Battery theory is a theory for xenomorph metabolism. In the films and most canon material outside the cinema, we rarely see xenomorph’s consume humans as food sources, do nor do we see them drink water. They also defy a particular rule for being an organism in the traditional sense in that they do not seem to need an oxygen intake, as they are capable of doing physical tasks like leaping, pouncing and manoeuvring with great speed and precision while inside of a vacuum. There are plenty of scenes of them ‘breathing’ but air does not need to be a particular need for them. Battery theory suggests that xenomorph’s do not eat, drink or require oxygen because all of their metabolic energy is acquired from an internal battery, similar to a sodium-sulphur battery, a battery with high energy density, high charge/discharge efficiency and long life cycles, which seems to match the xenomorphs’ physical behaviour (rarely seem to tire, unknown lifespan, but can survive up to 10 weeks without any food or water). Hives and rogue nests are important as when the xenomorph ‘sleeps’ in its ‘wall pit’ it is recharging itself.
Likewise, their ‘acidic’ blood may be a chemical of importance to their metabolism, much like some car and automotive batteries make use of sulfuric acid, making their blood not just an excellent defensive measure but also a helpful means of staying active.