You're probably slowly killing your phone.
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.