There are many myths and facts on various discusion forums about laptop batteries and other electronics devices which operates on rechargeable batteries. The raising question is that should we use laptop plugged-in all the time or on battery. The answer is not straight forward and depends on situation and you can conclude by knowing battery functionality.
Now a days LiON batteries are mostly used in notebooks and laptops. These batteries are also called smart batteries because they have internal protection circuit which helps protect them from damage and manage battery energy and usage. When the laptop battery is charged 100%, battery protection circuit stops taking power from adapter and bypassed the adapter power to laptop internal processing. If you think the battery is taking power after it is fully charged then it’s untrue. So running laptop plugged-in after the battery is fully charged has no effect on battery but the heat produced by the laptop internal circuitry and processing can have effect on battery to some extend.
Even, most manufacturers suggest leaving laptop plugged-in all the time. All batteries have their limited number of charging and discharging cycles. Simply, more you charge and discharge lesser the lifespan of battery. Most batteries works fine for about one year on regular use on adapter and battery. But you shouldn’t leave the battery without use for a long period of time. If you use your laptop plugged-in all the time you should discharge laptop battery once or twice in a month.
The only factor which can affect LiON battery is heat. Heat produced by laptop internal processing affect the battery to some extend. You can check it yourself or some already know that when we charge the phone using power adapter, phone becomes hot at the back where the battery is installed during the charging. But when battery is fully charged and stops charging, phone temperature goes normal. This concludes two things first, more you drained and recharged, more their internal structure warps and they eventually becomes unusable and second, it shows that there’s no charge coming to battery after it is fully charged that means, power source has no effect on the battery whether the device runs on adapter or on battery.
So it’s all about heating. If you are facing that your laptop becomes hot while using it plugged-in, you shouldn’t use it plugged in all the time because excess heating can affect the battery.