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This is obviously an objective criteria, so the mods are always right. Or maybe mildly right? Ahh.. what do we know?
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Picture of a disassembled Duracell 9v battery. Below the terminal assembly is a clear plastic case where you can see six sets of stacked rectangular terminals and fillings.
All 9v batteries are just a collection of smaller cells.
That's the case for anything above 4 V, right?
I had an old 12v power tool battery die, so I took it apart to find 8 generic AA rechargeables wired together. I suspect lots of batteries are multiples of 1.5v (9/12/18) because they're just stacked smaller cells that are already mass produced.
Battery chemistry produces fixed voltages depending on what you use. It depends on where the active components sit on the electronegativity table.
The typical ones are:
Zinc-carbon and alkaline - 1.5 volts per cell.
Lead acid - 2 volts
Nickel Cadmium - 1.2 volts
Nickel Metal Hydride - 1.4 ish.
All the Lithium ion combos - 3.4 to 3.7 volts.
Some are also 6x little tiny aaaa batteries
I actually took it apart hoping to find the AAAAs.
Cheap low-capacity 9V batteries are still 6 AAAA cells. The flat cells allow higher capacity in the same space, so you find them in the batteries that advertise themselves as long-lasting.