>Current IBM supplies are 16V, and old Apple
>supplies are 24V. There seems to be less
>variation from model to model from a single
>manufacturer than their used to be... (about time!)

I suspect this is mostly driven by the available battery technology now. You
need a certain voltage level to be able to charge them efficiently, and the
switching technology is now cheap and efficient enough that getting from the
battery voltage to the required internal voltages has resulted in a common
battery voltage.

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