|Mar 01, 2009, 01:27 AM|
Joined Jul 2008
Question about C Rating on Batt and Motor Amps
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!
|Mar 01, 2009, 02:25 AM|
Letchworth, Great Britain (UK)
Joined Jul 2004
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, 06:37 AM|
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, 07:35 AM|
Joined Nov 2003
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, 08:20 AM|
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.
|Mar 01, 2009, 02:35 PM|
Please remember that the motor doesn't set the amount of current alone! Your propeller will also determine the draw from the battery.
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