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My own experience of replacing a smartphone battery
Continuous charge cycles of a phone battery will deplete is useful life.
The more you charge and discharge a battery, eventually the battery will not be able to hold as much charge as before.
Modern smartphones now have the battery inside the case and is designed to be “non-removable”.
However depending on the model of the phone, these so called non-removable batteries can be swapped quite easily with simple tools.
An app I have found will measure the amount of charge going into your phone's battery and use it to estimate the battery's health.
The app is called Accubattery - https://play.google.com/store/apps/details?id=com.digibites.accubattery&hl=en_GB
If the app shows that your phone's battery is not holding as much charge, you may need to replace the battery to extend the life of the phone.