If the battery is removable, I would suggest using it for #BOINC (boinc.berkeley.edu). BOINC enables you to offer your spare computational capacity to scientific research, there are projects in all areas of science from open source drug discovery to finding pulsars to math and chemistry. Android tablets are more efficient-per-watt-of-compute than any desktop or laptop will be, even if the total output isn't huge. Not all BOINC project support Android, but you can consult the project list to find those who do. I believe #SiDock, #WorldCommunityGrid, and #Einsteinathome do.