The reason overall battery life can shrink as time goes on is because these power sources often have a particular number of charging cycles in them before they start to lose effectiveness. And every time a device goes from zero to full, that counts as one complete cycle. Eventually, it will hit that limit, and you’ll start to notice that your device may not hold a full charge for as long as it used to.
But if you’ve been leaving your Switch in its dock for extended periods of time, don’t panic. Nintendo has directly stated that the Switch’s battery will not degrade or take damage when the console stays docked past the point of a full charge. This does make sense seeing as the Switch was originally designed as a hybrid console that could work either as a more typical connected-to-the-TV affair or as a portable handheld device. If staying plugged in damaged the battery it would undercut its entire gimmick. Nintendo also offers assurances that plugging in or docking your Switch when there is still some juice left in the battery will count as a “partial” cycle when it hits 100 percent.
It’s unfortunate but inevitable that all of the batteries in our electronic devices will start to dwindle someday. But your Switch won’t suffer any ill effects if you leave it in its dock.