I have been using my old laptop (Dell Vostro 1400 with a 9 cell battery) in my home for the last 4 years continuously connected to AC power with battery in the laptop. Sometimes I check the battery without AC power and I notice, the battery still have enough power to last 4 hours.
The latest Lithium Ion batteries have intelligent technology (digital circuits) built-in and in conjunction with digitally controlled power supplies, do not get overcharged so easily. Once the battery gets fully charged, the charging current into the battery will be reduced to a trickle to keep the battery floating (means not charged/discharged).
This is similar to our mobile phones with Li-Ion batteries when connected to the chargers, show red color LED while charging turns to green when fully charged.
The laptop battery over a period of time tends to lose the charge, and when this happens, the digital circuits inside the battery senses the reduced battery voltage and again increases the charging current from AC power supply. This is how the charging cycle continues.
So the Li-Ion battery connected to the laptop always keeps fully charged and ready to supply power when power supply is disconnected.
So to answer your question, the automatic disconnection after fully charging and start charging after discharging is already built-in to the modern laptop batteries and power supplies.
You need to worry for the battery only when it gets heated up inside the laptop. The heat buid-up in the battery will damage the electronic circuits controlling the charging cycle. This will cause the battery to get overcharged and damaged.
As long as the battery is properly ventilated and kept cool there is nothing to worry.
If your battery becomes old and does not retain charge, it is time to replace the battery. Like everything in this world .... even the batteries have a limited lifespan .... Follow these tips and maximise your laptop battery's life .... Long Live Your Laptop Battery!