Battery C rate: do I need a lot of it?

One of the values that describe LiPo batteries used in RC crafts is so called C rating. For example: 3S 1300mAh 45-90C. Or 3S 5000mAh 25-30C. Or 4S 1300mAh 65-95C. S is simple, it tells the voltage. mAh or Ah tell capacity.

But what about C rate? According to Wikipedia is can be described as:

The C-rate is a measure of the rate at which a battery is being discharged. It is defined as the discharge current divided by the theoretical current draw under which the battery would deliver its nominal rated capacity in one hour. A 1C discharge rate would deliver the battery’s rated capacity in 1 hour. A 2C discharge rate means it will discharge twice as fast (30 minutes). A 1C discharge rate on a 1.6 Ah battery means a discharge current of 1.6 A. A 2C rate would mean a discharge current of 3.2 A.

So, if we take a look at Turnigy Nano-tech 3S 1300mAh 45-90C we will know that:

  • Constant discharge current on 45C is 58.5A
  • Burst discharge current (10 seconds) on 90C is 117A

Neat, yeah? Almost 60A constant current! That gives 15A per motor. And that a lot on 250 or smaller quad. So, I should be fine!

Well, yes. And no. Drone pilots likes to assume that battery will not show negative symptoms when current load it’s still within limit. Symptoms like voltage drop when pushing throttle up. Or swelling. Or reduced capacity. Manufactures understands C slightly different. For them, it is maximum load that is safe for battery. As long as current is within limit, battery should not ignite or explode! But that does not mean that it will not swell, loose capacity or have low voltage drop.

So, do I need as much C as possible? It depends. More C means bigger and heavier battery. And higher price.

For gentle flying (aerial photography or video for example), it does not make much sense to have huge load buffer. One usually flies there around half throttle, when battery load is not very high. Flight time might be more important than performance. So, check max motor current with a propeller you are using, multiply by number of motors and choose battery that has slightly higher C that you need and you should be fine.

For aggressive acrobatics, battery performance is very important. Higher C usually means lower internal resistance and lower internal resistance means lower voltage drop. My 250 requires around 36A on full throttle. Nano-tech 1300mAh 45C barely satisfies my flying style. With almost 60A of max safe constant load! After few seconds of full throttle I have to slow down for a moment to let battery rest. So, more is better. I really hope Turnigy Graphene 1300mAh 65C will solve my voltage problem.

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