Well, I do have some history for you, on this:
One, the batteries in my flashlight/torch (which are either D size, C, AA or AAA, and god only knows what happened to "B" and single "A".
Well, the Battery history, goes back to the old days of radio, with vaccum tubes, and before there was rectifier tubes.
"A" Battery refered to the low voltage battery that powered the tube filaments. This was often a lead acid wet cell, often between 2V and 6V.
"B" Battery refered to the high voltage battery that powered the tube plates. Usually these ranged from 45 Volts to as high as 108 Volts, with 45V, 67.5V and 90V being most common. If you don't beleive me that batteries of this high of voltage exist, see
www.tubesandmore.com, although they are rare these days.
"C" Batteries refered to smaller batteries used for grid bias. The current draw of these is near-zero, and are used to produce negative voltage used for the grid control of the tubes. These typically range from 4.5V to 18V.