C Rating Insanity
Dec 18, 2012 10:57:00 AM
A few weeks ago David wrote a post about our experience testing batteries with high C ratings. Today I'd like to follow up on that and take a moment to reflect on what C ratings actually mean.
A 1C discharge rate, by definition, means that you're going to discharge your battery in an hour. 2C is half an hour, 10C is six minutes, 30C is two minutes, etc. 65C, which by this point seems to be almost a baseline in the market, means your battery will be drained in 55 seconds.
That's how I usually think about C ratings, in terms of time. Yesterday, though, I started thinking about it another way, in terms of current draw. Looked at this way, a 1C discharge rate means you're pulling amperage equal to the capacity of your battery. For example a 5000mAh battery at 1C is pulling 5A, 2C is 10A, 10C is 50A, 30C is 150A, etc. 65C would be pulling 325A. Are your connectors ready for that? For a point of reference, EC5s – one of the best connector options available - are only rated to around 120A continuous.
There are batteries out there, though - I won't name names - that make 65C-rated packs look like paragons of honesty. Yesterday I was looking at an 8000mAh pack that was rated to 150C. 150C. Not only would that discharge the battery in 24 seconds - and blow it up in the process - the current draw would be a ridiculous 1200 amps!
The marketing is mind-blowing, but the math is simple. And the nice thing about math is that it doesn't care about marketing. So the next time you're shopping for batteries, run the numbers, figure out what your model actually needs, and see if what's being advertised makes any kind of rational sense.
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