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Old May 30, 2012, 12:30 AM
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wasilla,alaska
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20c..30c what does it mean when looking at battery

Hi
I see different prices for batteries some say 20c and 30c etc I know it means constant discharge but how does that relate to what I need ?
thanks
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Old May 30, 2012, 12:43 AM
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Discharge rate ("C") multiplied by capacity (mAh or Ah) gives you the maximum rated current you can draw from the battery. For a 30C 2200mAh battery, we have 30C x 2.2Ah = 66A of current. This number isn't really a hard limit, and most sport and trainer planes will never come near an appropriately-sized lipo's discharge limit.

Higher discharge ratings mean less internal resistance, which means the pack will maintain greater voltage under load, which results in more power at the motor.

Note that a battery doesn't "push" current through a system. The motor will draw exactly what it needs to spin the prop at a given RPM. Even if the battery is capable of putting out those 66 amps, it can be used with a motor rated for like ten amps as long as that motor has an appropriate prop.
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Old May 30, 2012, 12:45 AM
"Landing" in a tree somewhere
Rochester, NY
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For more information on lipo batteries and their use, give this a read.
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Old May 30, 2012, 02:55 AM
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wasilla,alaska
Joined Nov 2005
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Hey thank you both,,,so for this motor what c would I want im still little lost on that part ?

http://www.hobbyking.com/hobbyking/s...50kv_785w.html
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Old May 30, 2012, 03:36 AM
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Joined Jan 2011
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"20c..30c what does it mean when looking at battery"
In reality, it means almost nothing. There are no testing standards to which manufacturers must adhere when picking a C-rating for their product's label, so they're essentially free to make it up as they go along. Naturally, that means vastly over-inflated C-ratings which are practically meaningless, except perhaps if you're trying to gauge the probable relative performance of different lipo lines from the same manufacturer.

This thread is technical, and not exactly meant for beginners, but if you really want to know the answer to the question in your thread's title, it's in there.

The short version is that even the very best lipos struggle to maintain a true 30C current rate (which means being completely flat in two minutes!), and the 50C, 60C, 100C () ratings are pure marketing fantasy. Also, the "25C" line from one manufacturer may easily be better than a "30C" product from another, again because the way the ratings are determined is completely arbitrary.

As C₄H₁₀ said, the battery's internal resistance (IR) is a much better indicator of its true potential and health, but only a handful of the more expensive chargers can measure IR presently. To make this more fun, manufacturers almost never publish their IR specs
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Old May 30, 2012, 03:53 AM
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Australia, New South Wales, Sydney
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Quote:
Originally Posted by jasonak View Post
Hey thank you both,,,so for this motor what c would I want im still little lost on that part ?

http://www.hobbyking.com/hobbyking/s...50kv_785w.html
Assume that all "budget" batteries - like every line from Hobbyking - are really no better than 20C, for real, especially if you're planning to drive them to the edge.

That motor has a 55A max current rating, with a 60A recommended ESC, so say the overall draw is going to max out at 60A, unless you over-prop it (not recommended!).

To get a 20C battery to supply 60A, its capacity must be at least 3000mAh (3 x 20 = 60). Anything less, and the battery may not be able to sustain 20C for very long, or it won't last long if you regularly subject it to borderline harsh treatment. Lotsa current makes the battery heat up (because of its IR), which in turn makes it vent methane ("puff up"), which causes further erosion of its capabilities... You don't want to routinely subject a lipo to currents at the edge of its performance envelope.

Another thing... Just because it was "20C" (real) when it was purchased, doesn't mean it's still 20C a few weeks or months later. A common mistake people make is to leave their lipos fully charged (4.20V/cell) and ready to go, so that they can leave home quickly on Flying Day. It's understandable, but the IR of a cell which is stored above 4.0V is rising all the time - more quickly if it's full or close to it. IR never comes down again, so that "20C" battery may in fact be 17C or worse a few weeks later, if it sat fully charged.

Charge your batteries the day you intend to use them, and discharge the ones which come home without being depleted for some reason. (3.85V/cell is a commonly accepted safe storage voltage.) This is especially important whenever you're intending to push a drive system to its limits.
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Last edited by H2SO4; May 30, 2012 at 04:17 AM. Reason: grammerz
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Old May 30, 2012, 03:53 AM
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wasilla,alaska
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Good on ya mate! I used to live in richomond
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Old May 30, 2012, 03:58 AM
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wasilla,alaska
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Seriously though I do appreciate you taking the time and effort to answer my questions and in such detail.Its very much appreciated.
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Old May 30, 2012, 04:02 AM
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Quote:
Originally Posted by jasonak View Post
Good on ya mate! I used to live in richomond
Richmond, NSW? That military airport over there might limit RC flight options in the neighbourhood

Quote:
Originally Posted by jasonak View Post
Seriously though I do appreciate you taking the time and effort to answer my questions and in such detail.Its very much appreciated.
No problem at all mate. My wife is thoroughly sick of hearing about RC battery management
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Old May 30, 2012, 04:16 AM
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wasilla,alaska
Joined Nov 2005
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Yeah wasnt doing much flying when I lived there. They did have gliders going up on the weekends somtimes at the RAFF base that you could go up in for like 50.00 or something cheap like that, I never did I so wish I would have. That was a really nice place to live.I was only there bit over a year,but what a beautiful country you have.
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