Q: How do the testers on battery packages work?
A: They are composed of two layers of special inks, one that heats up when it conducts electricity and a second that changes color when it is heated. Usually, the color changes from black to transparent. If the battery has charge in it, a current flows through the first layer of ink and warms the second, making it become transparent and revealing the design (printed with conventional ink) underneath. The conductive layer is tapered so that the entire strip can be heated only when the battery is fully charged.