Laptop 'power' lights convey a couple of different states with a single light. Besides the typical on/off, there's also the 'sleep' fadeup/fadedown sequence.
One could also add a few blinking states. Slow blink = charge, fast blink = problem, two quick blinks = alert, etc.
One could also add a few blinking states. Slow blink = charge, fast blink = problem, two quick blinks = alert, etc.