TimothyGao,
I know this is an old thread, but I think a lot of people may be seeing this issue and have this question so I am adding my 2 cents for the knowledgebase.
I have the same issue and the same question. I know for sure that the battery is related, although I am not sure if it is 100% of the cause. I have a 15" 2012 rmbp and never had any issues until about 6 months ago. The initial symptom was that kernel_task would jump to 100+ % (my record was 3500%) when I was using my built in display and also had multiple displays connected via thunderbolt. If I closed the macbook lid or unplugged one display the kernel_task would go back to normal levels. Eventually this started happening more frequently during normal use with no external displays connected.
I reset my smc and pram many times.
I deleted the plist the controls the power management (faulty fan config bug).
I forced the GPU into low graphics mode always (turn off graphics switching and hack some plist file -forgot which one)
I did a clean install of Yosemite.
I turned off chrome plugins loading automatically
Eventually I realized that the issue seemed to be related to heat. The kernel task spins up CPU cycles to stop heat generating processes from maxing out the CPU and overheating the macbook. About the same time I also realized that the problems seemed to start happening about the time my battery reached 1000 cycles. I have still not replaced the battery so I am not sure it will fix the issue completely. I do know that when the power is not plugged in the kernel_task seems to stay relatively low on the CPU usage list. My battery only lasts about 30 minutes now on low usage so I keep it plugged in all the time. When the kernel_task process pegs my CPU I unplug the power and the process drops back down to normal.
I will replace the battery sometime last week and report back here to say if it resolved the problem.
