I just started having this problem -- kernel_task is using around 250% CPU. Can it be due to running the computer with no battery at all? Surprisingly, I don't hear the fan going.
I took my 2011 MacBook Pro to an authorized service provider to get the battery replaced (the computer was shutting down at 30%). They didn't have a replacement battery, but the tech noticed that the battery was swolen, and wouldn't put it back in because he said it could start leaking and damage the computer. He said it would be OK to run on AC.
I did an SMC reset yesterday (before removing the battery today -- I was on chat with Apple Support and they recommended it when we were doing initial trouble-shooting of the battery depletion problem).
Should I try BadgerMonkey's procedure?