Yes . if u always keep your laptop in charging mod then your battery performance will decreased.

<p class="MsoNormal" style="margin-bottom: 0.0001pt; line-height: normal; background-image: initial; background-position: initial; background-size: initial; background-repeat: initial; background-attachment: initial; background-origin: initial; background-clip: initial;">There is not much of a direct harm that can cause
to your laptop if you charge or plug-in always though what can cause damage is
increased temperature. When you plug in the laptop for all day, it might
increase the temperature of the laptop and eventually shorten the battery life.<o:p></o:p>

<p class="MsoNormal" style="margin-bottom: 0.0001pt; line-height: normal; background-image: initial; background-position: initial; background-size: initial; background-repeat: initial; background-attachment: initial; background-origin: initial; background-clip: initial;">So, in case you notice that the temperature of your
battery have heated up quite a much, you can put it in a water-tight bag, in a
fridge, not a freezer, a fridge for a while. It will run it down to 40% of the
temperature. Also if you are using, Lithium-ion or Li-ion battery, it should
not be frequently discharged fully and recharged, so you should charge them
regularly.<o:p></o:p>

If a laptop is plugged in and charging always can it cause any harm to the battery or laptop