3 cell vs 4 cell- pros and cons? Why would anyone use a 3 cell?!

[stevec]

Sorry read it back,  the wing is the same thickness as a 3s pack so if you want to run 3s it cannot be flush.  2s hides neatly below the airfoil section.

A 4s would stick out like nuts on a bulldog.

/Steve

[RaySky]

One reason 4S is more efficient is due to Internal resistance of batteries and current through them.

For a given power output W , then W = V * I . IOW lower voltage means more current is required for the same power output.  The current runs through the battery and its like there is a resistor sitting inside the battery connected inside  to positive terminal. You can only get at the terminal after the resistor. That is called the internal resistance. So Lets say the internal resistance is 2 ohm . The current is 2 A, the nominal battery voltage is 10 Volts. Power in therefore = 10 volts * 2 amps == 20 W

The voltage lost in the internal resistance of the battery  is found from the equation  V_lost= I*R_internal_resistance;
so V_lost = 2 amps * 2 ohms == 4 volts.

Therefore the voltage now seen by the motor is only 6 volts. Power out is 6V * 2A == 12 Watts

Now lets say the internal resistance is doubled to 4 ohms ( higher voltage battery) voltage is doubled to 20 volts and current is halved to 1A. Input power is same == 20 W

V_lost = 1 A * 4 ohms   == 4 Volts

so we now have voltage seen by motor is 16 volts. Power out is 16 V * 1 A = 16 Watts.

In both cases total power used is the same but the motor gets much more of it in the second case, as its running at lower current.

That siad many motors are designed to run at low voltage. Putting a higher voltage causes too much current to flow and saturets the magnets causing motor to overheat.

If you have a low powerd plane its prob fine on 3S. Only starts to be important when you draw a lot of current! But then you need some way to get 12V so you have to start using voltage regulators etc..

regards
Andy

Andy

The internal resistance of a cell is so low as to be negligible... its almost considered as being next to zero.
if it was 2 ohms per cell then a 4 cell pack would be 8 ohms so if you shorted it out there would only be 1.85 amps flowing,

In my honest opinion more cells would increase the pack resistance minimally, I will agree that less volts means more amps so the circuit resistance comprising of pack, esc and motor will generate more heat and make it less efficient to degree.

Ray

[skyscraper]

l resistance of a cell is so low as to be negligible... its almost considered as being next to zero.
[...]

Lipo internal resistance is prob of the order of a few milliOhms. May seem negligible but will drop voltage proportional to current. E.g With 40A through bat an internal resistance of 0.025 ohms drops 1 Volts. etc. which that feels about the correct order of magnitude to me.

You can see the effect if you have Voltage on your OSD. Turn full power on and watch battery voltage drop. The only thing( apart from lead resistance which is pretty small) causing that is battery internal resistance. In fact if you have a current sensor you can calculate the intenal resistance.

The higher internal resistance in example was just to simplify calculations!  :)

Lipo internal resistance also increases with age apparently, so replacing your old 3S with new 4S will give astounding results!

regards
Andy

[RaySky]

RaySky- I (think) understand that the airframe determines the thrust required and motor weight. But my issue is there seems to be several different options to achieve the same task.

Perhaps I'm over thinking this. Maybe it's as much of an art as a science? That's the impression you gave me RaySky anyway.

Please take the above apart guys and educate me :)

Billy

It is so difficult to work with pure number crunching and science regarding best prop/esc/motor and pack... not to mention airframe aero dynamic/lift and coefficient drag blah blah blah lol
It would drive us all nuts so what I do is use an online Rc motor calc to get me somewhere near and try props changing pitch and diameter either side of the size and see if the craft performs better or worse.

The best piece of kit to help getting a sweet blend of motor and prop is a watts meter,
If you know the max amps for a motor and use that as the limit then you are 90% the way there,

Andy

Too True  ;) We are both singing from the same sheet  ;) but I have to say that some motors wont take a 4S voltage so swopping from a 3S to 4S needs a change of prop for an already installed motor and even then it may not fly any faster or better and may not climb as well as it did on 3S with a larger prop.

So to answer the original question of "Why would anyone use a 3S pack?" the answer is... Its the original designers concept or opinion as a starting point to suit general criteria of motor and prop :D

Ray