Originally Posted by Aros
My confusion is that I would think 4S trumps 3S any day of the week on this setup, even at a lower C rating...
Here's how i understand the whole battery thing -- and how to compute what's sufficient for your setup -- (anyone else please step in and correct me if I say something wrong).
Your "C-rating" is an indicator of how rapidly you can either charge or discharge your battery without damaging it. If you have a 2200mAh battery and want to charge it at 1C, you charge it at 2.2A. If you want to charge it at 2C, you would set your charger for 4.4A.
The discharge works the same way... a 2200mAh battery, discharging at 20C, is capable of supplying 44A. A 2200mAh battery, discharging at 40C, is capable of supplying 88A.
So, with your batteries, a 3300mAh battery that's rated for 30C can supply 99Amps... and when you are pulling more than 99A out of the battery, you run the risk of hitting the LVC on your ESC and/or damaging the battery. With your 4000mAh battery that's rated for 45C, you could theoretically run a load of 180A on that without damaging the battery. The hobbypartz battery that you linked to lists a max discharge rate of 160A (4000mAh x 40C = 160A).
So... to know what battery you should be using, you need to know how much power your particular ESC/motor combo is pulling. If you need to supply 120A, you could choose some combination of capacity (mAh) and C-rating that multiplies together to meet/exceed 120A.
Hope that helps clear things up...