Generally, a 3.7v lithium battery needs a “protection board” for overcharge and overdischarge. If the battery does not have a protection board, it can only use a charging voltage of about 4.2v, because the ideal full charge voltage of a lithium battery is 4.2v, and the voltage exceeds 4.2v. Damage to the battery, while charging in this way, it is necessary to monitor the condition of the battery at all times.
If there is a protective board, you can use 5v (4.8 to 5.2 can be used), the USB5v of the computer or the 5v charger of the mobile phone can be used.
For a 3.7V battery, the charge cut-off voltage is 4.2V, and the discharge cut-off voltage is 3.0V. Therefore, when the open circuit voltage of the battery is lower than 3.6V, it should be able to charge. It is best to use the 4.2V constant voltage charging mode, so you don’t need to pay attention to the charging time. Charging with 5V is easy to overcharge and cause danger.

1. Float charge. Refers to charging while working online. This method is often used in backup power supply occasions. If it is lower than 12 volts, it cannot be charged, and if it is too high, it will affect the operation of the circuit. Therefore, when the floating charge works, the voltage is 13.8 volts.

2. Cycle charging. Refers to fully charging the battery to restore the capacity. When fully charged, the charger is not disconnected for measurement. Generally, it is around 14.5 volts, and the maximum does not exceed 14.9 volts. After disconnecting the charger for 24 hours, it is generally around 13 volts to 13.5 volts. About 12.8 to 12.9 volts after a week. The specific voltage value of different batteries is different.

The usual lithium battery cell is 3.7v, the voltage is 4.2v when fully charged, the nominal voltage after series connection is only 7.4v, 11.1v, 14.8v… the corresponding full voltage (that is, the no-load output voltage of the charger) is 8.4v, 12.6v, 16.8v… cannot be 12v integers, just like the interval of lead-acid storage battery is 2v, full is 2.4v, correspondingly only the nominal 6v, 12v, 24v… full voltage (The same is the output voltage of the charger) respectively 7.2v, 14.4v, 28.8v… I don’t know what kind of lithium battery you are?
The output of the charger is generally 5V, and 4.9 volts is also a non-standard. If you want to use this charger to charge the battery directly, it will definitely not work, but as long as it is charged by a mobile phone or a dock, it has a control circuit inside. It will be limited within the allowable range of the lithium battery, unless the circuit is damaged, don’t worry about this
The usual lithium battery cell is 3.7v, the voltage is 4.2v when fully charged, the nominal voltage after series connection is only 7.4v, 11.1v, 14.8v… the corresponding full voltage (that is, the no-load output voltage of the charger) is 8.4v, 12.6v, 16.8v… cannot be 12v integers, just like the interval of lead-acid storage battery is 2v, full is 2.4v, correspondingly only the nominal 6v, 12v, 24v… full voltage (The same is the output voltage of the charger) respectively 7.2v, 14.4v, 28.8v… I don’t know what kind of lithium battery you are?