Haha , sorry about the wall of text there . Yes you would need a device with both a current sensor and voltage sensor , as well as some sort of display to show you the info . Measuring current values at fast intervals , dividing them by the time interval between each measurement sample rate and adding up those values is how the amount of mAh consumed is calculated although for integer math reasons the calculations may not happen in exactly that order . This is what is occurring in the battery charger when it tells you how many mAh it has put back into the pack , and how it is done by minimosd or your flight controller if current sensor data is provided to it . The calculated number may not be exactly precise , but can give you an approximate idea of how much battery is used . Voltage is what will really tell you when a pack is dead and must be changed out . if this is something that will be used for FPV then using a simple OSD to show voltage , current , and mAh consumed is all you need . if its for a handheld system where you ' d want that info displayed on an LCD or something I ' m not aware of any good off the shelf solutions for that . Simply knowing voltage is alright for a guesstimate of how much battery life is left if you know the full voltage and the empty voltage . Here ' s a good e x a m p l equadbatteryreviewtesting 1300 mahbatteriespart 2 of several 3 s lipo discharge curves under constant current . you can see how if its up near 4 vcell it has almost full charge , 3 . 7 Vcell is close to the middle somewhere roughly , as it starts getting close to 3 . 5 Vcell it drops off very quickly to empty .