A flashlight includes a light powered by a battery, and a circuit having: a memory for storing a battery life information and voltage output information versus time for the battery to power the light, a controller operating a count down timer to accumulate an amount of time that the battery powers the light, a voltage measure circuit for monitoring the voltage output by the battery while the battery powers the light and supply the voltage output to the controller. The controller determines: a first remaining battery life by comparing the accumulated time to the battery life, a second remaining battery life by comparing the voltage output to the voltage output information, and the lesser of the first and second remaining battery lives. The flashlight includes a display for receiving a command from the controller to display the lesser of the first remaining battery life and the second remaining battery life.