Hello everyone,

Consider the following situation,

I have two batteries
1) 5000mAh  2S
2) 5000mAh  3S

I am using Same motor lets say, 1000kv with same prop (1045), I need same amount of thrust (lets say 400grams) to keep my plane in air.

so which battery will last long?

The 3S will have higher Watt-Hours than the 2S for same capacity. So, if both have same capacity but one is 2S and one is 3S , the 3S one will last longer.

Watt-Hour is basically Voltage(V) X Capacity(AH).

For 2S, you have (2x3.7)V X 5Ah = 37 Watt-Hours (Wh) at nominal battery voltage
For 3S, you have (3x3.7)V X 5Ah = 55.5 Watt-Hours (Wh) at nominal battery voltage

Basically, it all boils down to the Watt-Hour rating which indicates the Power output per hour.

Hope that helps.

-Regards

Thanks for the reply.

Ok now consider some other facts,
3s battery will weigh little bit more than 2s so the plane will need more power to stay in the air.

So will it affect the flight time?

Yes that will reduce the flight time by a bit. But 3S will still longer than 2S IMO since there is a significant difference in watt-hours. To get exact times you can either do a controlled experiment where you use them till they get drained or you find the amount of extra amps required by the motors when using 3S compared to 2S at some specific throttle (and altitude?). By knowing the amps required when using 3S and 2S you can then calculate the time it will take for both. Now, that is assuming that there is linearity in Amps drawn.

There might be some online calculators for doing this calculation.

Whats the exact weight of both the batteries?

Btw, Where do you stay in Thane(you can PM me)? I am from Navi Mumbai.

-Regards

As you said I searched for a calculator online.

here it is: http://multicopter.forestblue.nl/lipo_need_calculator.html

But it shows same flight time for 2s and 3s battery with same mAh

kindly go through it and read the 4th point at the bottom of page.

I am still confused....

Go for high C rating 30-60c Battery and with good capacity 3000-4000 mah 3s  11.1V
lipo batteries

It seems that is general "calc". What you addressed is a specific case. I have assumed the motors are giving same power output in both the cases, however the calculator doesn't seem to assume that (I'll check it in detail later, I might be wrong).

Lets say your motors output 100W at some throttle, with 2S you will require more current compared to 3S. If you are increase the power output in 3S then it will drain a bit more quickly but will give you more performance.

If same amount of amps is drawn in both cases then both will drain at the same time. But the 3S will give you more performance(if motors can handle) as I mentioned above, since the power consumption has increased.

I'll go through the calculator and flying times in detail this evening.

I appreciate that.

Any other views??

It seems he(the one who made the calc) is assuming constant current draw and hence he is also assuming that you have your batteries,ESC,Motors matched.
IMO Power consumption, rather than current consumption is better to get flight times since it also includes voltage (i.e. you battery series configuration). In my calculations I have assumed constant power draw.

Lets Consider your example:
You have a 1000KV motor, with 1045 props and want to compare the battery times for a fixed 400gms of thrust.
To get the same amount of thrust the motor will draw more amps from 2S battery compared to 3S, for a specific power output that gives 400gms of thrust. Hence, 2S will obviously drain faster than 3S in the example considered. I don't remember the complete working of ESC's with emf feedback (I was away for long from RC) but to spin the motor faster using 2S battery your ESC will suck more AMPs from battery as required by motor to attain higher RPMs as compared to 3S setup. So, with 2S the ESC will try to compensate for RPM by increasing the current draw as needed by motor.

Hence,to the best of my knowledge, my argument is still valid in your case. If someone can prove me wrong I'd be glad to learn about my mistake

I agree with Power_user_EX. Electrically, motor and ESC are going to be happy(lesser current = lesser heat + lesser loss + lesser noise + lesser magnetic field) with 3S 5Ah than a 2S 5Ah.

Regarding the additional weight of a 3S, my theory(disclaimer: not tested) is that the no load motor rpm(as well as the electric power) is also increased with this increased weight(2S 1000kv = 7400 rpm @ 7.4V; 3S 1000kv = 11100 rpm @ 11.1V). So, I can only imagine that this additional weight may not be a problem.

I'm not a plane guy and cannot comment on effect on CG and other things which can matter.

Yes I agree with both of you.

Thank you for taking time to write.