Replies to This Discussion

T3Permalink Reply by Richard Boyhan on February 11, 2013 at 2:34pm

You can only fly with either a 3S or 4S. The 'S' means the number of cells. Each cell holds 3.7 volts, so a 3S has 11.1 volts and a 4S has 14.8 volts. The other number 2200 or 3000 are the amps or how long they can run. I recommend 3S and 5000mAh, which should get you 15 minutes of flight. Your copters APM is also configured for the 3S. If you go with the 4S you will need to adjust the PIDs. Also the 4S will make your copter fly faster, but not necessarily longer.
Remember to start slow and fly low for the first few flights. Keep your controls in stabilize mode and learn how to fly.
Have fun and fly safe.
Richard

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Permalink Reply by Shyam Balasubramanian on February 11, 2013 at 3:34pm

Hi Richard,

Thank you for your response. My requirement as you said, would be to keep it simple.

Let us make this more interesting! From my understanding there is a trade-off between (net) weight of quadcopter Vs. Flight time. I have been quite interested in this trade-off between both, and later I will present a figure from one of the earliest papers on quadcopters (link) which explains this trade-off nicely.

In our discussion, if we use a battery of 5000mAh - we fly longer as compared to say 2000mAh (weight is decreased). Now, let us see this figure below, where it shows the an increase in weight presents an exponential increase in power consumption. I am looking for a feasible design choice. Please note, I am not trying to be exacting or looking for a scientific answer but of course, experts in flying here can help me guide me with their experience.

Here is what I figured out from HobbyKing website for 3S battery choices:

Turnigy 2200mAh 3S 25C Lipo Pack: 227g
Turnigy 2650mAh 3S 30C Lipo Pack: 332g

Turnigy 3000mAh 3S 20C Lipo Pack: 306g

Turnigy 4000mAh 3S 20C Lipo Pack: 423g

Turnigy 5000mAh 3S 25C Lipo Pack: 489g

Questions:

1. 5000mAh weighs more than twice of 2200mAh. Considering the weight of quad to be 1.1Kg (incl) battery, did you say you flew better with 5000mAh. There is also this term 3S (20/25/30 C) along with it, 'C' termed discharge. Which one should I choose (20/25/30C) and WHY? How does discharge play a role here?

2. How about 2650mAh. Is that also a choice?

3. Have you used anything greater than 5000mAh batteries. Did you fly longer or was it is a turnaround?

Looking to hear more from you!

- Shyam.

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T3Permalink Reply by Richard Boyhan on February 11, 2013 at 5:26pm

The C is what max current the battery can push. It's better to have a higher C. 20 is to low, 25 OK, 30 better. I fly with 45C. I have not use any batteries larger than 5000. I have flown with 2 - 5000 batteries mounted in parallel. It only gave me an additional 6 minutes of flight. But then again I'm flying a Hexcopter with a camera gimbal attached.

Richard

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Permalink Reply by Shyam Balasubramanian on February 15, 2013 at 4:43pm

I see that the weight of my quadcopter would be 1.4 kgs in total (with a small camera and additional hardware for my project).

@Richard, what is the weight of your copter which has the complete setup of camera gimbal?

Also, from what I understand from this discussion. Having a battery with higher 'C' discharge rate means the more power my ESCs and motor will get and eventually a greater thrust??

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T3Permalink Reply by Richard Boyhan on February 15, 2013 at 8:42pm

I fly a Hex with a GoPro camera and small gimbal mount. Total weight is about 2kgs 11 minutes of flight. The 'C' does not mean more thrust, it means it can handle more load or amps. Higher voltage will give you more thrust. So a 3 cell battery gives about 11 volts and flys nicely. A 4 cell battery gives almost 15 volts and flys like a rocket.

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Permalink Reply by Derek Ernewein on February 16, 2013 at 3:17pm

C rating is the discharge rate. This is the number of time the batteries average that you can safely use in one 'cycle'.

For example, if you have this battery: Turnigy 4000mAh 3S 30C Lipo Pack

It can handle 4000mAh * 30C draw. That is 4000*30 = 120000 mAh draw. That means it can take 120 Amps being drawn form it. With a quadcopter (4 motors) the max draw the battery can take is 30 Amps (120 / 4 = 30).

Drawing more than that can cause damage to the battery. Drawing too much less than this is also not good for the battery. That is why they sell low C batteries for transmitters.

Hope that helps!

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T3Permalink Reply by Richard Boyhan on February 16, 2013 at 3:46pm

Thanks Derek, nice info.

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Permalink Reply by Shyam Balasubramanian on February 16, 2013 at 3:53pm

Hi Derek,

When you meant 'one cycle', do you mean in terms of the loop frequency? 

You said " With a quadcopter (4 motors) the max draw the battery can take is 30 Amps (120 / 4 = 30)" : Do you mean the max draw the motor can take is 30 Amps or were you referring to the battery taking 30 Amps/ cycle. I am a bit confused here;)

If I am using 20Amps ESC's with 850kV motors, can I use 30C or more for battery too? Some more explanation, can get me enlightened! 

@Richard, the description on 4 cell battery flying like a rocket was perfect. I understood the difference between 3S and 4S ;)

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Permalink Reply by Derek Ernewein on February 16, 2013 at 5:26pm

Cycle does not have to do with the board's loop frequency. I believe it has to do with the chemical process in the battery.

I think a few more examples will help you apply the calculation to your build.

Turnigy 4000mAh 3S 30C = maximum current draw of 30A times 4 motors.

The math to get there was 4000 * 30 = 120,000 mAh. 1000mA = 1Amp. So, thats 120A divided by 4 motors. 30 amps per motor.

Turnigy 4000mAh 3S 40C = maximum current draw of 40A times 4 motors.

4000 * 40 = 160,000 mAh. 160A divided by 4 motors. 40 amps per motor.

When you have the ESC's maximum draw (20 Amps in your case), you di the opposite to find the battery you should use.

Remember, the C rating is multiplied by the mAh value of the battery to get the maximum current draw. It is hard to just come up with a mAh value because it depends how long you want to fly for, what you can afford and how much the battery weighs.

Lets do the calculation backwards using two possible battery sizes for you.

2200mAh battery with 20A ESC/motor combo. 20A times 4 motors = 80Amps total. That is the same as 80,000mAh.

80000mAh divided by 2200mAh battery means that the battery has to have a 36.364C rating or higher.

Next, lets do a 3000mAh battery with 20A ESC/motor combo. 20A times 4 motors = 80Amps total. That is the same as 80,000mAh.

80000mAh divided by 3000mAh battery means that the battery has to have a 26.667C rating or higher.

Lastly, a 4000mAh battery with 20A ESC/motor combo. 20A times 4 motors = 80Amps total. That is the same as 80,000mAh.

80000mAh divided by 4000mAh battery means that the battery has to have a 20C rating or higher.

Let me know if that helps!

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Permalink Reply by Dave on February 16, 2013 at 5:35pm

The C rating specifies the maximum continuous current a battery can safely deliver.  So a 20C battery can deliver 20 x the amp-hour rating of the battery continuously.  Batteries often have a rating of say 20C / 30C.  The 30C rating is the maximum short-term current the battery can deliver (e.g. at full throttle).

Some examples.  A 4000mAh (4Ah) 20C/30C battery can deliver 80A continuously and 120A in bursts.  A 2200mAh (2.2Ah) 20C/30C battery can deliver 44A / 66A.

So, if you've got 20A ESC's, then your quad is not going to draw more than 80A.  It's going to be less than that so I'd pick a battery capable of 80A sustained current and then you'll probably have some margin and the battery will run cooler.

BTW, I favour Zippy batteries.  I bought some several years ago and they are only now losing capacity.  Much more recently, I bought Turnigy and have suffered cell failures in several of them. 

 

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Permalink Reply by Shyam Balasubramanian on February 16, 2013 at 7:55pm

@ Derek: I think that was a wonderful explanation step by step. I really enjoyed it and now things are looking prettier for me.

@Dave: Thanks to your inputs too.

I am trying to unravel the mystery. Pardon me for the upcoming verbiage.

In my case. I bought the unassembled kit (Quad-C) to begin with Arducopter  (Although, I have built custom quadcopter before by using standard battery but w/o calculations;) ). In this Arducopter, I have already chosen the ESC/motor configuration (Can't change it now)

I come to the following analysis with: (Quad-C Arducopter kit)

20A ESC's and 850kV motor: (Quad configuration: 4 motors)

I go backwards in calculation:

1. Current draw: 20A x 4 motors: 80A (Max. current that can go through all ESCs combined and fed to all motor(s), i.e. max of 20A/ ESC).

2. This max current (80A) needs to be supplied by my battery (in worst case - under max Throttle).

3. Now, let me come to what battery to choose. I am going to choose 3S (3 cell) battery because it is easier to tune that Arducopter. I am left to choose 'battery capacity X (mAh)', constant discharge ('C'), weight (W) of battery parameters.

4. 80A (i.e. 80,000mA) / Battery capacity (X in mAh):

Available battery capacities(X): (1800mAh, 2200mAh, 2650mAh, 3000mAh, 3600mAh, 4000mAh, 5000mAh).

- 80,000/1800: ~45 'C' rating required

- 80,000/2200: ~36 'C' rating required

- 80,000/2650: ~30 'C' rating required

- 80,000/3000: ~27 'C' rating required

- 80,000/4000: ~20 'C' rating required

- 80,000/5000: ~16 'C' rating required

5. Now that we know the required 'C' rating for our possible batteries, lets ask hobbyKing what combination it likes.

Based on my requirements, it says possibilities are:

(2200mAh, 35C): 247g, 15 $

(2200mAh, 40C): 267g, 15 $

(2650mAh, 30C): 332g, 16 $

(3000mAh, 30C): 355g, 22 $

(4000mAh, 20C): 423g, 26 $

(5000mAh, 20C): 497g, 23 $ (strange, cost is lesser here)

However, ZIPPY Compact 2450mAh 3S 35C: 220g, 21$ ??

6. Performance: Now, that we have the set of batteries to choose from, let us apply Weight Vs Power principle. (see my post in this page with graph for a quadcopter)

Theoretically:

Power (P) ∝ Weight (W)

The propellor/ motor combination produce 'thrust' (lifting capacity) to lift a given mass.  (Units of Kilogram-force (kg-f or Newtons(N)). Thrust would depend on the 'speed of flight' and 'weight' it tries to lift. My reasoning is on the battery choice w.r.t. weight mainly under hovering condition.

Power required to develop thrust: Power (P) ∝ (cube root) Thrust (T)

Due to the non-linear nature of this graph (rapid increase in power requirement for a given thrust will occur, with increasing weight), so it is wiser to go with lighter batteries (total weight = constant weight of quadcopter + battery weight).

Practically:

Total time the arducopter will run for can be calculated if we know the 'total power' it consumes during its flight (via Mission Planner log), for a given battery: 

Time of flight = [mAh / 1000] * [battery voltage * # of batteries (in || )] / power consumed

In my opinion, all of the given battery combinations can be used, as higher mAh increases the time of flight. However, it would be a matter of applying the above equation with different battery combinations (having the weight of quadcopter otherwise to be constant), the best battery amongst these can be found.

- Now to some questions:

• There is a term called 'Peak Discharge(10sec): I have taken 'Constant Discharge(C) into account. How do we account for that?
• Isn't it true that we can choose a battery with ANY parameter for 'C', higher the better (30C, 45C, etc). I say this because, the board (APM) would guide what PWM to be generated, that ESC computes as power to be supplied to the motor (P= VI). The current, each ESC would generate is max 20A, and this guides the battery to give it the current it demands. ESC is still safe!
• Would you prefer Lipo, Nano-Tech or this kind of battery and why/ why not?
• How come Zippy weighs lighter than LIPO for the same configuration. Why not go with the ligher one then?

Thanks for having read so far!

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Permalink Reply by Shyam Balasubramanian on February 17, 2013 at 7:14am

Hi Tim,

Is that from your experience or do you have some stats to state that? ;) What was your quad net weight (w/t and w/o battery), coz that would depend on your statement.

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