This Macbook Pro (15" circa 2012) is giving me trouble. After a slight water spill incident, I removed and replaced the hard drive, optical drive and battery (warning sticker ignored). After another day drying I turned it on. However, the battery is not recognised, everything's happening slower and the mouse is really jumpy. I've also noticed the CPU load from the Activity Monitor remains steady at 80%-90% for the System (kernel_task). I'd like to reset the SMC to see what happens, but it doesn't seem to be resetting. The PRAM/NVRAM reset is working, just not the SMC reset (I'm using shift-control-option + power). Is there another way to do the SMC reset? Maybe open it up and poke around some more?
In the Apple Support article on SMC reset it reads "On MagSafe power adapters, the LED might change states or temporarily turn off when you reset the SMC.". Other than that SMC reset does nothing visible. So probably the reset is working.
It seems like the battery is dead. In that case the Mac will throttle the CPU to reduce power consumption. This explains also the high load. Since the battery is necessary to buffers peak loads, the MacBook relies on it to operate properly.
You must replace your battery.