Replace the battery when the unit does not turn on, display does not illuminate, or the display goes blank when attempting to power the laser. Use a good quality 3-volt lithium battery (CR2). Insert into the compartment with the negative (flat) end first (positive end facing out towards the battery cover).

Your initial battery should last for a few months. If you start to experience any issues with the performance, replace the battery with a high quality (Panasonic, Duracell, or Energizer), CR-2 3 Volt Lithium battery that is right out of the package.