The Arduino mini pro (8MHz) operates at 3.3V and has a 10-bit ADC, i.e., a read on an analog pin can return a value from 0 to 1023. Each value corresponds to 3.3/1024 volts or 3.22 mV. If you’re running Arduino on a battery, then you would want to get an idea of the remaining level of the battery. One way to do this, is to use a voltage divider (you don’t want the voltage input to the analog pin to be greater than 3.3 V, hence the need for the divider) to provide about half of the VCC as input to the analog pin. I use a 3.6V battery, and based on a voltage divider with two 1.2KΩ resistors, the voltage associated with each of the 1024 point values is 3.27 mV. This value can then be used to get a reasonably accurate estimate of the current battery voltage level. For instance, if the value of the analog pin is 544, then 544*0.00327*2 will give you the current battery level.