A good and working battery voltage is approximately 3.7 volts DC(direct current), below that maximum voltage range the phone will not be able to power on.
Checking the battery voltage by an analog multi-meter is so easy, just set it to a DC range 10volts then attach the both test probes to each corresponding polarity, positive probe to positive terminal, then negative probe to negative battery terminal. Check the maximum battery voltage reading 3.7 volts.
A battery voltage lower than 3.7 volts can power on the handsets but it shows Battery Low on a screen.
A battery voltage lower than 2 to zero volts will hardly charge the phone and won't power up the phone.
A drain battery voltage approximately down to zero volts will not be able to accept charge when a charger is being plug-in. A drain battery will be shock by a a higher voltage range from 6 to 12 volts for a quick charging point as possible to avoid damaging the battery, a DC regulated power supply. is the equipment used to shock a drained battery.just connect both probe to each corresponding polarity for a quicker period of time, prolonging it will damage the battery itself. You may ask an experience technician in your neighborhood about this method for assistance and safety guidelines.