[KenWittlief]

I think the application being described is a battery charge meter. The best way to do this is to measure the current being supplied by the battery along with the voltage. Think of it more as an electron counter. For any given capacity battery you can 'pump' a fixed number of electrons into it. If you then keep track of the current it is supplying while being used you have a pretty good idea of how much power the battery has left.

This is very tricky stuff if you want to do it well. Some of the things that make it complicated are:

1. The voltage across the battery terminals is a function of both the amount of charge remaining and the current being drawn at that instant. The more current you draw the lower the voltage across the terminals. When you stop drawing current the voltage will go back up! So a simple voltmeter is not enough to tell the whole story.

2. The temperature of the battery affects its capacity. At -20F a lead acid battery looses half its capacity. Depending on the current level the battery will heat itself up to some degree. This makes it very difficult to measure or calculate how much energy remains in the battery.

3. Every time you charge and discharge a battery its voltage curves drop slightly. As the battery gets older its capacity drops, and the voltage at each point of its discharge curve will be lower. This also makes it difficult to know how much energy is remaining.

As you might have guessed by now, a good battery charge 'meter' is not a trivial design. In fact, for things like cell phones and laptop computers a lot of design effort goes into that little icon on the corner of the screen to be able to accurately say "you have 29% remaining".