"low on battery part isn't quite true"
But when you computer is not plugged in, the GPU will most likely downclock itself to reduce power usage. Some HDDs will have power saving options (spin down when not in use). As for CPUs, Intel CPUs have "Intel speed step" (except for some celerons) which will adjust clock speed accordingly depends on your usage (plugged in or not).
There are softwares you can use to limit your CPU speed when you are on the go so you can conserve battery life. The screen also sucks up alot of power, so dimming the screen will give you a nice chunk of battery life in return.