Comment by mardifoufs

Batteries can technically be used until their voltage is 0 ( it would be hard to get any current under >1 volt for lithium cells but still). The cutoff is either due to the BMS (battery management system) cutting off power to protect the cells from permanent damage or because the voltage is just too low to power the device (but in that case there's still voltage).

Running lithium cells under 2.7v leads to permanent damage. But, I'm sure laptops have a built-in safety margin and can cut off power to the system selectively. That's why you can still usually see some electronics powered (red battery light, flashing low battery on a screen, etc) even after you "run out" of battery.

I've never designed a laptop battery pack, but from my experience in battery packs in general, you always try to keep safety/sensing/logging electronics powered even after a low voltage cutoff.

Even in very cheap commodity packs that are device agnostic, the basic internal electronics of the battery itself always keep themselves powered even after cutting off any power output. Laptops have the advantage of a purpose built battery and BMS so they can have a very fine grained power management even at low voltages.