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.
Related to Battery Level Icon
Related icons are the icons with matching tags, as well as all mobile icons. All icons are in the same flat style. You can either download free PNG icons or buy SVG vectors