A thermochromic apparatus for testing dc voltage sources, particularly small, low voltage disposable batteries, employs a very thin resistive heater layer deposited onto a flexible dielectric substrate. A series of thermochromic elements are "stacked" (mounted on top of one another) at a single thermochromic site proximate the resistive heater. During testing current from the battery flows through the resistive layer heats the substrate, causing a series of thermochromic elements mounted on the substrate to undergo visual "temperature events" (such as color changes) at progressively higher temperatures. The thermochromics may be stacked in a series of discrete films, each film containing a dye for one temperature event. Alternatively, the thermochromics may be contained in microcapsules in a common film.