I would go to whatever the recommended “storage charge” is, say 75%

[Marine359]

I’ll be on the wrong side of the pavement even if my LiFePo4 gives up at 3,000 cycles. Then it drops to 80% for who knows how long. They’ll use it then to power my Hearst. I see no rational reason not to charge to 100% and store at 60-75%. People may be shortening the life by continuing to use a FLA profile charger after switching. LiFePo4 does best, and charges shorter time when receiving its max rated charge amperage. So, if you drop the cash for the battery, don’t close your wallet until you get a good charger.

[DavidEM]

I have a moderate size Li battery on my eBike. The manufacturer says that for long term storage, 70% gives best life. So, since I use my eBike while camping, I try to store it afterwards at 70% but before I use it again, I charge it up to 100% first.

I get the best of both worlds: full capacity when I use it, longer life when I store it.

David

[bobwilcox]

Thanks for the feedback.

I actually mostly agree with all of the posts', if you think about it, you either give up capacity at the end, or throughout the life of the battery. Hopefully the price of the batteries (or capacity per $) will continue to improve over time, and it will be a moot point if the batteries don't outlive us...

The source of the data was a Will Prowse video from several years ago.

https://www.youtube.com/watch?v=Yf9N9zBgyB8&t=418s

Thanks, everybody!!!

[xrated]

Originally Posted by Marine359

I’ll be on the wrong side of the pavement even if my LiFePo4 gives up at 3,000 cycles. Then it drops to 80% for who knows how long. They’ll use it then to power my Hearst. I see no rational reason not to charge to 100% and store at 60-75%. People may be shortening the life by continuing to use a FLA profile charger after switching. LiFePo4 does best, and charges shorter time when receiving its max rated charge amperage. So, if you drop the cash for the battery, don’t close your wallet until you get a good charger.

I disagree with your statement about using an FLA charger. An example of the WFCO 9800 series converter/charge will provide 14.4V for up to 4 hours to the battery in bulk mode. Depending on the A.H. size of the LFP battery and the size of the charger, that 4 hour window will possible charge the LFP up to approx. 99% SOC. After the 4 hour bulk charge is done, it drops down to a 13.6 volt absorption charge and at that voltage level, a LFP battery will be charged to at least 95% SOC. I am in the process of building a 4 cell 12V LFP battery that is rated at 304 AH and have no intent on changing out the OEM converter charger that is made for FLA. Taking the battery to 95% SOC is easier on the battery and will allow for a longer life that hitting that 3.65V fully charged area of the battery charge curve.

Check out this thread. There are many others that agree with what I've stated....
https://www.irv2.com/forums/f258/pow...er-565116.html

[Marine359]

Sorry X,
Just posting what I’ve read and researched, like this article:
https://glider-battery.eu/post4-1/

Although it’s altogether possible that certain FLA chargers like the one you mentioned may be able to do the job well, in which case, you’d be absolutely correct.

[Isaac-1]

That is not exactly the best article, there are some truths there, some half truths, and some possibly irrelevant truths depending on the application.


Yes it is true some lead acid chargers have equalization modes at over 15VDC which can not be disabled and should not be used on LiFePo4, I really doubt that it is most. Not all chargers that have equalization modes can't be used, only those that equalize at over 14.6-14.8VDC, My PD9200 series converter does an equalization charge for 15 minutes every 20 hours at 14.4VDC which can't be disabled, the chance of this causing harm to my 420AH LiFePo4 battery bank is negligible, and under normal circumstances with storage float at 13.2VDC that roughly 15 minutes per day at 14.4 just about negates parasitic losses.


As to that need for voltage to drop down to "return to bulk", their math assume only low amp draw from the LiFePo4 battery bank, most people that seem to post on here want to run high amp draw appliances, like inverters running microwave ovens with their Lithium battery pack on a semi-routine, simply microwaving anything for a few seconds would cause enough voltage sag in a typical LiFePo4 battery bank to trigger re-bulk mode. In my 420AH LiFePo4 install the microwave running for more than 5-10 seconds is enough to drag the battery voltage as measured at the battery terminal down to about 12.4VDC when I have a fully charged battery bank, that voltage of course recovers back to above 13.2+VDC once the microwave stops.

[757driver]

Originally Posted by xrated

I disagree with your statement about using an FLA charger. An example of the WFCO 9800 series converter/charge will provide 14.4V for up to 4 hours to the battery in bulk mode. Depending on the A.H. size of the LFP battery and the size of the charger, that 4 hour window will possible charge the LFP up to approx. 99% SOC. After the 4 hour bulk charge is done, it drops down to a 13.6 volt absorption charge and at that voltage level, a LFP battery will be charged to at least 95% SOC. I am in the process of building a 4 cell 12V LFP battery that is rated at 304 AH and have no intent on changing out the OEM converter charger that is made for FLA. Taking the battery to 95% SOC is easier on the battery and will allow for a longer life that hitting that 3.65V fully charged area of the battery charge curve.

Check out this thread. There are many others that agree with what I've stated....
https://www.irv2.com/forums/f258/pow...er-565116.html

Depending on the size of the lithium bank and the size of the charger that “up to a 4 hour bulk” may well charge the lithium bank to 100% SOC. The point is what ever SOC it gets to dropping to an absorption charge of 13.6v will keep the lithium bank at that high SOC which could be 100%. If you keep a lithium bank charging at 13.6v long enough it will get to very near 100% SOC.

[Isaac-1]

I agree, the important thing to remember here is that the LiFePo4 voltage curve is very flat and then bumps up to 14.1VDC within the last 1% of charge, therefore 13.6VDC float will eventually get you to over 98% state of charge. Also that eventually is likely measured in hours not in days as LiFePo4 does not need that big voltage difference acceptance rate like lead acid does to push current into the battery due to their extremely low internal resistance.


So sure an optimal LiFePo4 charger might hold at 14.4VDC longer, but even at 13.6VDC it will still charge fairly fast, possibly still close to max amp output of the charger.

[Marine359]

Thanks gang. Very informative, and I learned a few things.
But, I’ll stick with my Xantrex so I don’t have to think much.

[Isaac-1]

I am not sure if i have said this in this thread or not, but if you have a Xantrex inverter/charger and use it on LiFePo4 batteries you likely need to disable the temperature compensation (unplug the temperature compensation probe) to keep it from over charging in cold weather. Check your owners manual to see how to do this on your model.