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.

1 Comments | Posted in Technical By Robin

Store Those LiPos Correctly!

Dec 5, 2012 3:31:00 PM

With winter rapidly approaching, us hobbyists begin to find it quite difficult to get any flights in! While it may be tempting to leave your batteries charged and ready to go at a moment’s notice (such as a 10 minute burst of sun in the middle of stormy Seattle day), this is quite bad for the life of your batteries. Winter is a good reminder to check all your batteries, and make sure they are at storage voltage!

Lithium batteries have come a long way, and have significantly propelled our hobby (pun intended) with more power, longer run times, and less weight. In order to maintain the quality of Lithium batteries, they must be taken care of appropriately.

When you buy our Spyder batteries, you may notice that all of the cells arrive at about 3.8 volts. The reason for this is LiPos like to be stored around half full. You can see from this chart that there is virtually no degradation when properly storing a LiPo, 40% full and at or below room temperature. If you were to store your battery fully charged however, after 1 year at room temperature, you will lose 20% of the capacity of the battery. So your 3S 2200mAh is now a 3S 1760mAh, and you can quickly see how easy it is to over-discharge the pack and things will degrade even more.

Image from www.batteryuniversity.com

So after a day at the field, track, or yard, check those batteries! If they are between 3.8 and 3.9 volts, they are ready for storage, any more or less, and use the storage function on your charger to bring them up, or drain them down to the appropriate voltage. Keep in mind if the battery needs to be discharged from full, this could take a while on a charger (that’s a lot of energy / heat to dissipate!) so running the battery down part way on your model is usually the quickest way to do it.

0 Comments | Posted in Technical By Yarden