Hi all,
I am using tarot 680 Hexacopter frame and avionic 880Kv motor(http://www.rcbazaar.com/product.aspx?productid=4744) and a 40A ESC(for safety). The propellers, I will be using is 11 x 5.5 Hobbyking (https://hobbyking.com/en_us/hobbyking-8482-thin-style-propeller-11x5-black-ccw-2pcs.html). Mostly I will be using this hexacopter for agriculture purpose and hence require heavy payload lifting. As per the manufacturer, the maximum current drawn by a single motor would be around ~30A.
Hence, the    Maximum (A) draw = 180A
At required flying load, Amp draw = ~100A

There even with 7000mah/35C lipo 3S battery, I will be getting only 4.2 mins flight time((7.0/100)*60).
For 10000mah/25C 3S lipo, it will be only 6mins.
But i need atleast 10-15 min flight time.

So, what is the suitable battery to meet my requirements?

Thanks in advance.

Try to copy the specifications from https://www.dji.com/mg-1. You can go for 2x10000mAh LiPo, please calculate the AUW including the batteries and payload and then calculate the flight time.

You can also use lead acid battery - pl see link.

You can try using li-ion batteries(18650) type. They weigh less so you can put on 20000mah and they will be able to supply the current you need as well as you get good flight time. They are even cheap to replace if any cell is damaged. Do not use lead acid they are heavy..

Li-ion batteries, including 18650(18mm dia & 65mm length) seems to be a good alternative in terms of mAh/gram for a given cell. But, where Li-ions lag is the discharge rate(C rating in Li-poly world) per cell. A very decent, easily available and affordable Li-ion cell with good battery life cannot get you beyond 2C-3C range, without violating chemistry, resulting in eventual dead cells.

Regards,
Raj Gunaseelan

Sent from my CP8676_I02 using Tapatalk

Hi Ajith,
I would suggest that you use 4s or more (if ESC & motor specs allow) batteries. This is because to lift a given payload, the motors require an electrical power which is P=VI(roughly). As we see, for a required thrust x, by providing more volt(4s instead of 3s), we will end up spending less current(cooler ESCs and motor windings), hence more flight time and more importantly, best utilization of the battery "mAh x C" capacity.

To make the point simply clear, if you refer to the motor specs from rcbazaar link that you had posted, for a 11x4.7GF prop, 1130g of thrust is attained at 15.6A @83% throttle on 3s; but for the same prop, 1240g of thrust is attained at 14.6A @66% throttle on 4s.

Do you have a rough estimate on your all up weight(AUW)? That can be a good starting point while deciding the battery.

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Thank you Santhosh, Yatendra, Balakrishna reddy, raj james for your information.
I can't go for 2 lipos connected in parallel or series. Since adding lipos each weighing ~800g will reduce the payload capacity.

Yes, Li-ion batteries seems to have high mah but discharge"C" is low. So, I cannot use it.

As Raj said, High volt batteries give good thrust for minimum current. Considering using 4S(max volt for the given motor) reduces the max current consumption to 25A(at full throttle), but I will get only ~7.5(at 60% throttle) mins even with 10000mah/4S battery.

I assume AUW would be around 2.5 kg (1.5kg drone + 1kg Payload).
Any other suggestions?

@ajith_kumar,
Let's run the numbers.
To hover, you'll need say 2500gms.
With low kv motors runnng slow on low pitch props, you can get 10gms/watt.
Say 250 watts
Or say 20amps on 3s
So a 10,000 mah 3s should give you 15 minutes using 50% capacity

Your estimate is 100amps!
Why?

Ajith,

First of all estimate your total weight. which includes...
frame, motor, esc, connectors,wiring, lipo, flight controller, and your agriculture equipment. and add 100-200 grams, for safe side.

Now divide the weight by six, as you have hexacopter.
you will get weight to be lifted by per motor.
now as per the data of the motor, given by manufacturer, see how much it draws, at that weight and what is it most efficient spot ( prop size on 3s or 4s )
Then calculate the time you get with your setup.
your motors are taking approx, 1 amp per 100 grams roughly.  for eg. at 12inch prop at 3s it takes 5.8 amp at 560 grams of thrust.

See its not that you on adding bigger lipo , your flight time will increase , because that extra weight will, increase the weight per axis on the motor, so more current drawn and less flight time.
You have to find the best spot .

If you want to get more time, there are two options, Either reduce the weight or find the most efficient motor like T-motors and others like that.

Thank you, Manoj and K K Iyer. Both of you are correct. I made a mistake in my calculation. I didn't consider the weight. I usually calculate the current needed by the motor when 60% throttle is given (So that battery will not blow up even if I give maximum throttle(for safety)). So, as per your suggestion
Let's calculate,
Assuming my AUW is 3Kg.
3000/6=500g need to be lifted by each motor.
As per datasheet, 5A will be consumed if it needs to lift 500g.

So, 5*6=30A will be the maximum current consumed by all motors.
Therefore, Theoretically, I will get nearly ~19min flight time with 10000 mah battery.

So, I can use 10000mah/3S battery to get my required flight time.
Thank you all.

Hi Ajith, the assumption that, to lift x gram of AUW, just x gram of thrust is sufficient is not practically true. In fact, as a rule of thumb, we need a thrust of at least twice the AUW just to hover the craft.

Since you are designing for a real world application(for agricultural purposes), I would suggest that you must be looking for a thrust of at least 5-6 times your AUW as a "bare minimum". This ensures maneuverability as well as durability of your design.

This brings us to a required thrust of 3kg per arm. This may require quite some power(and may be heat sinks too:)).

Lets just wait for experienced members provide their insight though.

Regards,
Raj Gunaseelan James

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@rajjames,
@ajith_kumar
Raj sir, i hope you won't mind if a disagree a bit

Quote from: rajjames on November 12, 2016, 05:38:46 PM
Hi Ajith, the assumption that, to lift x gram of AUW, just x gram of thrust is sufficient is not practically true. In fact, as a rule of thumb, we need a thrust of at least twice the AUW just to hover the craft.

Actually, it is true that to hover x gms, you need x gms of thrust
However, you cannot do an actual useful flight with just hover power.

Lifting that is a different issue.
With thrust equal to auw, you can probably lift off. (Due to ground effect). But you can't climb!

If your thrust was twice the weight, you'd have 1g acceleration. So you could climb 16ft in one second, or 64 ft in 2 seconds.

So the power reqmt for hover is only a starting point. Your total power reqmt depends on how much you need to accelerate/ decelarate, and how often.

But Thrust 5-6 times the auw? Seems excessive to me.

Also remember that:
1. You wont be able to use more than 75% of the battery capacity
2. Lipos that we usually use, will have severe voltage sag if run at over 10c or so...

Incidentally, do you have a wattmeter?

@KK Iyer. I really can't understand the point "Lipos that we usually use, will have severe voltage sag if run at over 10c or so..."
Do you mean If the Lipos which rated 40C or 60C may cause voltage sag when we run over above 10C?

Yeah, I have Wattmeter. But, I didn't purchase the battery yet. So, I can't check any parameters from the battery now.

[Inference from your previous reply:
-If the thrust is below the weight, we can't takeoff
-If thrust is twice as the weight, I will be able to lose 1G acceleration(which the vehicle experiences).
-As the vehicle moves upwards, requirement decreases.
-When moving downwards, G increases in steps(1G, 2G, 3G). After that, a sudden upward pull requires more thrust, hence the battery power consumption will increase.
-Therefore, the power needed to accelerate and decelerate does not depend on the initial power requirement.
-Using 5-6 times or more of thrust results in wastage of our power. This pull is usually required by vehicles which require attaining escape velocity]

Thank you, Raj james for your suggestion. Yeah, We cannot simply depend on the thrust data provided by the manufacturer. So, the current consumption may differ(increase) for the required thrust to produce, resulting in more power requirement.
The battery which I planned to buy is 10000mah/25C 3S. The reason is that this battery is available in India. But the problem is using 25C instead of 35-40C. The slow discharge rate may result in the problem if a sudden turning movement is performed by the vehicle. This battery won't be able to discharge such Amp requirement. Theoretically, 10*25=250A is suitable for my system. But, still using buying 25C seems to be the risk. Any suggestions?

Why don't you use this to calculate http://www.ecalc.ch/xcoptercalc.php

Hi santhosh,
There are two reasons
1.) I don't have the premium account on ecalc.
2.) We can't find the required components on the ecalc dropdown menu. Therefore, the calculation is not exact.

Ajith,

Since you're gonna invest more than a few hundred dollars for the Hexa, I will recommend to buy the ecalc Pro membership starting at $2 per month.

With your current setup, better go with a 8 to 10Ah 4S battery.