¿full cpuspeed?

Hi all i have a Dell inspiron 8500 Notebook that have SpeedStep
when i am on AC power i just love my notebook, but when i gen on Battery
i can't even see a dvd & and my X run verry slow..

is there and way to force fbsd to use 'hw.acpi.cpu.current_speed=8' when i am 
on battery.. by the way 'hw.acpi.cpu.current_speed' is read only.. 

-- sysctl --

--AC Power--
hw.acpi.cpu.max_speed: 8
hw.acpi.cpu.current_speed: 8
hw.acpi.cpu.performance_speed: 8
hw.acpi.cpu.economy_speed: 4

--Battery Power--
hw.acpi.cpu.max_speed: 8
hw.acpi.cpu.current_speed: 4
hw.acpi.cpu.performance_speed: 8
hw.acpi.cpu.economy_speed: 4