Question about C Rating on Batt and Motor Amps - RC Groups
Thread Tools
Mar 01, 2009, 02:27 AM
Registered User

Question about C Rating on Batt and Motor Amps

Simple question...

I have a Hacker a60-22s. 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!

Last edited by Hemoglobin; Mar 01, 2009 at 02:38 AM.
Sign up now
to remove ads between posts
Mar 01, 2009, 03:25 AM
Registered User
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
Mar 01, 2009, 07:37 AM
Registered User
ggcrandall1's Avatar
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.

Mar 01, 2009, 08:35 AM
Registered User
Originally Posted by Hemoglobin
I have a Hacker a60-22s. 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.
If you don't want to end up destroying motors, ESCs or batteries it's really important that you realise the difference between a current of 50A and a C-rating of 25C. The guys have given it to you but just to emphasis it :

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 C-rating so the bigger the battery the lower C-rating 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 C-rating. E.g. a 4000mAh 15C battery would supply 60A so that would work for you but even with the much higher C-rating a 1000mAh 35C battery wouldn't be any use.

Mar 01, 2009, 09:20 AM
Space Coast USA
hoppy's Avatar
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.
Last edited by hoppy; Mar 01, 2009 at 09:37 AM.
Mar 01, 2009, 03:35 PM
2dogrc's Avatar
Please remember that the motor doesn't set the amount of current alone! Your propeller will also determine the draw from the battery.

Mar 01, 2009, 05:07 PM
Registered User
Thanks, I understand now.

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Question about C rating. jpwkeeper Power Systems 5 Aug 02, 2007 04:45 PM
? about C rating on Poly's BlueDevil Batteries and Chargers 1 Aug 26, 2005 10:52 AM
question about autorotate hub on eco 8? SKYWARN Electric Heli Talk 11 Jan 23, 2003 12:12 AM
Question about Heat Sink on Tiger Moth? glenclif Parkflyers 10 Jun 20, 2002 08:06 PM
question about pinion gear on 3.8 geardrive Ripley Electric Sailplanes 1 Jun 10, 2002 11:10 PM