That’s according to Battery University who claim if your phone is kept charging after reaching capacity, the battery’s chemistry could damage it.
This means it would be in a constant ‘high-stress’ state. They argue it’s actually better to never fully charge your smartphone.
They recommend you do it at intervals as this extends the Lithium-ion (Li-ion) battery’s lifespan. These batteries are common in phones, including iPhones.
‘Li-ion does not need to be fully charged, nor is it desirable to do so,’ they wrote.
‘In fact, it is better not to fully charge, because a high voltage stresses the battery.’
You can read their full argument here.