I think you are definitely on the right track here. The duration of the bulb staying lit would be an identifiable quality, as well as the brightness of the bulb. I say that because I recently conducted a lab with my 7th graders that involved building a simple circuit with a small bulb (flashlight bulb) a strip of aluminum foil, and one AA battery. As the lab wore on with different classes, I noticed some of the bulbs weren't lighting up quite as brightly as they did at the beginning of the day. When you complete the circuit, the chemical reaction inside the battery commences, and the longer you have the circuit connected, the more chemicals are depleted within the battery. This will result directly into a dimmer bulb, perhaps even a flickering bulb. These qualities of bulb luminescence could be organized into a chart that could indicate battery quality or strength.