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Thread: C Rate vs C Rating

              
   
   
  1. #11
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    Quote Originally Posted by teddillard View Post
    But if you can point to an industry standard that references cycle life, then please do...
    An industry standard? I'm surprised many of these manufacturers can even make more than a dozen without burning down their factory yet alone actually do something sensible.

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  3. #12
    teddillard
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    So, where are you getting this impression of C-rating testing standards, then? Or did I misunderstand your question?

  4. #13
    Seņor Member podolefsky's Avatar
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    My understanding is that the C-ratings tell you what you can pull from the cell without doing damage (at least significant short term damage, whether it is due to heat or low temperature plating or whatever). The continuous rating tells you what you could discharge the cell at until it is dead. The peak or burst rating is usually what you can do for <10sec without causing significant damage. There is some damage done every time you discharge the cell, so it is all a matter of degree.

    Keep in mind that the C-rate (and rating) are based on the nominal Ah capacity. It gets complicated since at higher C-rates the effective capacity is lower - if you discharge a 100Ah cell at 3C you won't quite get 100Ah out of it. But it's just a mathematical definition - 300A is 3C for a "100Ah" cell, even if you only get 80Ah out of it.

    The ratings are kind of circular in logic. First you build a cell, then you have to test it for capacity by discharging it. That's often done at like 0.5C, so a 100Ah cell can deliver 50A for 2 hours. At 0.1C it might do a little more, at 3C a little less. At 10C it will heat up and you might only get a few Ah out of it before it pops.

    The peak C-rating can also be the C-rate at which you hit low voltage cutoff. That gets messy too, since sometimes different cells of the same chemistry have different LVC. Most LFP cells have LVC=2.5, but Headways have LVC=2.0. So Headways get a higher peak C-rating, even though they sag just as much, because they can sag more without hitting their rated LVC. Whether Headways can actually tolerate hitting 2.0V better than other types of cells I don't know - but I suspect that having a small metallic casing helps, since it dissipates heat better than a large PVC casing.
    Last edited by podolefsky; 15 August 2013 at 1518.
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  5. #14
    Not to be taken seriously DaveAK's Avatar
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    This is pretty fundamental stuff. Anything that is made is rated by the manufacturer at a level less than its absolute capability. Be it batteries or bridges. When it comes to cheap Chinese electronics they tend not to use much if any of a safety margin. As a result you can run their stuff at their max rating but you'll see the magic smoke a lot sooner than you'd like.

    I think the difference between rate and rating is pretty clearly defined. Think tires. If a tire has a speed rating of 100mph the expectation is that they can do that for the normal life of the tire. That's not to say that you can't run them at a higher rate, but they won't last as long and you risk catastrophic damage. If you run them at an extreme rate you're almost guaranteed spectacular catastrophic damage.

  6. #15
    Seņor Member podolefsky's Avatar
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    Yup - stuff *should* be underrated, but with the stuff we're buying it's usually the opposite. Almost every large format LFP cell is rated 3C continuous, 10C peak. We know they won't all do that. It's as if the manufacturers just stamp that number on there because that's what LFP should do, regardless of whether their cells actually do it.

    Tires, well, if you get a 100 mph rated tire that doesn't mean you can run it at 100 mph continuously every time you drive your car and expect to get the rated mileage. It just means you can do 100 mph without the tires disintegrating in a matter of seconds. Same with batteries. They might be rated 3C, and they'll do that without burning up, but that doesn't mean you'll get 2000 charge cycles if you always run them that way.
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  7. #16
    Senior Member billmi's Avatar
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    If I recall correctly, I've also seen the term "continuous" used rather loosely in the past, with CALB using it to describe a 50% duty cycle, alternating - 30 seconds discharging at the c-rate and 30 seconds resting.
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  8. #17
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    Quote Originally Posted by billmi View Post
    If I recall correctly, I've also seen the term "continuous" used rather loosely in the past, with CALB using it to describe a 50% duty cycle, alternating - 30 seconds discharging at the c-rate and 30 seconds resting.
    Here's what the CALB 70Ah cell datasheet says about cycle life
    Life Cycle (0.3C Charging-discharging,80%DD C) 2000

    Note the 0.3C...note 3C rate. Don't expect 2000 cycles at 10x the spec!

    They also say
    Maximum short-time (Burst) Discharging Current (period 10s,A) 700

    They don't say what hitting a few 700A or even 350A bursts will do to cycle life, but I doubt it will increase it...

  9. #18
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    Many of the various colors of plastc shell large format prismatics will spit up electrolyte from 10C discharge for 10s. I've heard there are some better than others, but I don't believe ive seen one that wasn't sagging below 0.5 of resting voltage under 10C loads. That both means your battery is eating more heat than what you're getting into the controller from it, and you can make more power output from reducing battery current limits.

  10. #19
    Seņor Member podolefsky's Avatar
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    Quote Originally Posted by liveforphysics View Post
    Many of the various colors of plastc shell large format prismatics will spit up electrolyte from 10C discharge for 10s. I've heard there are some better than others, but I don't believe ive seen one that wasn't sagging below 0.5 of resting voltage under 10C loads.
    I think it's pretty damn rare to actually pull 10C for 10sec. If you size your cells correctly for your controller, then at WOT your cells will only be putting out max current over a few hundred RPM (near base speed). You should get out of that band in a couple of sec at most.

    Point taken though - 10sec is the absolute max. Most spec sheets don't say 10sec, they just call it peak and figure you're on your own figuring out what is a good idea.


    That both means your battery is eating more heat than what you're getting into the controller from it, and you can make more power output from reducing battery current limits.
    I don't quite understand this. Power into the controller is I*V. Take a 3.3V cell sagging to 2.8V at 600A. That cell is producing 1980W, with 300W lost to heat. You still get 1680W out (2.8V * 600A).

    Reduce current a little bit, say 500A, and say sag is now to 2.9V. You get 1650W out, with 200W lost. 1450W to controller (2.9V * 500A). That is, less.

    I could be missing something...but according to the mathz you would have to have sag > 1/2 of rest voltage to have more power wasted than going to the controller.
    Last edited by podolefsky; 16 August 2013 at 0914.
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  11. #20
    teddillard
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    WOT = ?

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