What are the formulas to use when determing a brushless setup?

[Finnster]

NP Gee.. I may go back and edit it a bit more to clarify a few things. It can be hard to explain thigns clearly and concisely.

To add to what Linc said, I too would get as high as C rating as I can get. However, I would not design the system to require a really high C rating unless I was being really tight on weight (for racing or max performance, etc.) and was ok with shorter runtimes.

For your example: is a 25C 3300mah batt good enough?
Ok, you are talking a 4S setup
W=V*A => (4S*3.5v/cell)*(3.3Ah*25C)= 14V*82.5A=1155W

You can expect that batt of delivering ~1200W reliably. Now is that enough power for your truck? Is it a 10lb E-Revo?
=> 1200W/10lb=120W/lb.
Sounds a bit light to me. So you will have to settle for low speed, or torturing your lipo by forcing more amps out of it than its happy making.

I tried to express some methodology in a way that can be adapted to any application.
Take that same batt, and say we want it in a 12S setup for a massive 45mph 17lb Cen GST.
(12s*3.5v/cell)*(3.3*25)=42V*82.5A=3465W

Sounds not bad, but good enough..? what about runtime?
We want 45mph out of it, its an extra large truck with large tires and heavy drivetrain.. lets say we need 200W/lb* to do what we want, and make sure we have some headroom. (*This is a totally made up number, I'm guessing more accurate figures are lower. I'm just taking these from the above post.)

200W/lb*17lb=3400W

Oh.. right on the edge then of what we want. This setup would require 25C batts then. We can prolly predict an avg runtime.
Say we wanted a longer runtime, screw the weight. We said our motor needs 82.5A, so if we use a factor of 20 instead of 25, where previously we figured:
--> 3.3Ah= (required amps, ie 82.5A)/25C
we instead say: X Ah= 82.5A/20C=4.1Ah
So 12S of ~4Ah batts should give us a much longer runtime.
Note: I am not saying don't buy 25C batts and you have to use 20Cs, we're just using relative rules of thumb.

At the end of the day, we can be pretty confident of a setup that can acheive at least 45mph for a decent runtime and not get too hot and overstress the batts. Maybe this will do 50+mph in the end, but that's fine. Better to aim too high than low. If you put the batts together and the wieght seems too much, scale down a bit, but be prepared to maybe drop the speed targets a bit or stress the batts a bit more.

You never really know until its all put together and running, but at least its some way to put you in the ballpark of what you want w/o buying something way over/underpowered or overly $ for what you need.