EPS: The Electric Power System (EPS) board receives current from the solar panels to charge the battery.  The battery then supplies current to two power supplies that power the satellite.  The EPS system monitors the currents and voltages, both of itself and of the battery, and has the ability to power each of the other boards on and off.  Right after launch, EPS is in charge, making sure to wait 45 minutes (to be a safe distance from the ISS) and making sure the battery is charged before booting up the rest of the satellite.  Once C&DH is booted up, it takes over control of the satellite.