Why are 12V, 24V and 48V widely used defaults?

Why are 12V, 24V and 48V widely used defaults?

These systems were defined ages ago through lead acid batteries (also referred to as LA, SLA for Sealed LA, or PbA). A lead acid cell has a nominal voltage of 2V, six of them in series makes a 12V battery, twelve make a 24V battery and 24 make a 48V battery. 36V does exists, but is less common. There is also 96V but is usually only in reference to server backup units.
With the introduction and the widespread use of other chemistries, notably lithium based cells, this poses some challenges because their nominal voltage doesn't always fit the existing 12/24/48V systems.

Standard Lithium Cobalt is 3.7V nominal. So adding 3 together is 11.1V, and 4 is 14.8V.
Lithium Iron Phosphate is 3.2V nominal. 4 would be 12.8V, so these work great for LA replacement.

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