



As plain as can be.
http://sites.google.com/site/tjinguy...arginghowtos 



Holy not searching for your answers in the 10 million posts this has already been explained in, but hey I'm all ears for this new one. I need some lessons on LiPo too.. come on experts chime in any time now.







Letchworth, Great Britain (UK)
Joined Jul 2004
13,381 Posts

I must take issue with the link in hoppy's post: It starts off good by saying that C is a variable (I prefer to call it a constant), but then it goes on to say, "Figuring out what C is for a particular pack is simple, just take the capacity of the pack in mAh, divide it by 1000 to get Ah and then drop the "h", that is C." That is completely wrong, C is not simply the Ah without the h.
C is quite simply a number that the manufacturer has determined, hopefully by design backed up by experiment. It is a constant for any particular pack, and can be used to calculate safe discharge amp rates. So, for a 2200mAh 20C pack, for instance, it should be okay to discharge it at anything up to 20 x 2200/1000 = 44 amps. Note that the headline C rating is not used to calculate charge rate: Usually it will say in the small print on the pack what C it can be charged at. It may be 1C, 2C, or maybe up to 5C or more. If it says nothing, you should assume a 1C rate for charging which, in the case of the 2200mAh pack would be 1 x 2200/1000 = 2.2 amps. 



C does in fact stand for Capacity and not only that but capacity stated in Ah or mAh.
Ever notice on the HK site number on thier nano tech LiPolys such as 2.2 or 3.3. That would be 2.2Ah or 3.3 Ah. The C45 indictaes it can be discharged at C X 45 or 2.2 X 45 or 3.3 X 45. C when used a statement of Capacity in Ah is often fairly accurate however when used with a multiplier such as 20  65 for stating possible discharge rates it is often just a bunch of hot air , marketing hype, used car salesmanship, fairy tales, etc... Charles 


Staffs, UK
Joined Nov 2003
12,174 Posts

I see we are still managing to get confused over the difference between the C rate and the Crating.
The C rate for a battery is not actually capacity, it is (and always used to be known as) the 1 hour discharge rate. It is therefore the CURRENT which will discharge the battery in one hour. By a bit of good luck it is the same NUMBER as the capacity but because it's a current it is stated in mA or A not mAh or Ah . The Crating is a multiplier which, when applied to the C rate, gives the (theoretical) maximum current the battery should be able to supply. So 20C means 20 times the 1 hour discharge current. Thus a 3000mAh capacity battery has a C rate of 3000mA (or 3A). If it has a Crating of 25, usually stated as 3000mAh 25C, it should be able to deliver 25 x3 = 75A. If it has a Crating of 40 it can in theory deliver 40 x 3 = 120A maximum. We could really have done with some clearer terminology. Using C for a current just tempts far too many people into thinking it means capacity and from there they go on to get even more confused . Steve 



Quote:
More correctly the Standard charge rate for LiPolys for years ,years ago was 1C. In order to charge a battery in 1 hour it must be charged at 1C ,C being the capacity of the battery in Ah. 1Ah (1,000 mAh) charged at 1A rate is a 1C charge / one hour charge. C used as a unit of capacity makes C charging rates and discharge rates much simplier for the masses to understand. 2.2Ah charged at 1C = 2.2A / charged at 5C = 11A Discharged at 10C = 22 Amps. ( 10 X 2.2 ) / discharged at 10C = 6 minutes (60 min . / 10 ). Long before LiPolys and their 1C charge rates came along Nicads were commonly charged at C/10 ( 1/10 C ). To get the charge rate for a 1,000 mAh Nicad one divided the capacity of 1,000 by 10 which equals 100 mA whic is thus a C/10 or 1/10 C charge rate. http://www.camlight.com/techinfo/techtips.html Snip: Quote:
Charles 



Letchworth, Great Britain (UK)
Joined Jul 2004
13,381 Posts

C can indeed mean "capacity", and I can see a logic for it being so in what we're discussing  i.e. you take the capacity and multiply it by 25, or 0.1, or whatever the appropriate number is. But, if that's the case, what do we call that number itself  the "discharge factor" or something like that would seem to make more sense than just referring to the "Crating".



Staffs, UK
Joined Nov 2003
12,174 Posts

Quote:
BTW it has never been possible to charge a battery in one hour using the C rate. It ALWAYS takes longer than one hour because charging is never 100% efficient. That's why it's sensibly called a DISCHARGE rate NOT a charge rate, because you can fully discharge a battery in one hour at that rate. But you're right it has existed for many years, well before Lipos. Indeed the capacity of NiCds was usually specified at a DISCHARGE CURRENT of 0.2C. And the same value (the current C) was used to describe the charging CURRENT as in 0.1C or for some of the better NiCds anything up to 5C. But C has always been a measure of CURRENT it's never been a measure of capacity. Fortunately the calculations using it are simple enough that most people manage to get them right even it they don't quite understand what it is they're doing . Steve 




I posted
Quote:
Quote:
Let me explain in very simple terms. C being the Capacity of the battrery in Ah thus a 2.2 Ah (2200 mAh) is charged at 1C obtained by taking the capacity in Ahs and multiplying by 1. Charles 



Staffs, UK
Joined Nov 2003
12,174 Posts

Quote:
If you multiply a value in Ah by 1 you still have a value in Ah. 2.2Ah times 1 = 2.2Ah. You can't just suddenly ignore the "h" and pretend it's gone away. And talking about a charging current of 2.2Ah is as wrong as saying "The voltage of a 3S pack is 11.1 watts". Completely the wrong units. But never mind, we've probably completely confused the OP by now so I'll let you carry on with your misleading information. As I say the numbers still work out right even if you don't quite understand what you're doing. Steve 

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