If you try to charge the battery yourself while it is still connected in the phone then you might damage the battery and the phone.
A rechargeable Lithium battery cell is 4.2V when fully charged and when its charging current has automatically dropped to a low amount. Then it MUST be disconnected from the charger circuit. It is 3.2V when its load MUST be disconnected. Then its average voltage during a discharge is 3.7V. Some newer Lithium battery cells (Lithium-ferrous something) have a little less voltage. A Lithium battery cell has a maximum and recommended charging current stated by its manufacturer.
You can learn about charging a Lithium battery cell yourself at
www.batteryuniversity.com and on datasheets of battery charger ICs from semiconductor manufacturers.