windows 7 - Laptop Running Slower Without Battery Attached

I have a Thinkpad T520 and I'm monitoring my CPU using CPU-Z.

For some reason, when I detach the battery, the CPU only run till 800MHz. Whereas when I run using battery power, it goes up to 2.3GHz. I notice the difference in speed when I take the battery out. This is the reason why I investigated in the first place.

Any ideas on what's going on?

Specs:
Windows 7
Thinkpad T520
6GB DDR3
Core i5-2410M