If you have a Jensen Sprinkler system you likely have an Irritrol rain sensor installed. The batteries are in the device mounted on your gutter.
To change the battery in your sensor:
1) Removed the bottom cover of the sensor by pressing down on the thumb-release lever on the side of the unit. Carefully slide the circuit board out.
2) Remove the battery cover and replace the two 3V CR2032 batteries. Install with the positive side of the batteries facing up.
3) Reassemble unit in reverse order.