Eli5: You can use AAA batteries for electronics that require AA batteries, as long as you fill the space with something that conducts the power. So why does both battery types need to exist? Why not just have one size fits all?
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Size of the component it’s going into plays a large part in the choice of battery by the designer. As for the batteries themselves I’m under the impression that double a batteries will last a little bit longer than a double a used as in the above mentioned fashion. Essentially they have a little bit more juice to them even though they’re output is the same
If not mistaken I believe there are other battery types like c and d for example that also use the same output voltage but obviously hold much more energy
If one were so inclined you could even use a c or a d battery in AA or even triple a device if you were willing to run external wiring out to the battery itself since it obviously would not fit into the battery slot but the outputs would still match up same theory as your you know bridging the material to the terminal just doing it in a much more roundabout way
There is more to batteries than voltage.
The various common batteries like AA, AAA, C, and D cells all generate about 1.5 volts. But they also have different amounts of the chemicals inside them so they last different amounts of time. It’s often measured in milliamp hours, or mAh. The exact amount of usable energy depends on how quickly it is drained, how much is being drained at a time, and more.
A single AAA cell might have about 500 mAh at one level of drain, a single AA cell might have about 2000 mAh at the same level of drain. A single C cell might be rated at about 8000 mAh, and a D cell might have 15000. You can look up specific batteries from manufacturers and their estimated energy capacity at various drain rates.
So if you put a AAA battery into a device expecting a AA battery, it will probably last about a quarter of the time. If you were to wire in a giant D cell, it might last seven times as long as normal.
Battery capacity – Because AA batteries have more battery capacity than a AAA battery. Filling the empty space with some conductive material with AAA batteries would mean this device will run out of juice faster.
Device size – If a device is small, it may be not be practical to allow a AA battery as opposed to a AAA.
In addition to the other answers, while you could potentially use multiple AAAs to replace one AA you’re adding additional points of failure and complication to the device. One battery means one set or connectors. Multiple batteries means multiple connectors, aka more complicated or having to track multiple batteries together, any one of which could fail and cause issues. The right approach depends on the device, how it’s used, power demands, etc.
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