Leaving it always plugged in normally means that the functional life expectancy of the battery is decreased. That is as far I know the only reason to run your laptop on battery power.
Perhaps if you have a very poorly constructed laptop, the heat from a CPU under (heavy) load can have an effect. Or if you don't clean out dust, hair and fluff (for those "smart" enough to use laptops on blankets or carpeted rooms) from your laptops at regular intervals. Once a year, actually open your laptop and be amazed about how much cruft has found its way into your device. I would even suggest to use a very small air compressor or canned air to blow out the vents on your laptop every 6 months.
Even if you think you are in clean spaces, a laptop always find ways to attract much more dust than you think it does. And as a laptop has hardly any room to disperse heat generated by CPU, RAM or hard disk, keeping it clean means that your laptop won't throttle or even turns itself off to prevent damage caused by a build-up of excessive heat. Whatever that source may be.
Keeping Lithium-based battery technology within the 20%-80% range of charge is supposed to extend the functional life of the battery. Some batteries in laptops have technology on-board which adjusts the way how it is charged, depending on the amount of charge it contains. Some laptops have similar on-board tricks, but you won't find either in low-end laptops and it not even a given in mid-range laptops.
Links to software that shows battery status for windows 10:http://www.makeuseof...laptop-battery-life/http://www.windowsce...ry-report-windows-10http://www.windowsce...-windows-10-settingshttps://batterybarpro.com/https://www.microsof...y-level/9wzdncrfhmwnhttp://www.thewindow...software-windows-8-7http://www.c-sharpco...-windows-10-uwp-app/
I have no experience with any of the software suggested at those links, mainly because I think that any type of computing on any laptop is a below-par experience at best. Having owned one, operated several others and repaired even more, I speak from experience.
However, if you must own or work on one, be nice to it and always use one of these: http://www.laptopmag.../best-laptop-coolershttps://www.amazon.c...ng-Pad/dp/B000NWIOM6https://www.amazon.c...&node=2243862011https://www.newegg.c...s/SubCategory/ID-319
These will keep the laptop cooler and reduces the build-up of cruft. Now that will extend the functional life of the laptop as a whole. Not nonsense about keeping a laptop plugged in or not.