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Thread: C Rate vs C Rating

              
   
   
  1. #1
    teddillard
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    C Rate vs C Rating

    This is just the stupidest thing. I've been pretty much totally confused by the whole C thing for a while now. I read about manufacturers determining a C-rating a while ago, and it seemed both kind of arbitrary and not particularly reliable, and the usage I saw was just flat-out inconsistent, so I more or less dismissed the term and ratings, at least as far as any reliable indication goes.

    Why is it so confusing? Because they are two different things. There's your C-rate, and then, you have your C-rating. Duh.

    Basically, the C-rate is a number calculated primarily to determine a discharge rating ("scale" the discharge or charge, according to Battery U) in a way that's independent of the cell capacity, and the C-rating is the maximum C-rate you can expect from a cell without damaging it.

    (Noah even made a comment on a passage in my book about my misuse of the term. I changed it to what he suggested - as I do with most suggestions my editors make without really understanding why it was correct.)

    Anyway, for what it's worth, I just wanted to point that out. I'm probably one of the few, um, special people who didn't realize this, but I put together a little more information on the blog, here: http://evmc2.wordpress.com/2013/08/1...-and-c-rating/

    Now, of course, it makes perfect sense. You want to determine a specific discharge rate, so you can compare cells of different capacities? It's "current / Ah, so a 100Ah cell at 100A is 1C. 200A is 2C, etc.". That's the C-rate at which you discharge (or charge) your cells.

    You want to figure out what your cells can do before they blow/burn/melt? Then pick a C-rate and test them. If they pass, pick a higher rate and see what they do. When they start to burn up, then you've determined the C-rate they exceed their safe discharge rate at, then you can give them a C-rating (of slightly less, presumably).

    When somebody is talking about the tests they're doing on cells and talk about doing a discharge at a certain "C", they're talking about the C-rate they've picked. When they say one pack vs another on a bike at a certain voltage is doing 10C vs 5C, they're talking about the actual rate at which the pack is being discharged. When you're looking at buying cells and they're rated at 10C or 15C, that's the C-rating the manufacturer has assigned the cells, (and should be subject to all the skepticism that any manufacturer rating is...) and if a user claims a cell is a 20C cell, it is really a C-rating, and may or may not be particularly valid, but would require them to do extensive testing of the cells at that C-rate.

    Yeah, I'm a little hung up on words. Sometime let me tell you about "Gamma"... Don't even get me started on Heidegger. ("What is the C-rating? It is the Rating of a Cell at a Rate of C which does not exceed the Rate at which the Cell can C without exceeding it's Thermal Capacity and thus earning it's Rating of C by applying the Rate of C.". Yeah, right. Like that.)

    And yes. Thank you Noah.
    Last edited by teddillard; 15 August 2013 at 0403.

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  3. #2
    Senior Member jonescg's Avatar
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    If a reputable manufacturer states a 'continuous' discharge of 5C, I would opt to never exceed 5C. If a hobby LiPo manufacturer says 40C 'continuous' discharge I would halve it and say don't exceed this.

    It's really a more comparable way of referring to amps per cell. Mostly just words though

  4. #3
    teddillard
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    Right, or to re-state: If a manufacturer states a "continuous" discharge C-rating of 5C, you'd never exceed a 5C C-rate.

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    Senior Member jonescg's Avatar
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    Sounds good to me. And if I decided to drag 300 A from my 60 Ah pack, one could say I was discharging them at 5C.

  6. #5
    teddillard
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    Right, and bear with me, I'm trying out my new brain: That's C-rate, or current/ah, so 300/60, which = 5. If it does that without getting hot, you can tell everybody your pack has at least a C-rating of 5C.

    Ted's Brain on Math:


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    Senior Member jonescg's Avatar
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    Mmmm delicious brains....

  8. #7
    teddillard
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    Quote Originally Posted by jonescg View Post
    Mmmm delicious brains....

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    You want to figure out what your cells can do before they blow/burn/melt? Then pick a C-rate and test them. If they pass, pick a higher rate and see what they do. When they start to burn up, then you've determined the C-rate they exceed their safe discharge rate at, then you can give them a C-rating (of slightly less, presumably).
    Is that really the case? I was under the impression that the C-rating most cells are given is the maximum that will still give them a given number of charge cycles.

    An 8C cell might be fine at 16C but you might only get 100 cycles instead of 1000. Very few people will buy it if it only lasts a 100 cycles so... It might not start smoking until 32C or much more.

    Have I missed something?

  11. #9
    teddillard
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    Literally as I was writing that, I was thinking someone would get on and post that it wasn't how batteries are tested for C-ratings.

    First, it was just an example. An illustration. However, it is, for sure, the way a lot of guys in their garages test for real-life C-ratings, I can point to any number of threads and videos.

    But second, as far as I'm aware, there are no industry standards for determining what's an acceptable rating method. Or is that methodology? I'm certain that any manufacturer's claims about cycle life in particular are pretty thin, (based on some reports I've had from people doing actual independent testing - which cannot be published, unfortunately). So if you don't have standards for cycle-life testing, or other capacity-related testing, then what do you have?

    This is one of my issues with C-ratings and why I've pretty much dismissed them in the past - they seem pretty arbitrary. But if you can point to an industry standard that references cycle life, then please do...
    Last edited by teddillard; 15 August 2013 at 0936.

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    Quote Originally Posted by teddillard View Post
    If it does that without getting hot, you can tell everybody your pack has at least a C-rating of 5C.

    This was the metric I once used as well, but it doesn't tell you the whole story. You want to look at coulombic efficiency as well (this is another way of saying electron quanity out vs electron quanity in.) If you see in your overlayed charge and discharge curves, that a significant amount of energy is being lost not just in the form of potential from imperfect conduction, even if it's not getting hot, you're putting energy into driving some processes that aren't as reversible as simply cooling down.

    For charging C-rates you should not use temp as your guide. Uneven distribution of lithiated material both increases volumetric expansion driven stress from the top of the layer to bottom of the layer and can break bits of the carbon away from the current collector where you can't access the energy stored in them. You can also cause metalic lithium plating if you're charging rapidly while the surface is already lithiation saturated. The formation of metalic lithium is something you strongly want to avoid. For that matter, excessive C-rates in discharge can also generate metallic lithium on the surface of the cathode while the cell isn't at an elevated temperature at all if the local surface of the cathode is saturated and needs more time for ions to diffuse away from the surface.

    The effects of making this metallic lithium form is kinda outside the scope of a silly elmoto post, but think gas production and anode/cathode structure damage and potential dendritic growths damaging separators etc. Generally bad juju.



    To really know what C-rate a cell is good for, you need to not just discharge it and see if it warms. That's a crude rule of thumb that will/can have success some of time, but actually putting some cycling time in at that C-rate will give you a much better idea of the long-term effects on the cells health.

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