High fans, slowness, and no battery detection

It sounds like you are facing a few different issues here. The first is the battery may need replacement the second is you have a sensor which is preventing the system from working at full clocking. SMC will put the system in CPU Safe Mode when it senses a thermal or voltage problem.

Lets run diagnostics. Restart your system holding the D key do you get an error message with an error code let us know what you discover.

To check the battery install this gem of an app CoconutBattery take a snapshot of the main window and post it here for us to see Adding images to an existing question