Battery C rating explained

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 inversely 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.


 

C Rating Calculation

Turnigy 1.3a 4s battery 1300MAH The perfect choice for high performance QAV250 Quadcopters

Turnigy 1.3a 4s battery 1300MAH The perfect choice for high performance QAV250 Quadcopters

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:

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

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

 \displaystyle 2A*20C=40A

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


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:

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

Determine discharge rate by multiplying Amps x C Rating

 \displaystyle 1.3A*25C=32.5 amp discharge.

Divide by number of motors:

 \displaystyle \frac{32.5amp}{4motors}=8.13A

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,

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

 \displaystyle 1.3A*50C=65 amp discharge.

 \displaystyle \frac{65A}{4motors}=16.25A

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.


IMPORTANT NOTE:

Battery manufacturers lie.  So a good rule of thumb is to engineer about a 20% margin into your calculations to make sure that you are well within spec.


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.

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:

  \displaystyle 10.4 amps*4=41.6 amps.

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.

 \displaystyle Capacity=\frac{40A}{25C}

 \displaystyle Capacity={1.6A}

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.


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.

Turnigy ACC-6 battery charger

Turnigy ACC-6 battery charger

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:

 \displaystyle {capacity in amps}*{charge C rate}={charge rate in amps}

 \displaystyle (1.3A)*(1)={1.3amps}

 \displaystyle (1.3A) \frac{1}{1000}=1300MA

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.


 Parallel charging C rates

Parallel charging board for multiple battery charging.

Parallel charging board for multiple battery charging.

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.

 

 


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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20 replies on “Battery C rating explained

  • russell holt

    this was excellent. glad i read to the end. i’m an newbie and i was looking for all the information given in this post PLUS the info on parallel charging (i had thought this was the case, but really needed it confirmed. thanks so much for this post.
    Russell Holt from Coral Springs, Fl.

    Reply
    • admin

      Hi Sam, You can do a few different things to figure this out- First off, check with your motor manufacturer to see if they have any published thrust data. This thrust data should show you 3 key things- battery voltage (3s,4s, etc) Prop type and size (Dal 6040 for instance) and amps drawn. From this published data, you should be able to determine if your prop/motor combination is ok or drawing too many amps. The other way to do it is to test it on the bench, but you need a way to test thrust and current.

      What is your motor/prop combination? We might be able to test your config here at quadquestions.

    • admin

      Hi kunal. 1400kv motors are slower than recommended for a 5″ or 6″ rig. I would recommend 7″ or larger props with those motors. With that being said the larger props can carry more weight so you should be able to lift a 3300mah battery without a problem. A good rule of thumb is to make sure that you have at least a 4:1 ratio of thrust to weight. So weigh your quad, figure out how much thrust your motor prop combination puts out and then figure out if you can get away with having a heavier battery.

  • Brian

    One thing that didn’t make sense to me was when you said that higher voltage would equal higher amp draw…..if all things being equal in terms of power demand, wouldn’t current draw in fact be less? You would be drawing the same amount of watts with a higher voltage, therefore your current flow would decrease according to P=VI.

    Reply
    • admin

      Brian, You are right, What should have been said is that at higher voltages, there will be more power output at full throttle. When running 4S batteries vs 3S batteries, The capability for producing more power(watts) increases. If demanded power is constant, than yes, the current required to deliver that power would be less when using a higher voltage setup according to Ohm’s Law.

      Thank you for the correction.

    • Bruce Williamson

      If you have a 20 amp ESC that can handle a 4 cell battery is the higher c rate out put going to over heat the esc?

    • admin

      Current flow is the main concern. A 20 amp esc with only 10 amps flowing through it will not overheat, however a 20 amp esc with 30 amps flowing through it most likely will overheat which is the main cause of escs failure. I think you are missing part of the equation. Think about the current (amp) demand of your motor/prop combo- then size your escs accordingly to handle the max amperage and then some. . Then make sure your battery has enough c rating to handle the amp draw of the combined demand. C rate is only the amount of current that a battery can deliver at one time, it has nothing to do with the esc.

      So for instance. If you know you have a motor/prop combo with an 18A max current draw, then you know you need to get escs to handle it. So you would get 20 amp escs. Then you have to figure that the total current draw if this is a quadcopter is 80A at max thrust when all 4 motors are at full throttle. (4 motors/props*20A).

      Knowing this, you need to size your battery to handle a current of 80a. So if your battery is a 1300mA, 30c, 4s, then it should be able to handle a max current of 39 amps (A*C-rating)- 39 Amps is not enough for this build because we know we have an 80A demand. If we use a 75C 4s 1300mA battery, it would handle 97.5 max Amps and would be a much better choice for this build because we know that it can flow enough current for the demand of the motors and props and then some. (1.3A * 75=97.5A). Does that make sense?

  • Pragati

    I m building a quad of frame 330mm and
    2212 1400kv motor
    Max current is 16amp
    With 30 ESC and 7 inch prop
    Could u tell which battery with c rating would be best???

    Reply
    • admin

      Hmm, let’s see. 2212, 1400KV, 7″ props, – If this is like the Sunnysky, then you could go off of the results posted here:
      https://www.rcgroups.com/forums/showthread.php?1874705-HQ-Propeller/page41

      Looking at the test results, this motor with 7″ props on 4s pulled around 16A in static air, but let’s say 17A to be safe.
      17*4=68A total. 2.2A*x=68A.
      x=30.9C. So, you would need to run a 31C battery or higher.
      In this case a 35C 2200mah battery would be fine.

      In general, if you have a 330mm frame, then you can run a larger battery, which requires less C rating than a smaller one, You could get away with a 20C battery…. If your frame, & motor thrust can handle it.
      4*X=68
      X=17C
      So if you can fit a 4000mAh battery, then you would only need it to be 17C or more.

      Figure out your thrust, (probably around 1150g with 7×4 HQ props on 4S) so
      1150×4=4600G of thirst total.

      A good rule of thumb is to have at least 4 times the weight of the copter in thrust more if possible, so weigh your copter without a battery. Let’s say it’s 400G- this would mean that you could run a battery that weighs 750G for an agile copter. A gens-tattu 4000mah 4S 25C battery weighs 428G and would be a good choice.

    • Anthony

      You are missing a major part of the equation- What prop are you using? How many cells is the battery? What is the known max avg. amp draw of the motor & prop combination? Only after you know that can you truly determine if your battery is adequate. That battery can discharge (\frac{5200mAh}{1000})\*35C=182A
      your 30A escs would only handle 120A total, so your battery is already adequate if you look at it that way. To do this right, you really should look at your props and motor current demand and then take it from there though, just to be sure.

  • Sir Ulrich

    I’m struggling to understand how to really take the “test results” for motor/prop combinations either by the manufacturer or a place like MQTB. The amperage draw seems very high, but I see that people are using these combinations in the field without issue. I have read that you should take 10-20% off the test total because of prop unloading. However, quads move “backward” a lot. In freestyle especially, it is very common to change directions of travel. Therefore, for a short time (<2 sec.), the prop would actually be MORE loaded than a static air test.

    Here is a specific example. The T-Motor F60III 2750kv shows on their website with a DAL5050 at ~42a, MQTB shows similar with a Buttercutter 5x5x3 prop. But I see people running these on 35a escs.
    I'm actually planning a build on these motors right now, I won't be using 50 pitch but something like the HQ 5×4.3×3 at 37a. Would a 30A esc work? MANY people use that setup. Should I move up to a 35a ESC? or is that not enough?

    Reply
  • Kevin Kull

    If i have 4 batteries with specs 100C 1500MAH (4S 14,8V (22.2Wh) Can i parallel charge these 4 with 6 Amps Because 1500×4 is 6000?

    Reply
  • Max R

    Hi, I have a wizard x220s 3 / 5S, ESC 30A, engines
    Eachine MN2206 2300KV

    KV: 2300
    Maximum thrust: 1100G
    Length: 35MM
    Cell no.: 3-5S
    Diameter: 28MM
    Framework: 12N14P
    Tree: 3MM
    Propeller: 5 inches
    Weight: 28g

    Which is the best battery to use and the maximum C that I can get to?
    Can a 4S 1.8A 100C battery fit?

    Thanks

    Reply

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