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Surprise! The "built-in battery" is actually a completely removable standard Nikon en-el11 battery. But what's more interesting is it's pretty low capacity - even Nikon makes a (slightly) higher capacity en-el11 ( https://www.nikonusa.com/en/Nikon-Products/Product/Batteries/EN-EL11-Rechargeable-Battery.html )

I found this kastar one that's almost 1.75x higher capacity.