Laptop battery liftime from Dell specs?

"Only if you know the power consumption of the laptop. The rate at which it uses the charge in the cells.

And presumably, as the number of cells changes, the output voltage is regulated by electronics in the laptop, or there are series/parallel connections so the voltage for the circuity stays the same. Circuitry likes that.

You would need to know what current is consumed, at what voltage, so you can divide the Wh (Watt hours) by voltage and current to give the number of hours.

It's obviously directly proportional to the Wh figure. More money, more hours.

My interpretation is that 4 cells gives 24 Wh in total."