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Old 06-06-2012, 12:49 PM
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Default TS Cycle life clarification

I wanted to see what everyone thought of the various cycle life stats. For the sake of keeping it simple, let's use the original TS spec of 2000 cycles to 80% at .5C.

When I first looked at cells a few years ago, I assumed the specs meant that I could expect 2000 cycles when discharging to 80% at .5C, but now I don't think that is what it means. Some specs say 2000 cycles at 80% DOD which is far different then 2000 cycles to 80% capacity. When they do these tests, are they doing full 100% charges and discharges, or are they doing a full charge and then discharging to 80% DOD? My new assumption is they are 100% charge and discharge. If that assumption is correct, then will there be a better chance of greater cycle life if avoiding the ends of the charge and discharge curve? (I do understand normal use is different than a constant .5C charge and discharge, and that you should de-rate them more because of it.)

For example, I will use maybe 50% DOD daily and will drive mostly freeway at around 1C rate with accelerations in the 2C to 3C range. This would give me about 500 cycles a year. If I de-rate the graph I saw of 8000 cycles at 60% capacity, due to my usage, to 5000, then I should still expect 10+ years of use, correct?

Sorry if this is not very clear. Let me know if I can clarify what I am getting at.
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Old 06-06-2012, 12:55 PM
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Default Re: TS Cycle life clarification

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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Old 06-06-2012, 01:46 PM
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Default Re: TS Cycle life clarification

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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Old 06-06-2012, 03:05 PM
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Default Re: TS Cycle life clarification

Quote:
Originally Posted by Ziggythewiz View Post
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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Old 06-06-2012, 03:16 PM
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Default Re: TS Cycle life clarification

Quote:
Originally Posted by Ziggythewiz View Post
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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Old 06-06-2012, 04:34 PM
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Default Re: TS Cycle life clarification

Just found this:

http://jackrickard.blogspot.com/2010...-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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Old 06-06-2012, 05:05 PM
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Default Re: TS Cycle life clarification

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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Old 06-06-2012, 07:53 PM
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Default Re: TS Cycle life clarification

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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Old 06-06-2012, 08:21 PM
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Default Re: TS Cycle life clarification

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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Last edited by Ziggythewiz; 06-06-2012 at 08:36 PM.
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Old 06-07-2012, 10:49 AM
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Default Re: TS Cycle life clarification

Quote:
Originally Posted by Ziggythewiz View Post
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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