C rating

If you need to read up on some basic battery theory,  read this post first.

C rating is a rating of how quickly energy can be discharged from a battery.  The rating is combined with a battery’s capacity to find determine the maximum discharge.   This is very important when you consider battery choices for Quadcopters and multi rotors because these machines draw very high amps from the motors and it is easy to choose a battery that can’t handle the currents required by brushless motors and steep-pitched props.

The C rating is important when it comes to choosing a battery for high performance multi rotors because our multi-rotor motors draw high amounts of current (amps) and it is very easy to choose a battery that is not properly rated for our current draw.

The C rating is proportional to the battery’s capacity,  so as you move up in capacity, you can move down in C.

When engineering a new quadcopter, we have to consider the amp draw of the motor/prop combination, and the max battery size we can fit on the quad to determine the right battery for the job.

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C Rating Calculation

It is important to calculate  what amperage a battery can output when considering it for a quadcopter build.  Let’s consider A 20C battery rated at 2000 mAh.

to calculate what this battery can do:

Convert milliamps to amps:

[latex] \displaystyle (2000ma)\frac{1amp}{1000ma} = 2amps. [/latex]

Calculate the amps times the c rating to get the total output current:

[latex] \displaystyle 2A*20C=40A[/latex]

This means that we can discharge 40 amps continuously without damaging the battery.

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Nanotech calculations

The typical nanotech battery that I recommend for the QAV250 is a 25c continuous, 50c burst battery.

We can calculate the safe amp output capacity of this battery by the following.

Convert milliamps to amps:

[latex] \displaystyle (1300MA)\frac{1amp}{1000ma} = 1.3A [/latex]

Determine discharge rate by multiplying Amps x C Rating

[latex] \displaystyle 1.3A*25C=32.5 amp discharge. [/latex]

Divide by number of motors:

[latex] \displaystyle \frac{32.5amp}{4motors}=8.13A [/latex]

This means that each motor (if building a quadcopter) shouldn’t draw more than 8.13 amps.

Looking at the graph of the current draw of 2300KV Cobra motors on 3s with 5030 props, we can see that the motor current draw is somewhat linear until about 80% throttle, after which it becomes logarithmic; the efficiency of the motor drops off and the current really rises.  Looking at this chart, I would think that it would be ok to use the nanotech battery in this application.

Burst Rating

These Nanotechs are rated with a burst capacity of 50C-

Again, Doing the math, but this time for a 50C Discharge rate,

[latex] \displaystyle (1300MA)\frac{1amp}{1000ma} = 1.3A [/latex]

[latex] \displaystyle 1.3A*50C=65 amp discharge. [/latex]

[latex] \displaystyle \frac{65A}{4motors}=16.25A [/latex]

That is 16.25 amps per motor at the burst capacity of the battery.

Burst is usually considered in 15-20 second increments, so in my designs, I like to make sure that I mostly stay within the main C rating.

Putting it all together:

We should use this battery on a craft that draws no more than 8.13 amps in most circumstances.  Acrobatic maneuvers & full throttle pullouts tend to use much more amperage, so you could use this battery on a craft that draws 8a per motor in full throttle, and then know that burst will cover acrobatic maneuvers.   The Gemfan 5030 draws about 8.4 amps total with a Cobra 2300kv motor on 3S, so I am within the design limits so long as I am not building this quad to be at full throttle all of the time.  The burst C rating should handle any current overages that are incurred.

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IMPORTANT NOTE:

Battery manufacturers lie.  So a good rule of thumb is to

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Higher Amperages

If we decide to go with a steeper pitch prop such as a 5045 or a a larger propeller, then we would need to reconsider this battery as it would not be adequate.

If we decided to go to a 4s battery, we would need to consider that the higher voltage would result in a higher amp draw.  The cobra motor draws about 13.3 amps with a gemfan 5030 on 4s.

The 2000v Cobra motor draws about 10.4 amps at full throttle.  It is a little bit more efficient at full throttle.

If we use the 2000KV motor for a 4S build (recommended) we can calculate it out:

 [latex] \displaystyle 10.4 amps*4=41.6 amps. [/latex]

This means that ideally, we would need a battery capable of a continous current at or near a 40 amp discharge.

We could achieve this by either increasing our battery capacity, or increasing our c rating.  So it would make sense to bump up our 1300mah 4s battery to a 35C rating.

25C batteries cost less than higher C rating batteries, but in order to use a 25C 4S battery at with Cobra motors at 10.4 amps, we would need to up the battery capacity to 2000mah or more.

[latex] \displaystyle Capacity=\frac{40A}{25C} [/latex]

[latex] \displaystyle Capacity={1.6A} [/latex]

So we would need a 1.6A (1600MA) or larger 25C battery in order to get the discharge amperage required for these motors.

You can find Amp draws on propeller manufacturers websites.  I will be posting real life data to this site soon.

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Charging C ratings

It is also important to consider C-rating when charging batteries.  I always stick to the rule of thumb that you should charge your batteries at no more than 1C even if they are rated for a higher Charge c-rating.  Zippy compacts are a good example of a battery that will work great until you start charging it at a high C-rating.

Charging these at a 5C charge rating will rapidly reduce their life because their cells become oxidized.  The batteries say on them that they can be charged at a 1-5C rate, but my real life experience says otherwise.

Doing the math for a charging rate of a 1300MA battery, we calculate:

[latex] \displaystyle {capacity in amps}*{charge C rate}={charge rate in amps} [/latex]

[latex] \displaystyle (1.3A)*(1)={1.3amps} [/latex]

[latex] \displaystyle (1.3A) \frac{1}{1000}=1300MA [/latex]

Essentially, charging a battery at a 1C rate means charging at the battery’s capacity.  So a 2000MA battery should be charged at 2000MA,  a 1.6amp battery should be charged at 1600ma, etc.

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 Parallel charging C rates

One way to charge many batteries at once is to use a parallel charging board.  These boards work great when charging many of the SAME batteries at once.  One thing that you have to realize with these boards is that as you add

batteries in parallel you are tricking your charger into thinking it is charging one large battery.  So essentially, connecting 4 3S 1300 MAH batteries to a parallel charging board fools the charger into thinking that you connected one 5200MAH 3S4P to it.  So using the logic above, you could charge all four batteries at once at 1C by setting your charger to 5200MA when charging.

 

 

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Conclusion:

Choosing the right battery when building a quadcopter is important!  Quadcopters draw large currents, and choosing the incorrect battery for the application could greatly reduce it’s life and reliability.

Make sure to do the engineering before you purchase.

 

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