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.
Oct 23, 2012 11:18:06 AM
Moving forward a couple of years to present day, Spyder packs have proven their performance and longevity. In the same period of time, other manufacturers have marched forward with 50C and even 60C+ rated packs. The manufacturer of our cells has also offered updated product lines, including “improved” 40C rated cells, followed by presumably better 45C rated cells. With increased C ratings the respective pack weights have gone up, and the capacity has gone down.
Rather than choosing to quickly package and release these packs to consumers, we take the unconventional next step of qualifying the manufacturer’s cells against their ratings. To provide a glimpse, here’s a snapshot from discharge testing at around a kW of power to illustrate this point. Three different cells are shown: a 45C 5000mAh sample from last week, a 40C 5000mAh sample from 2010, and an original 30C 5300mAh sample tested three years ago:
Lo and behold, much to our surprise we found that the newer 40C and 45C rated cells did not perform any better than our high performing 30C rated cells under load. Testing shows the 30C cell held the highest average voltage under load and provided the greatest capacity, while ringing in at a lower weight than the higher C rated packs.
So we’re not just going to slap a 45C label on them and put them up for sale. We’re committed to marketing our batteries honestly. Until we see higher rated packs that outperform our current generation, we’ll stick with the 30C packs that even now, outperform optimistically higher C rated packs.
Testing is forever ongoing here, and we’ll keep you posted when real progress is made in this arena. But for now, it would seem more Cs on the label are not always better, and it makes me wonder what is being sold out there…