The operating time of a laptop on batteries depends on how many watts the laptop consumes versus how many watts the batteries contain. The average consumption over time is fairly fixed, although the brightness of the screen, especially large ones, does have a notable impact.
As others have addressed, laptops have lithium batteries and to get more operating time you need more power (Watt hours) so you need more or larger capacity batteries. The size of the laptop generally limits the battery size so more power is obtained by using more batteries and generally those batteries are put in series (less circuitry needed (=cheaper) to properly charge when the batteries are in series rather than in parallel) which then results in the raw operating voltage of the laptop. Internal DC/DC converters then take that raw unregulated voltage and produce the regulated low voltages (3.3VDC etc) that the electronics need.
To charge those batteries the internal charging circuit needs an input voltage that is about a volt higher than the fully charged voltage of the lithium batteries. Also the Chinese made external power supply has an output tolerance that is typically +/-5%. It is worth noting that the actual output voltage must be measured at the operating load. It will always be higher with no load because of the IR (current x resistance) drop (loss) in the DC cable and the load regulation of the external power supply which is generally negative a bit.
Power supplies for critical applications have a feature called "Sense" which measures the output voltage at the load or connector and automatically compensates for the IR loss but I've never seen it in an external power supply. (although we are building a custom one for a 5V/80W application for the military because the IR losses are notable with 18A flowing through just a few feet of copper wire)
Factor in all that and with the commonly used 4 lithium batteries in series for "larger" or longer running on batteries laptops and you end up needing a nominal 19VDC external power supply which actually could be any where from about 17 - 20 VDC. The internal DC/DC converters for generating the lower DC voltages and the the battery charging circuitry easily accepts that range plus probably another few more volts. You can test the lower acceptance voltage by using a variable output power supply and turning the voltage down until the "charge light" goes out. However, you'd have to measure that voltage at the connector. Do NOT test the high acceptance voltage as you can easily blow out the DC/DC converters making your laptop kaput and that is generally your only indication that the input voltage is too high.
BTW, the 19VDC is also needed to get watt-hours up for longer run times and the current down in the larger laptops because the ubiquitous barrel connector is only rated to handle 5A - and that is for a really good one. Most are 2-3A. That is the main reason you do not want to be plugging and unplugging that connector when your PC is powered on as you'll burn the contacts eventually making for unreliable contact in that connector.
To learn more about PC connectors, see: https://en.wikipedia.org/wiki/DC_connector
BTW2, PCs also have a battery "gas guage" which tells you how much run time you have left when operating on batteries. That "gauge" has to track power going in and out of the batteries. While they are fairly accurate in real time, they do have errors that accumulate over time and the capacity of the lithium batteries declines with age, operating temperatures, and charge cycles. This often results in your PC "telling" you that you have no run time reamaining and it is going to shut down when, in fact, the battery may still be at 50% capacity, which then causes you to go out and buy a new (and expenssive) battery pack. When that replacement battery is plugged in the PC recognizes that new battery and resets its battery capacity settings. Deep down in (some/many/most?) PCs there is a battery capacity calibration routine. If you can access that, the PC will go through a routine of discharging and recharging the battery a couple times to re-calibrate the battery capacity giving you another year or two on the original battery pack, albeit with declining operating time.