Joe, what you are asking is at least partially possible.
If you read the voltage of most batteries, that will tell you how much it is charged. For example a car battery will read about 14.4 Volts when fully charged (yeah, even though they are called 12 V batteries). If the battery voltage falls towards 12, it's pretty much dead and won't deliver much more power. A "D" battery will normally be about 1.5 and is considered dead as it approached 1.0. Of course, the actual numbers depend on the battery itself.
However, the fact that a battery is for examle 80% charged doesn't give a clue as to how Long it will last. If it isn't being used at all, it will last a while. If it's being drained by something attached to it, the remaining life (in time) depends on how hard it is being drained. If you have a fully charged 1000mAH battery and are only pulling 1mA, it will last a Long time. If you are pulling 3000mA, you have less than 20 minutes.
One doesn't usually measure Generator capacity (total ampere of the Generator), one measures the amps of the load. Even though a Generator may be able to produce a theoretical 10 Amps at 12 Volts, at any given Point in time it may only be truly "producing" 1 Amp if that is how much is needed to power the load.
Provide more Details of what you are attempting to do.