My father never trashes any old batteries because he knows that combined with another used one, a few weeks later, it can juice up the wall clock any such device. I know its not recommended, but bottom line is that people DO it! So if you can’t break them, may as well join them; provide them with a Digital Measuring Paper. It features soft OLED screen and paper-thin IC that displays the amount of backup juice in a cell. At least two like-powered batteries can be used together more effectively.
Designer: Wang Gang