Why High C Rate Cells Live Less than Low C Rate?

Batteries, Chargers, and Battery Management Systems.
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rg12   100 kW

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Why High C Rate Cells Live Less than Low C Rate?

Post by rg12 » Feb 02 2020 7:05pm

The datasheets show that cells with 2C have a much higher cycle number compared to say 8C cells.
but isn't the test being done at the same low discharge current for all cells?
It's obvious that a high current will lower the number of cycles a cell has but in datasheets the cycle number isn't relative to the C rate but is constant.
Am I missing something?

john61ct   100 GW

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Re: Why High C Rate Cells Live Less than Low C Rate?

Post by john61ct » Feb 02 2020 10:39pm

If you are comparing different lines from the same mfg the specs **might** be somewhat comparable.

Between makers likely not at all.

And neither will have much to do with what happens IRL.

The default traditionally was highest energy density, longest life, low C-rates.

To tweak a given formulation for higher power density (C-rate), definitely sacrifices energy density

and IMO likely longevity as well, **even if** the cell never actually sees those high C-rates.

And the price goes up, so value (aka mAh range & cycle lifespan) goes way **way** down.

Moral of the story, only pay the extra for higher C-rates if your use case really needs it, you're likely sacrificing a lot more than just the greater up-front acquisition cost.


goatman   10 MW

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Re: Why High C Rate Cells Live Less than Low C Rate?

Post by goatman » Feb 03 2020 1:02am

john61ct wrote:
Feb 02 2020 10:39pm
If you are comparing different lines from the same mfg the specs **might** be somewhat comparable.

Between makers likely not at all.

And neither will have much to do with what happens IRL.

The default traditionally was highest energy density, longest life, low C-rates.

To tweak a given formulation for higher power density (C-rate), definitely sacrifices energy density

and IMO likely longevity as well, **even if** the cell never actually sees those high C-rates.

And the price goes up, so value (aka mAh range & cycle lifespan) goes way **way** down.

Moral of the story, only pay the extra for higher C-rates if your use case really needs it, you're likely sacrificing a lot more than just the greater up-front acquisition cost.
Im just going to type to see if I can figure out what C means. 1c = 1xah of battery

25r 20a 2500 mah, 1c=2.5amps, 20 amp cell/ 1c= 8c cell

40T 35a 4000 mah, 1c=4amps, 35amp cell/ 1c= 8.75c cell

50E 9.8a 5000 mah 1c=5amps, 9.8amp cell/ 1c= 1.96c cell

if people recommend running at 2c or 3c, lets pick 2c

25r 2c is 5amps

40t 2c is 8amps

50e 2c is 10amps

a 50e after 500 cycles at 5 amps has 3802mah, at 0.2c/0.98 amps the cell delivers 100% capacity and 2c gives 95% capacity

a 40t after 250 cycles at 35amps is 2400mah, at 10 amps the cell delivers 100% capacity, is that the happy discharge rate of the cell?

if I look at that

the 50E was tested/cycled at 1c/5amps or half its amp discharge rating

the 40t was tested at 8.75c or its full 35amp rating

I wonder how the 50e and 40t would compare if they were both tested at 2c?

if I wanted to run a pack at 40 amps or 2c I would need 5p of 40t/20ah or 4p of 50e/20ah

the more I look at a 40T I wonder if its really just a 25r that grew up a little bit, a little taller and a little wider and a little stronger.

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jonescg   1.21 GW

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Re: Why High C Rate Cells Live Less than Low C Rate?

Post by jonescg » Feb 03 2020 2:40am

The main reason why high C rate cells don't last as long is because the ratio of active material to current collector is much lower. This means with less active charge storage matrix to begin with, cracking and de-activation of the cathode and anode means less lithium ions are available to move with each cycle.
So basically, less room to move when you have little to start with.

goatman   10 MW

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Re: Why High C Rate Cells Live Less than Low C Rate?

Post by goatman » Feb 03 2020 3:38am

so the high c are made for a short lifespan? with the info I gathered I can run 4p of 40t/ 10amps is 100% discharge capacity but I would need twice as many 50E, 8p at 5amp/cell discharge

seems like you need to be ready for a big battery that costs twice as much but should last atleast twice as long or pay half as much for half the battery but be ready to replace the pack more often so no matter what you are going to wind up paying

leffex   1 kW

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Re: Why High C Rate Cells Live Less than Low C Rate?

Post by leffex » Feb 03 2020 1:42pm

viewtopic.php?f=14&t=102682&start=75#p1504990

Check the reply above. or https://i.imgur.com/FRRahFf.jpg

and to the right, check the ratio-.
Compare the cells 29e, 35e and 50e and dcir acir ratio is about 1,2.
Compare 25r and 40t and the ratio is about 1.4

I would say that it can be a measurement of "power" so some cells do have same power capabilities if that ratio is the same(hypothesis) and also possibly that the material the cells are made of or if they are ncm, ncr, ice cell type can be assumed by the ratio as well.

What do you think?
Last edited by leffex on Feb 03 2020 2:08pm, edited 1 time in total.
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leffex   1 kW

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Re: Why High C Rate Cells Live Less than Low C Rate?

Post by leffex » Feb 03 2020 1:50pm

I would love to use 50e cells for my battery builds but I can't stand they are weak in one sense. I mean 10p 40t cells can be replaced by 35p 50e

21s35p is to big of a battery. (to run them at lower power for better cycle life)

What I do to fix this is? Fix a fast charger. is 3000w fast enough? Sure so then is adaptor to charge at charge points. Check: Mennekes type 2 euro standard. Pretty sweet. Also any charger more than 1000-1500 would be a good one in comparison to 10a or 2-4a standard chargers.

I like to find out stuff so what I will do this year of 2020 is to test 10-20 different old used 18650 cells. Used 50-250 cycles in average and se how they perform in real life and not lab test style, sorry.
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john61ct   100 GW

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Re: Why High C Rate Cells Live Less than Low C Rate?

Post by john61ct » Feb 03 2020 5:19pm

Were you serious about not grokking C-rate? Could not slog through the rest of that post sorry.
goatman wrote:so the high c are made for a short lifespan?
Well not designed that way on purpose

but as a consequence of the "unnatural striving" toward the high-power goal

Some swear that the ability to put out a high C-rate at lower temperature (lower ESIR) also implies longer life cycles as a general indication of "higher quality".

Maybe IMO for some at the peak of the industry, but they will of course cost a lot more per Ah.

> seems like you need to be ready for a big battery that costs twice as much but should last atleast twice as long or pay half as much for half the battery but be ready to replace the pack more often so no matter what you are going to wind up paying

Yes going for a smaller high-power pack is great for performance, but will need to be replaced in maybe 1/4 the time of a larger pack with longer-lasting cells combined with a lower C-rate.

Like 100 IRL cycles as opposed to even 1000 is entirely possible. And the more energy-dense cells might even be a lot cheaper too.

So again, only pay for the lightweight more expensive option if your use case actually requires it.





goatman   10 MW

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Re: Why High C Rate Cells Live Less than Low C Rate?

Post by goatman » Feb 03 2020 6:15pm

john61ct wrote:
Feb 03 2020 5:19pm
Were you serious about not grokking C-rate? Could not slog through the rest of that post sorry.
goatman wrote:so the high c are made for a short lifespan?
Well not designed that way on purpose

but as a consequence of the "unnatural striving" toward the high-power goal

Some swear that the ability to put out a high C-rate at lower temperature (lower ESIR) also implies longer life cycles as a general indication of "higher quality".

Maybe IMO for some at the peak of the industry, but they will of course cost a lot more per Ah.

> seems like you need to be ready for a big battery that costs twice as much but should last atleast twice as long or pay half as much for half the battery but be ready to replace the pack more often so no matter what you are going to wind up paying

Yes going for a smaller high-power pack is great for performance, but will need to be replaced in maybe 1/4 the time of a larger pack with longer-lasting cells combined with a lower C-rate.

Like 100 IRL cycles as opposed to even 1000 is entirely possible. And the more energy-dense cells might even be a lot cheaper too.

So again, only pay for the lightweight more expensive option if your use case actually requires it.
im trying to learn C rate because I read people using c rate and I think. no that's wrong and the other person doesn't say, hey that's wrong they just carry on the discussion

so if I think I finally figured out c rate, I make the above post hoping someone says, hey that's wrong, if what I said is wrong

im new to this, im still in my first year of apprenticeship training if you know what I mean. im a plumber but I don't plumb anymore, got hurt so now im a heavy equipment operator. it might sound funny but plumbing is very similar to this, its just a different language

john61ct   100 GW

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Re: Why High C Rate Cells Live Less than Low C Rate?

Post by john61ct » Feb 03 2020 6:30pm

Ok to keep math simple

10Ah battery

2A is 0.2C

20A is 2C

200A is 20C

1A is 0.1C

10mA is 0.01A is 0.001C

rg12   100 kW

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Re: Why High C Rate Cells Live Less than Low C Rate?

Post by rg12 » Feb 04 2020 4:35am

jonescg wrote:
Feb 03 2020 2:40am
The main reason why high C rate cells don't last as long is because the ratio of active material to current collector is much lower. This means with less active charge storage matrix to begin with, cracking and de-activation of the cathode and anode means less lithium ions are available to move with each cycle.
So basically, less room to move when you have little to start with.
Thank you! summed it up perfectly for me :)

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