I've searched for an answer for this question for a long time. Specifically I've found this thread which is closely related to my own question: Why are batteries measured in ampere-hours but electricity usage measured in kilowatt-hours?
Here is my question: I would expect that the electrical energy that a battery produces would be constant, but according to tyblu's answer to the thread above, the charge is the one that's approximately constants. It seems odd to me, because it seems to violate the law of conversation of energy, because if a battery's charge is constant, but its voltage output decreases over time, then the electrical energy must decrease too, because E=V*Q.
What am I missing?