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Old Aug 06, 2012, 07:46 PM
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Quick Battery Discharge Rate Question?

Hello
So discharge = C * mAh

I'm buying a model that requires a 4s 30C 3300mAh battery

Will a 4s 20C 5000mAh work safely ?

Since 30*3.3 = 99 A
and 20*5 = 100 A
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Old Aug 06, 2012, 08:47 PM
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Exactly right. The only penalty for the 5000 mAh battery is the extra weight. But actually, not so much -- because high-C batteries tend to be heavier anyway.

Looked at the other way: the main reason to use a high-C battery is to reduce the battery weight (at the expense of mAh capacity.)
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Old Aug 06, 2012, 08:56 PM
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Indeed, the 5000 mAh bat is like 150 grams heavier. Fortunately though I should have no weight issues with this model.

Cheers.
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Old Aug 07, 2012, 08:49 AM
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One thing to be aware of is that the C-ratings on many batteries are pure marketing fantasy, especially the ones that claim 30C and above. There is no set standard for how those numbers are to be measured, so in order to stay ahead of the competition many LiPo manufacturers print labels with extremely questionable (read: "invalid") maximum discharge rate claims.

A 30C battery must theoretically be able to dump its contents in two minutes flat without sustaining damage in the process, again and again. Few batteries can do that for real. The ones which claim 60C or even 100C+ () should be able to do the same in a minute or less - without melting or catching fire.

The 20C battery is probably OK, at least when new (effective C-rating decreases over time, especially if cells are abused, most commonly by leaving them charged). Anything rated 30C will most likely be unable to live up to the promise. Think 20 to 25C, maybe, if on a good day.

That disparity between marketing hype and reality needs to figure in the calculation when approaching the limits of what the battery can supposedly handle.
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Old Aug 07, 2012, 03:16 PM
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A 2200mAh pack is almost never seen with anything larger than 12AWG leads. If the pack could REALLY be discharged at 65C safely, you'd be running 143A through those 12AWG leads for 48 seconds before you hit the 20% capacity threshold.

Somehow I doubt the leads and/or solder joints would make it more than a few seconds.

100C ratings are laughably absurd if you think about it: 220A from that 2200mAh pack. How many seconds of runtime is that? Can the aluminum cell tabs even handle that much current?
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