Hey guys, check out this chart I found. You just plug in your setup and it charts what your performance should be. Pretty cool.

http://www.qcda.com/emaxx/speedchart.asp

If you increase the number of cells, the motor will draw more current (ie. shorter run times). In other words voltage is proportional to current. Another interesting fact is if you double voltage, power will quadruple: So adding an extra 6 cells will almost double power.

i dont think thats to acurate.....

I just set up my 8xl & 9920 w/ 12 IB 4200s. It is really fast!! I will be fine with 12 for a while - I will probably increase some time down the road. If you have any weak points in your build, this setup will find them!

maxxdude;

Current is opposite proportional to voltage.

With 6 cells more on 12 cells, you won't double the power either. It will be 50% more.With more cells, comes more runtime as well, if you keep running on the same speed as with the lower amount of cells.

Serum, let me show you some formulas:

V=IR ; potential difference (voltage drop) = current x resistance (a constant)

Hence voltage is directly proportional to current.

P=VI ; power (watts) = current x voltage

If we re-arrange the first formula to give ; I=V/R , and substitute this into the second forumula we get;

P = V x V/R or P = V^2R .

Again say R is a constant, if voltage doubles, power will quadruple. This happens because as V doubles, I doubles, and P=VI, so P quadruples.


So if 18 cells are used rather than 12, this will give a ratio of 1.5:1.

1.5 x 1.5 = 2.25, so theoretically power will increase by 125%.

Hope this clears things up


*PS. To answer your last thing, adding more cells will decrease runtime, because more voltage = more current. This can be combated by a higher turn motor that will draw less current at a given voltage (in other words its resistance is higher), however the sacrifice will be power.
In theory a 7XL on 20cells will be more powerful than a 12XL on 20cells. OF course all this is providing the batteries can provide the current.

I am sorry, but 12 cells can do 1200 watts and 18 cells can do 1800 watts. (that's no 100% boost, but 50)

And if you need 1200 watts on 12 volts, it is 100A and on 18volts that is 66.67A, (P=U.I) and like i said, if you use the same power, runtime WILL be larger, not smaller. Adding cells gives you more energy, so runtime will increase. I know this law of Ohm, but in this, you forget a few things, the cells can deliver a maxximum amount of power each. this will not increase if you add an extra cell.

That is quite true, and why I said at the end of my post, that it all depends on whether the batteries can supply the power.

That is incorrect, if you increase voltage you increase current - its proven scientific fact. Its not a matter of 'if you need 1200watts', motors do not work like that. I could spend hours explaining why, but its a Sunday and I can't be bothered.

I'm happy to agree to disagree :)

I won't mind clearing one thing up; (even though it's sunday)

from the beginning i stated 'if the same power is needed' (with that i meant the power on the wheels) so, knowing this, you simlple have to admit that a larger number of cells will give you MORE runtime (not to mention the setups efficiency will increase on a higher voltage)

If you want to run 50 mph on 12 cells, your batteries will be empty FASTER than with 18 cells going 50 mph. Got it?

You have got your theory right, but you need to translate it back to the limitations of RC, (which includes the fact that the batteries simple don't do a 225 percent increase going from going from 12 to 18) Power isn't unlimited in these, so you are not right if you say that 6 cells will add 125% of power. It just isn't right period.

My goal was to disagree with that, open your eyes and see.. ;)

No worries though, i still can drink a beer or two in the pub with you... :p (as long the beer is not hand warm...)

Fair play :dft003:

I take it that when you say you running a higher voltage you also mean a higher turn motor. I think thats what I didnt quite realise. I thought you meant on the same motor.

LOL..

Brushless motors do work like that..

Charming thing this Brushless is...Actually i did meant on the same motor... like i said, if you need a certain amount of power to go 50 mph, the amount of power to go 50 mph on more cells will be the same, so with that, you get more runtime.. (you don't need to use the entire power range)

Have a good sunday buddy!

I think we are both thinking on two different wavelengths. What I said is correct on a scientific scale, but what you have said is more on the practical scale - and hence more appropriate for rc's. There is no denying that, providing everything else remains constant (gearing, amount of throttle applied, temperature) a motor will draw more current at a higher voltage than at a lower voltage.
The beauty of running at higher voltages is you can gear down, and therefore reduce the amount of current the motor will need to draw - which is effectively what you said, and I fully agree with.

Good talking with you Serum, and I hope we have reached an agreement.

Yeah, sure.. We are not too stubern to see eachothers right.. ;) And heck.. It is a nice sunday..

What's up with that smiley flapping his tongue out?!

:032: