The LCD on my digital thermometer faded away, so I knew that it needed to have the single AAA cell replaced. I bought the indoor/outdoor thermometer probably more than two decades ago from Radio Shack. This time, I removed the AAA cell and I had marked it as installed in Nov of 2005, so it has been working with this cell for more than ten years.
I guess I can’t complain. Most of these come with one or two button cells, so they last for a year or two before needing replacement. There is plenty of room for the AAA cell, there is no need to reduce the size of the battery. The alkaline button cells may be cheaper than a AAA cell, but I doubt they are cheaper than a zinc carbon cell, which should last five years or more.
The LCD draws very little current, tens of microamps. But the thermal sensors have to have enough current to measure their resistance. Fortunately the temperature changes very slowly, so a measurement can be taken for a small fraction of a second every several tens of seconds, which means the duty cycle might be less than 1 percent, and the average current is very low. The digital circuit itself is doing very little, so it also can have a very low duty cycle.
This same effect is what allows a crystal controlled clock movement to tick off the seconds for more than a year using a single AA cell. Intermittent operation makes the duty cycle very low.
