Battery life: longest; extending??

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Hi All,

What's presently the best battery life you can get with a notebook,
using a screen no smaller than 15"? (Models that allow using 2
batteries might be of interest, too.)

Besides reducing speed and brightness, and wireless conections - if
you don't need them - what else can you do to extend how much time you
can use the notebook without having to look for a power plug?

Extra batteries: proprietary versus universal, what the best and the
worst of each, in your experience?

Thank you,

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