This Battery Level icon is made in Material Design style developed by Google. This style is based on extremely simplified shapes, thin two-pixel lines, and is optimized for 24x24 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 4,300 icons, all in the same style and quality.
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.