I’m thinking of things like clocks, answering machines, and smoke detectors. Considering that the backup power source is used very infrequently, and usually not for long at that, why do they bother using such a beefy battery? Wouldn’t a AA or AAA fare nearly as well?
AA and AAA batteries are only 1.5v, not enough to operate most TTL circuits. To use them would require multiple cells in most cases which is more cumbersome and expensive in the long run.
Because they’re 9 volts in a relatively small package. It would take 6 AA’s to yield 9 volts. Of course the amphere-hour rating is smaller, but you don’t need long life for back up.
Peace,
mangeorge
For backup power, the objective is small size, small volume, convenience and simplicity; it is not electrical efficiency or mA-hours. It is very simple to integrate a 9-volt battery into a circuit as a back-up power supply, since cheap linear regulators and/or diodes can be used to get the voltage you want (6V, 5V, etc.). With a 1.5 V cell, on the other hand, you have two ugly options: 1) Use multiple cells in series to get the voltage you want, or 2) use a DC-to-DC converter to get the voltage you want.
#1 consumes too much volume and weight. #2 is simply too costly, and makes for a more complex circuit.
9V batteries cannot output high amperage. They can’t power motors or LCD screens or even incandescent bulbs. But they can output a steady 9 volts at low amps for a very long time (compared to 1½ volt cells) which is why they’re used as memory backup power.