compaq mini 704ea on battery power stops without warning after an hour

When using my compaq mini 704ea on battery power the machine stops without warning after an hour - i.e. the power goes off with the result that the screen goes black, the lights go out, the fan and disk stop. This seems wrong on two counts - one hour seems too short, and surely I should get a battery low indication, Indeed the battery status was way better than 50% a half hour into my session.
Is this a fault, or am I expected to make a special system configuration to get the expected 3 hours life - if so what?
TIA