1 Charging: 24V should be used for charging.
Need to pay attention to the problem, charge to the unit voltage 4.2V needs to stop. If the 24V power supply can be stepped down in series with 3-4 diodes, be sure to select a tube with sufficient current. The charging time should be controlled and should not exceed 5 hours.
For the protection board, you can use the used mobile phone battery. It is recommended to have one battery for each battery. Safety first.
Regularly check if the battery is behind, and charge the backward battery with the universal charge to the same voltage as other batteries.
4 about 6 batteries
It is recommended to connect the voltage regulator circuit. Just like the LM317. The output is set at 20V. Pay attention to the treatment of heat dissipation