

USA
Joined Jul 2008
95 Posts

Discussion
Question about C Rating on Batt and Motor Amps
Simple question...
I have a Hacker a6022s. The specs say it pulls 40A continuous, 50A burst. Does this mean I have to purchase batteries that are at least 40a/50a? Was thinking about getting the Zippy's but they are only 20c/30c or 25c/35c. Please explain!! I am confused! Thanks! 



Letchworth, Great Britain (UK)
Joined Jul 2004
13,418 Posts

You don't have to run the motor at 40A/50A  you can put on a smaller prop to limit the amps
But, assuming you've bought that motor because you want to use it to its full capacity, you need to get a battery that can comfortably deliver at least those amps. To determine the safe amps rating of a battery you mulitiply its C rating by its mAh rating divided by 1000. So, if your 20C Zippy is 2500mAh capacity, it should be able to comfortably deliver 20 x 2500 / 1000 = 50A I don't know about Zippy, but some manufacturer's are known to exaggerate their C ratings, so it's always best to have some margin. Higher C or greater mAh, or a combination of both, will give you more amps 



abenn's calculations are correct but don't go far enough. Your 50 amp motor needs a battery that can supply more than 50 amps or the battery life can be considerably shortened. You should not run a battery at greater than 80% of it's capacity. So in theory you need a battery that can supply about 65 amps.
Also just increasing the "C" value may not be the way to go. For example a 1000mah 10c battery can supply 10 amps and do it for 6 minutes. A 1000mah 20c battery can supply 20 amps but only for 3 minutes. So you can see that increasing the "C" does increase current availability but at a cost of flying time. Glen 


Staffs, UK
Joined Nov 2003
12,301 Posts

Quote:
The battery needs to have the ability to supply the CURRENT you need (40/50A). But to find out how much current you can expect to get from a battery you have to do the calculations. Current capability is capacity times Crating so the bigger the battery the lower Crating needed. So a 1000mAh 25C battery can only supply 25A, 2000mAh 25C battery is 50A but a 4000mAh 25C battery could supply 100A. And it's the current you need not just the Crating. E.g. a 4000mAh 15C battery would supply 60A so that would work for you but even with the much higher Crating a 1000mAh 35C battery wouldn't be any use. Steve 




Other considerations
1. Even the best batteries operated continuously at their maximum continuous duty rating will only last 50 cycles or less. Cheaper batteries will not last as long if at all. 2. A pack operated at 20C continuous will give a max 3min flight. A pack operated at 30C continuous will give a max 2 min flight. 3. To get a 10min flight you need to limit the draw on the battery to an average of 6C. Get a big enough battery (mah) so that you can fly 10 min which is an average 6C draw. I don't have any idea of how you fly with the throttle, wide open 100% or mostly 50%, but that's the key. Let's assume you use 100% 5% of the time and 50% the rest of the time. For the 100% throttle you need 50A and for the 50%, let's say 15A. A 20C 3000mah battery will give you 50A for the times when you need 100% and loaf along at 15A. 15A is a 5C draw. 15000/3000 = 5C. 60min/5C = 12 min flight 25A average draw would be 8C. 25000/3000mah = 8.33 60min/8C = 7.5min If you wanted to operate full throttle for 100% for 10min you would need a 6666mah 15C pack. And that's how it goes. 



Also,
Please remember that the motor doesn't set the amount of current alone! Your propeller will also determine the draw from the battery. 2dogrc www.2dogrc.com 
Latest blog entry: New 2dogrc Race Flags, In Stock!




USA
Joined Jul 2008
95 Posts

Thanks, I understand now.

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