Comment by voussoir

Sure, it must be possible. You just need to give the right voltage to the +/- pins, and most phone batteries have at least one or two additional pins which you'll have to reverse engineer. Usually a thermistor for temperature reporting, which you'd replace with a fixed resistor / voltage divider to report a constant temperature.

Apparently some manufacturers put the phone's NFC coil inside the battery instead of on the phone's back housing, so there could be a fourth pin for that. Or perhaps the battery contains its own microprocessor that answers a DRM challenge from the phone specifically for the purpose of blocking out third party battery manufacturers. Etc. etc.

Still there is the problem that the phone might have peak power usage above what its charger can provide, so you'd have to use a wall adapter with a higher amperage rating.