How to choose an Electronic Speed controller?
I'm having a go at making my self a quad-copter and was wondering how one would go about choosing an ESC?


Your ESC (Electronic Speed Controller) and motor are of the brushless DC type (which is the norm for modern RC models).
 Key parameters for the ESC are   


the ability to switch the maximum current required by the motor
(or from several motors if more than one is used on the same controller.)    
the ability to withstand the voltage from the chosen battery. 


Some ESCs may have a lower battery voltage limit as well as an upper one.
 Other features of relevance which may be offered can include   


Braking control (using motor shorting to produce regenerative braking),   
Maximum motor current limiting and   
Low battery cutoff capability.
Low battery cutoff is vital to ensure reasonable battery life. 



  I've chosen this motor and this ESC,
   Is the ESC suitable for that motor? 



The chosen ESC will work with the selected motor if a single motor is used per ESC.
Up to 2 or these motors can be safely run in parallel by this ESC.
3 would be marginal. 
Use 2 x LiIon cells or 6 x NimH or NiCd cells.




Specifications given are about zero on the seller's page.
 Fortunately there is a manual of sorts here

Your motorhas key specs for this purpose of


2~3S Li-ion battery  
Internal resistance: 0.52 ohm  
Max effective current: 2~6A, >70%  
Max working voltage: 8.5V  


It's somewhat disturbing that they say "3s LiIon" but 8.5V max as 3 x LiIon can provide 12+ Volts. You'll need to ensure the ESC is programmed to not exceed motor ratings. Motor effective current is 6A so if you allow 10A to be safe the ESC's 25A peak is usefully higher.



The ESC spec below says 25A, and the chart suggests 2 x LiIon or 3 x LiIon are OK - so that suits the motor battery wise and is very comfortable current wise.
 The 18A rated version would be adequate - only useful if it's significantly cheaper as size and weight are the same. 


A look through the mini-manual does not seem to indicate that peak current or voltage are controllable so it sounds like 2 x LiIon max is the limit for this motor. 

If using NimH or NicD cells then 6 cells max would be normal (6 x 1.2V = 7.2V nominal) but 7 cells could perhaps be used (7 x 1.2 = 8.4V), although this would exceed motor rating for a short while when fully charged. Caveat Emptor / 6 cells is fine.