Idea of Battery Level Icon
The logo for a "half-charged battery" is a battery on its side. When the battery is full it will have a number of vertical lines to fill up the space inside the battery. As the battery life drains, the lines representing the percentage of battery life are left. When the battery is half-charged, only half of the battery has these vertical lines.
This page shows the different variations of the Battery Level from an Icons8 icon pack. Feel free to resize and recolor the icon, and then download the icon in PNG format for free.