This little tool does nothing but measure how long last the main battery of your device.

You can run the test measuring how long take the battery level to go down for a few % and based on that time the programs calculates how long will take to go from 100% to 0 considering that the battery life vs time is a lineal function. This consideration is not real, that function is not lineal so if you need the real value in minutes the best thing to do is to select a % bigger 95 %.