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Old Oct 17, 2014, 01:48 PM
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Why using "C" instead of existing units ?

Hi All,

in many cases I know that I am a bit slow please feel free to laugh at the question at hand.


I have studied many threads and many pages about lipo cells and there is one thing that it seems that no living man or woman on this planet can answer to.

Why have this sort of "new" unit "C" been created for Lipo cells ?
whats wrong using Ampere or even Watt both have existed for quite some time and C have not.

I have done the math many times and what "C" actually states is the maximum continous or burst current ( example could be a 3S @2200mAh marked with 40C/80C ).

Whats wrong with marking it 3S @2200mAh 88A/176A which would make a lot more sence to slow people like myself.

if anyone have any additional input on this topic, please help out.
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Old Oct 17, 2014, 02:02 PM
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C is a unit, capacity in Ah (amp hours)
The numbers 15, 20, 25 etc. are factors in the equation.

If the capacity is listed as 2200mAh it is 2.2Ah

Using a factor (part of a multiplication equation) of 20 the pack is listed as 20C which means 20 times the capacity in Ah and can also be written as 20 x C (capacity in Ah)

20 (the factor constant) times 2.2Ah = 44, which is how the supplier rates the maximum amp draw.
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Old Oct 17, 2014, 02:18 PM
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Quote:
Why have this sort of "new" unit "C" been created for Lipo cells ?
whats wrong using Ampere or even Watt both have existed for quite some time and C have not.
The correct term is C-Rate and 1 C-Rate is defined as the amount of current the battery will deliver for 1 hour.
It's been around a long long time and is used for all batteries, not just LiPoly.


http://batteryuniversity.com/learn/a..._is_the_c_rate
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Old Oct 17, 2014, 05:55 PM
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Ira NZ's Avatar
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As for why use C rate rather than the actual peak current? Not sure really, probably because C-rate is chemistry and construction related and independent of actual capacity.
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Old Oct 17, 2014, 06:37 PM
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C-Rate is both current and time related.
Peak current is only amps.

For instance:
1C is the current the pack will deliver for 1 hour.
2C is the current the battery will deliver for 30 min.
4C is the current the battery will deliver for 15 min.

A 60C battery will deliver capacity x 60 for 1 min.

Time and current, hand in hand.

So if you got a plane that draws 60A, you could buy a 60C 1000mah pack which would give you 60A (but only for 1 min). Knowing only the current wouldn't give you the whole story.
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Old Oct 17, 2014, 10:14 PM
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IMO, the biggest advantage of C rates is that it is capacity-independent, unlike amperage ratings.

C-ratings are only suddenly a big deal as most other battery chemistries are pretty unimpressive in terms of maximum discharge rates (when I had NiMH sub-c batteries years ago, I never knew about C anything). And most other battery chemistries aren't quite so dangerous if you push them too far.

Also interesting is that there are very few scenarios other than RC which have such low run times and high C demand so that terminology will be less common elsewhere.
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Old Oct 18, 2014, 02:05 AM
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"C" doesn't really represent anything -- it's simply a number that you multiply the battery's Ah (ampere-hour) rating by to get an indication of its maximum amps capability.

They could simply show the maximum amps rating as well as the capacity but, as pointed out in post #6, the C rating is independent of the battery's capacity, so it gives an indication of the battery's capability irrespective of its size.
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Old Oct 18, 2014, 03:16 AM
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Quote:
Originally Posted by hoppy View Post
C-Rate is both current and time related.
Peak current is only amps.

For instance:
1C is the current the pack will deliver for 1 hour.
2C is the current the battery will deliver for 30 min.
4C is the current the battery will deliver for 15 min.

A 60C battery will deliver capacity x 60 for 1 min.

Time and current, hand in hand.

So if you got a plane that draws 60A, you could buy a 60C 1000mah pack which would give you 60A (but only for 1 min). Knowing only the current wouldn't give you the whole story.
´first off, thanks for the link that really straigthened my head.. still

If I need 60A I could use a 1000mAh battery of which can deliver a 60A current ( and yes it would still last for a minute )

I get that C is a function of battery capacity and allowed current
but if a battery is marked 2200mAh I know the capacity and If it is also marked with max continous and burst current we are still in the game.

I understand what C represents, but I still have a hard time using it. Ah, A and V is much more common in every electronic device apart from lipo cells. sure they are a bit explosive, but a 12V 65Ah lead acid battery in your car can deliver 1500A for a few seconds before it turns into flames.. and thats dangerous too. Still you NEVER see that a car battery can deliver whatever "C" ....

is there something missing or something that needs to be changed?
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Old Oct 18, 2014, 03:31 AM
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The C-rating is how the manufacturers make and test the cells. If you ever see anyone selling packs with maximum Ampere ratings then all that's happened is they've done the minimal arithmetic for you. And to be honest if you're not capable of doing simple multiplication you probably shouldn't be messing about with lipos anyway .

OTOH the marketeers probably like them precisely because it obscures things slightly and the theoretical current capability isn't right out there "in your face". If instead of 2200mAh 40C-65C battery you stated outright that it was capable of 143A some fool would probably try it and then complain because he'd blown his battery (and possibly his house) sky high .

Steve
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Old Oct 18, 2014, 06:00 AM
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c rating isn't something 'new' or something invented for LiPos. It's a standard way to rate a battery charge and discharge rating that's applicable to all types of batteries and always has been.
Just because you never heard of it before doesn't mean it's new in fact the concept of 'c' rating goes back to the 1700's: http://batteryuniversity.com/learn/a..._is_the_c_rate
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Old Oct 18, 2014, 06:52 AM
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Hi, I like to think in terms of supply and demand.

For a 2200mAh-3s-20c battery, I think of it as 2.2x20=44 Amps Available Now (supply), and then compare to the demand from the ESC and Motor (load). This 2200mAh-3s-20c battery is adequate for a 40 amp ESC and Motor (under normal cruise throttle, more later).

I learned about “Batteries” from that …
A few years ago I bought a SkyArrow at a swap meet, I put in a battery and tried to fly it, and wondered why it would take-off, then say “beep-beep-beep”, and then crash. On investigation my battery was not keeping up with demand/load.
Battery (1300mAh-3s-20c) 1.3x20=26 amps Available Now (supply), and
30 Amp ESC and Motor (demand/load).
The excess power-draw, pulled from the battery faster than it could supply, the voltage dropped, and the airplane receiver reset, and the plane crashed.

Unfortunately the limitations of the battery do not also limit the ESC/Motor (load).

Also an issue that has become one of my pet peeves is “Burst Mode”, where your Motor, ESC, and even batteries will attempt to make a heroic effort in order to save your airplane from such things as; pulling out of a dive to avoid the ground, a strong take-off to avoid a tree, you get the idea. Commonly Motors and ESC's can give 50% more effort for about 30 seconds, and I think that batteries are good for 25-50% more effort for less than 10 seconds.

For my SkyArrow with its 30 amp Motor and ESC that becomes a heroic effort of 45 amps of demand. Even if my battery (with 1.3x20=26 amps Available Now) gave a heroic effort of 39 amps (supply) it still could not keep up for more than a few seconds (voltage drop > receiver reset > crash).

Your mileage may vary, good luck
JD

P.S Another situation where this “C-Rating” will bite you is if you try to use a lower c-rated battery from your trainer in an EDF electric ducted fan, they want lots of power right now.

P.S. After I learned all this I invested in a inline volt-amp-watt-meter and I currently think that the best way to limit the power draw (demand) is by carefully choosing the propeller for power draw and thrust. If you get into bench testing power-trains, then wear a face mask for safety.

P.S. HobbyPartz dotcom usually gives very good information on the KV, volts, amps, watts for their motors.
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Old Oct 18, 2014, 10:23 AM
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All good info above, what I would add is batteries have limitations when compared to a tank of gas. When I am planning an electric power installation I usually start by considering how much power for how long? The more installations we do the better "feel" we get. Additionally, software like motocalc and ecalc are very helpful when researching your setup. Do not expect 100% accuracy, Do not expect to use 100% of your battery capacity, DO PLAN for safety margins!

Michael
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Old Oct 18, 2014, 10:44 AM
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C is a constant. The number before it is a coefficient. It’s nothing new. Math terms have been with us as long as man has been using it to describe the value of anything.
You see it in description of the electric motor on your plane too. Kv is the constant for rpm / volt applied.

So C simply means the capacity of any (not only Lipo) battery. The number before the C is what the manufacturer claims it can be charged or discharged.
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Old Oct 18, 2014, 11:34 AM
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Quote:
Originally Posted by ahlis_se View Post
Still you NEVER see that a car battery can deliver whatever "C" ....
Actual car batteries tend to be very light on specs. Or at least light on the specs that you find on a typical battery. For example, finding an actual capacity for a car battery can be difficult. That does kind of make sense since 99% of users just crank them for a couple of seconds and drive away, but you shouldn't use the specs of a car battery as the basis of what to expect from any battery that is meant to power things for long periods.

On the other hand, the manufacturer of the deep cycle lead acid battery that I use at the field recommends charging it at a max of 1/6C. It also came with a graph of capacity vs C rate for discharging. Sorry but it's just not a LiPo invention.

Here's a typical chart I googled up for a lead acid battery:



I have also heard usages of C at the field with respect to NiCDs, NiMH, etc. For example, I recently charged a receiver pack for someone and since I hadn't charged a NiMH in some time, I asked how fast I should charge it and all the suggestions came back in C.
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Old Oct 18, 2014, 05:35 PM
PGR
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Quote:
Originally Posted by ahlis_se View Post
is there something missing or something that needs to be changed?
I started flying electric RC planes in 2003 which was right around when the first pre-made RC LiPo packs became commercially available. Back then the packs were rated at 6C maximum discharge and a 1C charge rate was a universal "never exceed" standard. I was confused by C-ratings for about the first 2 hours after I got my first LiPo packs, but I sought help and it was explained to me in language I could understand - and I haven't had any trouble with C for a little over 11 years now.

So no, I don't think there's something missing and I don't think it's something that needs to be changed. What I do think (hope?) is the proverbial light will eventually come on for you too and you'll end up with the same understanding of the concept as we do.

My suggestion to that end is don't make it more complicated than it is. The number is a multiplier, C is the capacity of the pack, and the product of the equation is your maximum theoretical discharge rate. The same is true for charging rates, but your multiplier will be a lot less than for the discharge rating.

See?

Pete
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