Life and cycles

[Don B. Cilly]

This has been addressed before, but no satisfactory answer was provided - that I can find.
Say an LFP battery has a life expectancy of... 5000 cycles.
Pretty good as it is, but... say one never "cycles" it. There's no real reason to do so either, is there?

From what I understand, a cycle is going from above 90% charge to under 90% discharge.
Me, I keep mine between 50% and 85% SOC. Does that actually mean I never cycle them?

And are there any studies on expected life for LFPs at... shallow cycles? No-cycles? Like, a LiFePO4 battery, if kept between [those parameters] is expected to last [so many] years?

 

[rmaddy]

Out of curiosity, why are you planning to only use 35% of your battery's capacity? You need to buy 2 or 3 times as many batteries to get the same use compared to making full use of a LiFePO₄ battery. Getting a few more cycles but spending that much more doesn't add up to me.

 

[Don B. Cilly]

No, it's what I have and it's meant for future "contingencies". Like my daughter spending some time here - my son can deal with low-consumption, my daughter... :·/
At the moment, I live here alone. At solar minimum (we're pretty much at winter solstice) I charge the batteries in three-four hours - even on cloudy days - discharge them in 16. Rest of the time, they basically "float".
I set the charge controller not to go over 27.6V. I never discharge to less that 26.2... the smart shunt says some 50-60%, the leds on the (Pylontech) batteries agree.

So, out of curiosity ;·) am I cycling? And even in a "worse" situation than mine, say you kept them between 25% and 95%, is that non-cycling? Almost-cycling? Life-span impact? Any studies/data?
.

 

[stok]

a cycle works as follows, say you have a 100ah battery, if you discharge 100ah out of the battery and recharge 100ah back into the battery that is counted as one cycle.
If you only discharge 10ah out of the battery every day and recharge those 10ah's back into the battery then only after 10 days have you completed one cycle.

 

[Samsonite801]

Out of curiosity, why are you planning to only use 35% of your battery's capacity? You need to buy 2 or 3 times as many batteries to get the same use compared to making full use of a LiFePO₄ battery. Getting a few more cycles but spending that much more doesn't add up to me.

In my eyes, I might see it as good peace of mind though, to know that I could get through a week of smoke and dim Sun from the fire season, or several days of overcast clouds or something.

 

[OffGridInTheCity]

I view a 'cycle' as a charge/discharge with a DOD % & C qualifier. I'm off-grid Solar so I have daily occurrences of this. 18650 has a wider voltage range so I can control the DOD easier than LifePo4. In particular I'm focused on seeing if this chart from Battery University ( https://batteryuniversity.com/article/bu-808-how-to-prolong-lithium-based-batteries ) has any truth.

I'm up to 1,194 cycles with a yearly average of 36.3% DOD with <0.1C charge/discharge on my oldest 18650 battery and going strong (no detectable loss). I'm currently closest to the 75-25% Soc line in the chart above due to the 25% cut-off (discharge curve knee) I use. However, I'm continuing to expand my battery bank to see if I can operate closer to the 75-45% line and "hope" to get 7000 cycles with only 15% loss of original capacity....

If I can get to 7000 cycles I might even get my money back

I do wonder...
1) When I see significant loss, will it be dramatic or gradual? The charge above indicates gradual.
2) When do dendrites (fire danger?) come into play - at a certain % of original capacity? or is that a red herring.

 

[Don B. Cilly]

a cycle works as follows, say you have a 100ah battery, if you discharge 100ah out of the battery and recharge 100ah back into the battery that is counted as one cycle.
If you only discharge 10ah out of the battery every day and recharge those 10ah's back into the battery then only after 10 days have you completed one cycle.

That's interesting. I wonder if it's actually the case...
See, this is the "history" from my Victron shunt. I got it almost exactly three months ago (Sep. 17th).


If I discharged (in three months) roughly 110 kWh, on a 200Ah battery, the batteries are around 26½ volts most of the time, so divide by 26, 4,200 Ah. - which tallies with Cumulative Ah drawn. Divide again by 220 - the battery capacity - some 19... cycles?
Now, the shunt says 4. It also says things like "Time since last full charge" that don't make sense - except I disconnected the battery a while ago, the shunt reset the parameters, and decided that "charged voltage" was 26.3. The Max at 28.9 is true, it happened twice - the BMS kicked in - after which I actually managed to tell the MPPT to "please don't do that". The min. voltage was a discharge experiment.

So I wonder.
Note that the battery is at 72% on a foggy morning, 9AM, been discharging since say, yesterday at 4PM, so some 17 hours.

 

[Don B. Cilly]

So - I've been reading around quite a bit - no one seems to know what a cycle is, exactly.
What I can infer, though, is that the shallower, the better. Both ways. Exactly by how much... no one seems to know.

It just seems that there is no such thing as a cycle, per se. There are deeper or shallower ones. and shallow ones count for... less than deep ones, we just don't know by how much.

 

[OffGridInTheCity]

So - I've been reading around quite a bit - no one seems to know what a cycle is, exactly.
What I can infer, though, is that the shallower, the better. Both ways. Exactly by how much... no one seems to know.

It just seems that there is no such thing as a cycle, per se. There are deeper or shallower ones. and shallow ones count for... less than deep ones, we just don't know by how much.

To me, it seems like the wider voltage range of lithium-ion vs LifePo4 makes DOD 'control' easier / offers more options but does it really matter in the long run?.

For example, the lithium-ion range is wide enough that one can think in terms like 30% DOD vs 50% DOD. Also, if one designs for a smaller DOD and operates at the bottom voltage range or middle voltage range or upper voltage range - does that make a material difference as well.

 

[robby]

A Cycle as stated by @stok is going from 100% SOC down to 0% SOC.
If you go down to 100 to 50 and do that for 2 days you have done one cycle.

 

[Don B. Cilly]

does it really matter in the long run?.

Well, that was basically what I was trying to work out... in the sense, since I never cycle my batteries even remotely deeply - they're always between 50 and 90%, what sort of life-expectancy was I looking at. The answer seems to be along the lines of "not too bad" :·)
.

 

[John Frum]

No, it's what I have and it's meant for future "contingencies". Like my daughter spending some time here - my son can deal with low-consumption, my daughter... :·/
At the moment, I live here alone. At solar minimum (we're pretty much at winter solstice) I charge the batteries in three-four hours - even on cloudy days - discharge them in 16. Rest of the time, they basically "float".
I set the charge controller not to go over 27.6V. I never discharge to less that 26.2... the smart shunt says some 50-60%, the leds on the (Pylontech) batteries agree.

So, out of curiosity ;·) am I cycling? And even in a "worse" situation than mine, say you kept them between 25% and 95%, is that non-cycling? Almost-cycling? Life-span impact? Any studies/data?
.

Yes you are cycling.
I float at 26.4 and charge to 27.4 and I plan to do it for the foreseeable future.
I would not count your usage as a full cycle though.
Call it a half cycle.

 

[OffGridInTheCity]

Well, that was basically what I was trying to work out... in the sense, since I never cycle my batteries even remotely deeply - they're always between 50 and 90%, what sort of life-expectancy was I looking at. The answer seems to be along the lines of "not too bad" :·)
.

Here's a short recap from a user on another forum that repeated full cycles on 4 different 18650 cells. Its interesting.
(Source - @Generic - https://secondlifestorage.com/index...apacity-testing-18650s-for-1-600-cycles.6868/ )

Capacity vs cycles, starting at 2000mah...


The question I wasn't able to answer was whether cells degrade linearly or exponentially. While the INR cells both fell off a cliff, the ASO cell looked exponential and the LG looked more linear.

T

 

[robby]

A Cycle as stated by @stok is going from 100% SOC down to 0% SOC.
If you go down to 100 to 50 and do that for 2 days you have done one cycle.

There is a video on this Topic by Fortress batteries and I should add a qualifier. A company may state that you get 6000 cycles if you go from to 80% DOD and only 4000 Cycles if you go to 90% DOD. So the Industry has kind of thrown in a relationship between cycles and DOD but a Cycle is from 100% SOC to 0% SOC.

 

[Samsonite801]

I personally don't think estimating lifespan based on cycles is even a super relevant value to worry about too much (as the end-user). Some manufacturers give estimated 'cycles' ranging from 2000-6000 or whatever kind of crazy range estimates, which may or may not be based on any kind of educated guess or based on a real value other than sampling and projection (as I doubt any of those manufacturers actually ran through 2000-6000 cycles in their testing).

If the bank was capacity tested at initial time of deployment (even as Will recommends), then later on one can run more capacity tests throughout the lifespan and compare the values, to get an estimation of where they are in the estimated lifecycle of the bank. Like say for example, now I am sitting at 93% of original baseline number, and based on how many years old it is, I might be able to expect xx value will be left in 2 more years.

For me, I just prefer not to worry about it, just size the bank initially with extra capacity and if it gets too weak to work well (at some point later), then decide what to do, like add some more capacity or whatever. That's just me though, I used to get all OCD with stuff like this years ago, but I've learned to loosen up and just not worry about it much anymore.

By the time it will ever be a problem, they will likely have better battery tech out by then and you will want to upgrade anyways at some point.

 

[RobertGreen]

Apparently, the life cycle count that you can expect can change drastically depending on how the cells are operated and stored during their lifetime.
Some variables include: operating/storage temperature, the charge/discharge rate or "C" rate, how long of a time of rest between cycles, and the depth of discharge.... and that's without getting into more esoteric variables such as compression fixture pressure etc.
These cells experience calendar aging, even when they're not being cycled or used. In other words, just because a certain testing condition yields a cycle life of 6000 cycles when the tests are run back to back like the EVE LF280K datasheet claims, doesn't mean that you would expect to see 6000 cycles when the cells are cycled once daily, as is normal in a lot of solar storage applications.

It's commonly recommended to just try to stay out of the steeper part of the "knees" of the charge curve, and these cells will probably last a long long time.

 

[Don B. Cilly]

Yes, fuzzy logic and common sense are good enough for me.
I have managed to tell my MPPT controller to not charge them fully. I'm in the privileged position, ATM anyway, to not have to discharge them much at all.

How long I can expect them to last... "a long time" is good enough. I just know I'm doing the best possible thing.