For example, this one looks really nice, but it only has 'battery' and 'battery-charging'. In an app I work on, I need battery with no charge, 25%, 50%, 75%, 100%, and charging.
Then I think that this would be small to add, but again I don't want to complain. Then maybe someone will need a battery with a sliding charge, so they can represent as many charge percentages as the pixels allow. And then this becomes a whole other things, and not just an icon collection..