Well, percent state of charge (how full the battery pack is) is not so hard to calculate, you just have a look up table for battery voltage vs %SOC. Although yo

Well, percent state of charge (how full the battery pack is) is not so hard to calculate, you just have a look up table for battery voltage vs %SOC. Although you may need to calibrate for the voltage drop caused by the internal resistance of the battery cells. The hard part is converting that into a "miles remaining" estimate, which requires a good estimation of how many kWh your vehicle uses per mile. I think some manufacturers even link this into the GPS and take into account the elevation change on the route you selected.