If you have to extend cables for brushless motors, extend the motor cables, not the battery cables. If that's not possible, you have to add extra capacitors to the controller. 

• Schulze controllers and battery lead length
  http://www.schulze-elektronik-gmbh.de/tips_e.htm
• Castle Controllers and battery lead length
  http://www.rcgroups.com/forums/showpost.php?p=1537846&postcount=28
  http://www.rcgroups.com/forums/showpost.php?p=4833040&postcount=5
  http://www.rcgroups.com/forums/showpost.php?p=1531363&postcount=25
• Bob Boucher (Astrobob, http://www.astroflight.com) on long battery leads
  http://www.rcgroups.com/forums/showpost.php?p=2988042&postcount=32
• As a rule of thumb, for every 4inch/10cm extra length/distance between battery and ESC, add an extra 220uF capacity (electrolytic condensators, voltage the same as the capacitors already installed, low ESR type) (Ludwich Retzbach, German e-flight author, the 'R' in LRK).
• How to add extra capacitors (English and German)
  http://www.schulze-elektronik-gmbh.de/guide/gfutc-de.pdf
• In depth discussion
  http://www.rcgroups.com/forums/showthread.php?p=1518827#post1518827
  

[size=1]Vriendelijke groeten Ron van Sommeren
• int. electric fly-in, Nijmegen, the Netherlands
• brushless motor building tips & tricks
• diy brushless motor discussion group
• Drive Calculator download & discussion group[/size]

Great. i am building a twin with two 2000w brushless motors. was going to put ECS in Nacelles behind motor and run battery wires from fuselage to Nacelles through the wing, about 30 inch. I will now run the motor cables in wing instead.
Thanks

Just make sure you twist long motor wires a bit, they might pick up noise.