They can. A number of well-built apps pause background data sync if battery is too low.
However, changing behaviour based on battery level is trickier than it seems.
* The point of reducing energy usage is to leave the user more juice for the actions that are most important to them. It could be that your app is the important one.
* Stopping expensive operations at 20% or 30% battery level means that 70-80% was already wasted. If there are good ways of optimising energy use, they should be applied at all times, and prolonging the time until you reach 20%.
* The appropriate threshold depends on what the user imagines doing for the rest of the day. If I'm travelling to another city, I disable background data in the morning to make sure I still have battery left for phone calls in the evening after heavy use of maps during the day. If I'm travelling to the office, it doesn't matter, since a charger is nearby. Predicting "time to next charging" and "use of other apps before charging" automatically for context-aware app behaviour is non-trivial.
However, changing behaviour based on battery level is trickier than it seems.
* The point of reducing energy usage is to leave the user more juice for the actions that are most important to them. It could be that your app is the important one.
* Stopping expensive operations at 20% or 30% battery level means that 70-80% was already wasted. If there are good ways of optimising energy use, they should be applied at all times, and prolonging the time until you reach 20%.
* The appropriate threshold depends on what the user imagines doing for the rest of the day. If I'm travelling to another city, I disable background data in the morning to make sure I still have battery left for phone calls in the evening after heavy use of maps during the day. If I'm travelling to the office, it doesn't matter, since a charger is nearby. Predicting "time to next charging" and "use of other apps before charging" automatically for context-aware app behaviour is non-trivial.