detecting life in batteries

[wkethman]

I want to be able to determine the life left in a batter. I am not sure if it is best to know current or voltage and if there is a way to predict how a motor will react. Say if you want to operate a vehicle over a certain speed but when the batteries are close to now being able to support that speed a warning is given or a system shutoff instead of dropping below that speed. the speed the motors can't go under would be the set point but you have to predict when its going to go below that speed so using maybe a voltage or current sensor woudl be best. I am really unsure about this so if i said something wrong dont hold me to it. please list specific sensors or link to them. does need to sense anything above 24 v but maybe a safe band so 32v or something. thanks guys