optimal mAH 18650 cell required for laptop
Yes, but only if you know the capacity of the original cells or you can get a measurement the average current draw.


The basic formula is time = capacity / current. However towards the end of the discharge voltage drops and the laptop may draw more current to maintain a constant power drain. It will probably shut down before the battery is 100% discharged, so in practice you might get 80~90% of the rated capacity (assuming that the rating is accurate!).


No, because 18650 cells with a real capacity of 4000mAh do not exist.


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