[morcheeba]

Requiring a battery can be a design decision. Let's say a computer uses 10W on average with a 40W peak for short bursts. If you've got a 20W power supply, it won't be able to supply those bursts. But add in a good battery and it will be able to. Of course, you could just make the power supply twice as powerful, but it will weigh more, be physically bigger, and become less efficient.

[somewheres]

Let's be honest, those 40W spikes are due to video playback and/or other GPU bound tasks. Sorry, wait for the battery on those. But booting the OS should be unconditionally permissible on AC power.

So design the OS bootstrap to never spike consumption past the load capacity of the power supply. Once booted, deny access to processes that will slam power consumption, until you have a sufficiently energized battery.

Hey, presto, a weightless solution solved in software without any additional added mass!