ATI Radeon HD 4650 poor performance in battery mode

I have an AHTEC Sense XHLB2 (a relabeled version of the Compal HLB2) that has a VGA ATI Radeon HD 4650.

When I switch off the power plug and run into battery mode the perfomance of the laptop is severely affected. After running some benchmarks I found that the VGA results are something between 35% and 200% slower.

How can I avoid the change of the VGA performance even If this makes my battery life shorter?

EDIT: Changed typo from 6450 to 4650.

ATI graphical cards on laptops are relying on the PowerPlay "technology".

It adjusts the frequencies from the GPU and its memory, to save power. You can deactivate this option in the Catalyst control center which should be installed with your drivers:

If you deactivate it simply, the card will be on maximum performance all the time. You can also enable it, and choose yourself which performance level should be used depending on the power status (if I remember good, there is also an intermediate step between max and min, on ATI power management, which could be eventually enough for some programs, and would allow to save power compared to the maximum, in battery mode).