They use coulomb counting, along with an estimate of how much safely-usable energy is left in the battery. That estimate is usually accurate, but it can drift

They use coulomb counting, along with an estimate of how much safely-usable energy is left in the battery. That estimate is usually accurate, but it can drift a bit over time if you never fully recharge or discharge the battery. Also, when the battery gets old it starts having trouble delivering full current when mostly discharged, which is why you'll sometimes see shutdowns at 5-15% remaining.