What do the "C" numbers mean?  
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All Forums >> Radios, Batteries, Clubhouse and more >> Batteries & Chargers >> What do the "C" numbers mean?
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What do the "C" numbers mean? - 5/31/2008 5:56:00 PM   
w4kv



Posts: 39
Joined: 6/19/2002
From: Pittsburgh, PA, USA
Status: offline 		I'm trying to convert a 6-cell NiCd power source to a 2-cell LiPo source. In doing this, (and I recognize this is terribly elementary), I find that I never understood what is meant by a, say, 20C battery. It is apparent that it is a current measure, and higher means more current. Why not just use amps? Is there a direct relationship between amp draw and "C" number?

Please elucidate. I've googled everywhere, but the C numbers are apparently so basic that nobody explains how they're derived.

Thanks

w4kv
       Post #: 1

RE: What do the "C" numbers mean? - 5/31/2008 6:15:35 PM   
Walt Thyng


 

Posts: 754
Joined: 7/27/2002
From: St. Charles, IL, USA
Status: offline 		W4KV,

In simplest terms "C" is the measure of the capacity of a battery to store electrons. There are a ton of variables involving resistance, etc, but for most purposes the capacity can also be used to set guidelines for charge and discharge. A 20C cell can be discharged at 20 times its capacity. Where it gets tricky is that capacity is usually expressed in milliamps (mah) which I suspect is a marketing ploy, while charge and discharge rates are expressed in amps. Here's an example: we have a 1000 mah battery rated at 20C. 20C= 20x1000= 20,000 divided by 1000 to convert from milliamps to amps gives us a discharge rate of 20 amps. Unless otherwise stated the charge rate would be C1 or 1000/1000 = 1 amp.

Hope this helps.

Now let me add a cautionary note, unless you are purchasing the very top of the line Lipos, the general wisdom is to use only 75-80% of the "C" rating. This, in effect reduces a 1000mah Lipo to 800mah with the resulting reductions in charge and discharge rates. this is both a safety and longevity issue.
Walt

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WT in Illinois

(in reply to w4kv)
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RE: What do the "C" numbers mean? - 5/31/2008 7:35:05 PM   
Rodney



Posts: 4782
Joined: 12/8/2001
From: FL
Status: offline 		The C rating when used to express maximum current you should pull from the battery at any one time is simply the capacity of the battery times the C rating. A 1200 mah battery with a 10 C rating would be capable of providing 12 amps for a short time, if the C rating were 20, it would be capable of providing 24 amperes for a short time. Most batteries will not sustain that magnitude of current for a very long period of time; to make your batteries last longer (more charge/discharge cycles) try to stay well below the C ratings. C used in this manner is not connected to the charge rate although most battery manufactures recommend that you stay at or below 1C on the charge rate for LiPo batteries.

(in reply to Walt Thyng)
       Post #: 3

RE: What do the "C" numbers mean? - 5/31/2008 11:14:49 PM   
Capt Lou


 

Posts: 299
Joined: 1/3/2007
From: Manahawkin, NJ, USA
Status: offline 		w4kv:

Take a look at this web site: www.batteryuniversity.com It may help you with understanding batteries

(in reply to w4kv)
       Post #: 4

RE: What do the "C" numbers mean? - 6/4/2008 12:02:36 AM   
w4kv



Posts: 39
Joined: 6/19/2002
From: Pittsburgh, PA, USA
Status: offline 		WaltThanks for the concise explanation. Just what I needed.

RodneyThank you for the practical tip. I guess I need to limit my need for speed.

LouAlthough I had looked at some of batteryuniversity, getting there thru a search engine, I never had the good sense to go to the index page of the site. From there, I found the explanation of C ratings. I've bookmarked the page.

John

(in reply to Capt Lou)
       Post #: 5

RE: What do the "C" numbers mean? - 6/4/2008 12:09:39 AM   
SavageX78



Posts: 1157
Joined: 2/26/2007
From: Laconia, NH, USA
Status: offline 		Ya batteryU has a lot of info.

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       Post #: 6

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