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I'm starting a project of modeling a quadcopter. For this I'm of course in need of a battery and I have red a bunch of threads to understand how to read the specifications of them. The most complete website I found was the battery university. http://batteryuniversity.com/learn/article/what_is_the_c_rate

I still have a question about the C-Rate. For exemple this battery: http://www.hobbyking.com/hobbyking/store/__8932__Turnigy_2200mAh_3S_20C_Lipo_Pack.html

Spec. Minimum Capacity: 2200mAh (True 100% Capacity) Configuration: 3S1P / 11.1v / 3Cell Constant Discharge: 20C Peak Discharge (10sec): 30C Pack Weight: 188g Pack Size: 103 x 33 x 24mm Charge Plug: JST-XH Discharge Plug: XT60

If i understood well, since the constant discharge is 20c, (so 20*2200 =44 A) this battery will have a good flytime if my motors are pumping less then the 44A? (more than 100%)

Thanks!

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  • \$\begingroup\$ Keep in mind that a quadcopter isn't a constant power load. You have a typical or average power needed to hover, but then you have peaks needed to accelerate upwards or arrest a descent. Your peak "C" rating needs to (practically, which may differ from on paper) accommodate the latter, while your battery life is hopefully more related to the former. This is a pretty busy field, so you have the option of just using whatever (brand as much as specs) others with similar mass vehicles and similar motors have had good results with, rather than trying to work it out theoretically. \$\endgroup\$ – Chris Stratton Mar 10 '16 at 20:01
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To first order, if the capacity is actually 2.2Ah ...

You would get nominally 3 minutes fly time at 44A = 20C.
You would get nominally 6 minutes fly time at 22A = 10C.
You would get nominally 12 minutes fly time at 11A = 5C.

Nowhere does the battery capacity exceed 100%.

A lower rate does trivially allow you more than 100% of the 20C fly time.

The actual capacity you get, and the discharge rate, are inversely linked. Not as strongly as the reciprocal, but you need to check what the capacity is at your discharge rate.

Unfortunately, the capacity is usually quoted at the 1C rate, for a 1 hour discharge. The capacity at 20C will therefore before lower than 2.2Ah. How much lower? This depends to some extent on the quality of the battery. This means that the nominal flight times above are really 'will not be exceeded' times.

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  • \$\begingroup\$ You could expect 3 minutes at 20C. You might not get that long, since the advert doesn't say what discharge rate was used to measure the capacity, but I bet it wasn't 20C. If the capacity at 20C is only 1500mAh, you'd better be prepared for a landing at about 2 minutes. \$\endgroup\$ – Brian Drummond Mar 10 '16 at 12:09
  • \$\begingroup\$ Hobbyking, while offering good value, is well known for overstating the maximum discharge rates. The battery capacities are generally as advertised. I would get a battery with at least a quarter more advertised constant discharge current than needed. \$\endgroup\$ – jms Mar 10 '16 at 15:37
  • \$\begingroup\$ Here is a quote from battery university: "By discharging the 1Ah battery at the faster 2C-rate, or 2A, the battery should ideally deliver the full capacity in 30 minutes. The sum should be the same since the identical amount of energy is dispensed over a shorter time. In reality, internal losses turn some of the energy into heat and lower the resulting capacity to about 95 percent or less. Discharging the same battery at 0.5C, or 500mA over 2 hours, will likely increase the capacity to above 100 percent." \$\endgroup\$ – ArkaFlo Mar 10 '16 at 16:47
  • \$\begingroup\$ If we are assuming that the value given by the seller are right. I understand that the discharging is not linearly fonctionnal to the current asked by motor... Am i wrong? If i follow what's said there, a 20C, 2200mAh ( so 44A) battery used on motors asking 44A will last (2.2/44)*60=3minutes while used on a set of motors asking let's say the half, it will last more than 6 minutes? \$\endgroup\$ – ArkaFlo Mar 10 '16 at 16:51
  • \$\begingroup\$ @ArkaFlo your direction of travel is right, but your starting point is not. While 22A flight can be expected to last more than twice as long as 44A flight, the latter will not achieve the nominal 3 minutes that the first order calculation might suggest. \$\endgroup\$ – Neil_UK Mar 10 '16 at 17:40

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