A thermo-responsive device for testing dc voltage sources, particularly small, low voltage dry cells, employs a very thin resistive heater film deposited onto a thin flexible dielectric substrate. During testing the ends of the resistive film engage the pole terminals of the battery. Current from the battery flows through the resistive film and uniformly heats the substrate by resistance heating, causing a series of thermo-responsive elements mounted on the substrate to undergo visual "temperature events" at progressively higher temperatures. The resistive film is uniform in thickness and uniform in sheet resistivity. The current density and resistive heating of the film is therefore uniform throughout the substrate and the thermo-responsive elements. The uniform heating permits the thermo-responsive elements to be mounted anywhere on the substrate. The length of the resistive heater may be varied to vary the heating current and the heating temperature. The atomic thickness of the resistive film increases the crease resistance of the film and reduces the effects of bending fatigue.