A little scrounging around at home might reveal a flashlight that requires D-cell batteries, a remote control that requires AA or AAA batteries, and maybe even an electronic toy that requires a C-cell. But when was the last time you saw an A-cell or B-cell battery?
Chances are you've never actually seen one of these, but they do exist! (Or at least they did.) You don't see them around anymore because they are obsolete. A-cells and B-cells were used to power the now outmoded vacuum tubes that were the height of audio-visual technology 100 years ago. You could find them in old-fashioned radios, crank telephones, and doorbells.
As electronic devices have become smaller, so have their batteries. When batteries smaller than the A-cell were created, they were given the names AA and AAA. There are even AAAA batteries out there powering (relatively) super-small devices like laser pointers and LED penlights. In fact, some brands of 9-volt batteries are really just six AAAA batteries bundled together.
So what does the future hold for batteries? If A- and B-cell batteries have become obsolete in our past, can we expect the batteries of today to become obsolete in the future?
To a certain degree, yes. C-cell batteries are already becoming unpopular, and as technology keeps shrinking our hi-tech gadgets and gizmos, D-cell batteries are increasingly relegated to an ever-smaller group of electronic devices (mostly to those big flashlights that could double as baseball bats). With the development of newer, cleaner, more efficient ways of producing portable electricity, there may come a time when the lettered, cylindrical batteries of today will be as far from memory as the vacuum tubes of yesteryear.
Cliffs Notes Online