# TS Cycle life clarification



## octagondd (Jan 27, 2010)

Here is an example. The Balqon page lists this spec:

Cycle Life @ 80% DOD : 2,000 Cycles - to me this implies if you discharge only to 80%, you will get 2000 cycles

But the TS pdf graph shows 2000 cycles to 80% capacity which I now assume means the capacity of the cell slowly gets lower with each cycle


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## Ziggythewiz (May 16, 2010)

It's usually 80% capacity after 2000 cycles to 80% DOD. That's one of the big benefits to lithium, you can go well past 50% without significantly hurting the cells.

Some people say the shallower the discharge the better, but the only good data I've seen had numbers that showed if you integrate the total WH consumed you'd do much better by discharging to 60 or 70% DOD rather than 10 or 20% (you get less cycles but much more WH per cycle).


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## octagondd (Jan 27, 2010)

Ziggythewiz said:


> *It's usually 80% capacity after 2000 cycles to 80% DOD.* That's one of the big benefits to lithium, you can go well past 50% without significantly hurting the cells.
> 
> Some people say the shallower the discharge the better, but the only good data I've seen had numbers that showed if you integrate the total WH consumed you'd do much better by discharging to 60 or 70% DOD rather than 10 or 20% (you get less cycles but much more WH per cycle).


I'm not so sure that is how the battery testing works. If you set a cell up to cycle, I imagine you would set it to charge to a certain voltage and then discharge to a certain voltage. After a certain number of full cycles, you would see how much capacity was left. After 2000 full cycles there is only 80% of the original capacity discharged and after 3000 there is only 70% of the original capacity discharged.

I don't think you could set it up to discharge to a certain DOD% because the capacity is a moving target. The more you cycle the lower the capacity.


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## octagondd (Jan 27, 2010)

Ziggythewiz said:


> It's usually 80% capacity after 2000 cycles to 80% DOD. That's one of the big benefits to lithium, you can go well past 50% without significantly hurting the cells.
> 
> *Some people say the shallower the discharge the better, but the only good data I've seen had numbers that showed if you integrate the total WH consumed you'd do much better by discharging to 60 or 70% DOD rather than 10 or 20% (you get less cycles but much more WH per cycle).*


I am very interested in this data though. Curious what people's thoughts are on why this is the case. Charge efficiency? Top balancing shunts? Battery efficiency when they are warmer?

Do you by chance still have a link to that info?


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## octagondd (Jan 27, 2010)

Just found this:

http://jackrickard.blogspot.com/2010/06/life-in-lifepo4-cycle-life-and.html

About half way down there are some graphs and Jack explains that the CALB tests are 100% charge and 100% discharge. I am now curious if cycle life would be even better by staying away from the ends of the charge/discharge curves as that may be where you lose the capacity the most. I know people have stated to keep away from the ends so as to not destroy the batteries outright, but it could also affect cycle life. Just a theory though.


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## bjfreeman (Dec 7, 2011)

You also have to specify the Temperature since that effects all measurements. Typically 25C.
Here is my take:
think of the battery as a bunch of electrons or charged atoms.
as you charge and discharge a certain amount are dissipated as heat.
Therefore the longer you charge/discharge, or the rate is higher on charge and discharge you will lose some capacity.
The battery has a knee (80%) under certain charge/discharge.
The higher the discharge the Knee moves closer to 50%.
the importance of the Knee, is you have to charge a lot longer to return to the normal voltages, if you discharge past the Knee.


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## MN Driver (Sep 29, 2009)

Towards the end 70% depth of discharge and lower the cells start their path of heavier sag and in the process generate quite a bit more heat than the rest of the discharge cycle. The higher your discharge rate is getting towards the end, the more heat gets created. I'd imagine at a lower rate towards the end isn't quite so bad in a cycle life test drawing at a low rate but I'd be hesitant to go 3C on a pack that only has 20% left. I'm personally planning to avoid anything over 1C if I ever drop down to 20% SOC, not just to try and make it home but to try to avoid getting them hot internally and stressing them.


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## Ziggythewiz (May 16, 2010)

The specs say 80% DOD and 70% DOD, not capacity. I think if they meant capacity they would have said that. It's well known that you get better life by staying away from the edges. I'm sure big companies like CALB have done more than one kind of test. They likely have done some to 100% DOD and others to 80 and 70. 

CALB manual says you should be alerted at 3.0V and should stop discharging at 2.5 They strongly recommend shallow charging and discharging.


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## octagondd (Jan 27, 2010)

Ziggythewiz said:


> The specs say 80% DOD and 70% DOD, not capacity. I think if they meant capacity they would have said that. It's well known that you get better life by staying away from the edges. I'm sure big companies like CALB have done more than one kind of test. They likely have done some to 100% DOD and others to 80 and 70.
> 
> CALB manual says you should be alerted at 3.0V and should stop discharging at 2.5 They strongly recommend shallow charging and discharging.


All I can go by is real data and the only real data I have found is what Jack posted from CALB. Spec sheets don't mean anything and especially spec sheets translated from Chinese. Remember the original TS spec sheets? They said to charge to 4.2. This caused all kinds of problems for the first buyers of the cells and those people completely reject them now based on a bad experience. They have since revised their charge spec to 4.0 and then again to 3.8. Also their discharge spec now sits at 3.0.

Here is what Jack said about the CALB excel spreadsheet data he got from the salesperson:

"The tests are of course at a very moderate 0.3C. But the thing that just blows me away is that all 500 cycles are to 100% discharge - not 80% or 70%. They run until the voltage at 0.3C breaks 2.0v which is their definition on the spec sheets of 100% discharge."

I would really like to see data from a 1C test and a 2C test for a significant number of cycles like 500-1000. I would also like to see data from a test where someone actually discharges to 70% or 80% for each cycle which would mean they would have to recalculate on every cycle how many AH to remove.


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## Ziggythewiz (May 16, 2010)

There's lots of data out there for 100% DOD cycle testing of various chemistries at different discharge rates and different temps. I don't know where the one I had seen was that showed the 10% 20% etc. It was similar to this: http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries

but for LiFePo4 so it was more favorable for cycle life, though not as much for the shallow discharge.

I don't think anyone would bother recalculating capacity during the test, you just base it off the spec number and the test ends when you can't hit that spec anymore. So if I want to test the cycle life of a 100AH cell at 80% DOD I'll take 80AH out and put about 80 back until I can't get 80AH. That's the spec. It's likely that cell started out with a capacity of about 105-110 AH, but you don't take 84 or 88 AH as 80% of that, you just take 80% of the spec.

When you test any random cell, approx half of cells will be better, and half will be worse. Obviously you don't want half your customers happy and half mad, so whatever you think is typical you'll add some measure onto that, and tell you users to treat them somewhat better than you did, and that is the spec you base your warranty on. That way you end up with 95-99% of product living up to the spec.


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## octagondd (Jan 27, 2010)

Ziggythewiz said:


> There's lots of data out there for 100% DOD cycle testing of various chemistries at different discharge rates and different temps. I don't know where the one I had seen was that showed the 10% 20% etc. It was similar to this: http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries
> 
> but for LiFePo4 so it was more favorable for cycle life, though not as much for the shallow discharge.
> 
> ...


Thanks for the link. I know it is talking Lithium-ion in general, and Table 2 seems to clearly show what you are saying about the shallow cycles not being as beneficial as the 50% DOD cycles. I wish they included a 75% DOD. It appears 50% DOD gets 3x the number of cycles of the 100% discharge which is better then just the doubling you might expect. I also like that they state they tested the batteries until they reached 70% of original capacity.

If these numbers are anywhere close to translateable to the CALB info Jack had, then if I discharge to 50% of original capacity on average, and derating for higher C discharges, I may see upwards of 4000-7000 cycles, depending on my driving habits, before hitting the 70% remaining capacity mark. I could get an additional 1000 or so before the capacity drops to unusable for my commute. Thats 5000 cycles. Very nice. I agree with Jack that these batteries are probably under-rated, which is great for the consumer and makes the producer look good as well.


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## Ziggythewiz (May 16, 2010)

The exact specifics are irrelevant for any chemistry not exactly the same as what you are using. Even the difference between CALB/TS types would affect where the ideal use point is. I expect the ideal is somewhere between 50-70%, but that doesn't factor continued use past the defined end of life, but there shelf life becomes a factor.

Imagine a pack where you can do your commute in about 40% DOD, but you only charge every other day (this would be my use case if I had a 144V 100AH pack). After your 2000 cycles you've driven the car for 4000 commutes, or around 13 years. Now your capacity is down to 80% of the original, so you start charging every day and put on another 3000 cycles or so before you can't do a full commute on a single charge. So that gives you another 10 years. Can the batteries really last 23 years? I doubt it.


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## jeremyjs (Sep 22, 2010)

Ziggythewiz said:


> The exact specifics are irrelevant for any chemistry not exactly the same as what you are using. Even the difference between CALB/TS types would affect where the ideal use point is. I expect the ideal is somewhere between 50-70%, but that doesn't factor continued use past the defined end of life, but there shelf life becomes a factor.
> 
> Imagine a pack where you can do your commute in about 40% DOD, but you only charge every other day (this would be my use case if I had a 144V 100AH pack). After your 2000 cycles you've driven the car for 4000 commutes, or around 13 years. Now your capacity is down to 80% of the original, so you start charging every day and put on another 3000 cycles or so before you can't do a full commute on a single charge. So that gives you another 10 years. Can the batteries really last 23 years? I doubt it.


I don't think anyone really knows that. The chemistry hasn't been around long enough. Although It seems that they actually last longer than originally thought; which would account for the steady updates to cycle life the manufacturers have been adding to their spec sheets. calendar life seems to be anyone's guess. At least what it's rated for, but possibly much longer. How much longer is anyone's guess. here's an interesting link to a thread about it on this very board.


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## Ziggythewiz (May 16, 2010)

It's not rated for a calendar life though. It may only be a 1-2% degradation per year, but after a decade even that becomes significant.

I suppose it doesn't matter too much, as in 10 years you'll be able to buy much better batteries for much less, but it would be nice to spend that $$ on a new pack for a new EV instead of renewing the old


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## batterytang (Nov 17, 2011)

How to say...

Data is only on paper not on real world.

If you be serious, you became lose...

5000 or 8000 cycles are only a joke..

Not be serious.


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## octagondd (Jan 27, 2010)

batterytang said:


> How to say...
> 
> Data is only on paper not on real world.
> 
> ...


So you are saying that de-rating the actual test lab data to better represent the real world use by 40% is not enough? If I went strictly with the test lab data of CALB cells at .3C, and correlated that with the lithium ion cell testing for cycle life at 50% DOD, I would end up with 7000-10000 cycles and still have 70% of original capacity remaining. I de-rated that to 4000-6000 for real world use at about a 1.5C average.

Do you have any test data you could share with the group that would suggest I should de-rate the cells even more? I like data, not unsubstantiated hyperbole.


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## dtbaker (Jan 5, 2008)

the crux of this is that the capacity is reduced by *some* amount after a couple thousand cycles, but how much is highly variable depending on usual depth of discharge, if you were ever 'unkind' with 100% DOD, temperature, extended 1+C use, etc, etc.

the take awy from this is that the aHr gauge is useful as an estimate based on nominal capacity, but only so long as you understand the capacity will go down over time.... meaning that you HAVE to watch pack voltage to be sure you aren't at 100%DOD. Problem there is that the sag varies under load and with temp; so again, you just have to watch to see if pack voltage recovers to 3.0vpc at rest to get an idea when you're approaching the knee...

i.e. with 100ah cells, you can count on being able to use close to 100ah on the meter... but as time/cycles go by, you might only get 90ah of use before pack voltage doesn't recover to 3.0vpc indicating you're close to 90% DOD.


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## batterytang (Nov 17, 2011)

octagondd said:


> So you are saying that de-rating the actual test lab data to better represent the real world use by 40% is not enough? If I went strictly with the test lab data of CALB cells at .3C, and correlated that with the lithium ion cell testing for cycle life at 50% DOD, I would end up with 7000-10000 cycles and still have 70% of original capacity remaining. I de-rated that to 4000-6000 for real world use at about a 1.5C average.
> 
> Do you have any test data you could share with the group that would suggest I should de-rate the cells even more? I like data, not unsubstantiated hyperbole.


Hi octa,

You know inside the LiFePO4 cells, the cathode is Carbon. Now matter what kind of carbon, the can not reach the cycle life of 3000, 5000, 8000.
So people developing lithium titanate, which is used for instead carbon for cathode.
So if some data show 5000 cycles, 8000 cycles, I do not believe.
I have some friend selling Li-ion battery materials, I just think that is so kind of data playing game. I am not mean CLAB or TS is NOT good, I just point out 5000, 8000 cycle is fake.
By the way, if, such big batteries, single cell can reach 200Ah capacity, and so good performance, so good price, why here is no widely commercial usage?
A123, Valence, Dow-Kokam, LG etc, Including most of Japanese company, all of them do not make cells of TS type. They have reason.


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## octagondd (Jan 27, 2010)

dtbaker said:


> the crux of this is that the capacity is reduced by *some* amount after a couple thousand cycles, but how much is highly variable depending on usual depth of discharge, if you were ever 'unkind' with 100% DOD, temperature, extended 1+C use, etc, etc.
> 
> the take awy from this is that the aHr gauge is useful as an estimate based on nominal capacity, but only so long as you understand the capacity will go down over time.... meaning that you HAVE to watch pack voltage to be sure you aren't at 100%DOD. Problem there is that the sag varies under load and with temp; so again, you just have to watch to see if pack voltage recovers to 3.0vpc at rest to get an idea when you're approaching the knee...
> 
> i.e. with 100ah cells, you can count on being able to use close to 100ah on the meter... but as time/cycles go by, you might only get 90ah of use before pack voltage doesn't recover to 3.0vpc indicating you're close to 90% DOD.


I agree with everything you are saying here. I am only going to discharge to around 50% on a regular basis. Obviously there will be times I will discharge further or less, but I have an AH counter and will be regularly watching my voltage when at rest.

I guess the crux of my original post is, I now believe I originally read the spec sheets wrong and that the battery tests done by the manufacturerers were full 100% DOD tests that went from a certain charge voltage to a certain discharge voltage. I don't believe they ran tests to varying depths of discharge. They just noted the overall capacity loss at x number of FULL cycles.

Not sure what the theoretical limit of the Carbon in the anode is, but I did see an interesting article about replenishing depleted lithium in a cell to gain some capacity back. I am not a member of that science site so I can't read the article, but the summary looked interesting.

http://www.sciencedirect.com/science/article/pii/S0378775311005155


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## dtbaker (Jan 5, 2008)

octagondd said:


> I am only going to discharge to around 50% on a regular basis.



me too.... so who KNOWS how long these things will last, or what the decay rate at end of life will be under 'real' use if you are kind to them ?!

One additional piece of info I want to throw out there is that I was very pleased to see that the Soliton controllers now have 'low-voltage' triggers you can set to catch low voltage at either no-load, or sag under load! very cool feature. Presumably this allows a 'safe' floor at full load with sag as well as watching to be sure the pack bounces back up under no-load.


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