BOINC

Use your phone or tablet to study diseases, predict global warming, or
discover pulsars! BOINC harnesses the unused computing power of your Android
device and runs jobs for scientific research projects. You can choose from
projects in several areas of science, including Yoyo@Home, World Community
Grid, PrimeGrid, Enigma@Home, OProject@Home, theSkyNet POGS, Asteroids@Home,
and Einstein@Home.

BOINC computes only when your device is plugged in and charged, so it won't
run down your battery. It transfers data over WiFi, so it won't use up your
cell phone plan's data limit.