The publish highlights a very simple circuit which could be employed for charging a cellphone 3.7V battery with the aid of a laptop battery.
Just create a LM317 voltage regulator circuit, regulate its pot for obtaining 4.2V at the output,……..that’s all…now it is possible to connect the 10V to the input of the circuit and the 3.7V cell across the output of the circuit for quick charging it.
The comprehensive circuit of the recommended cellphone battery charger by means of a laptop battery could be researched below:
The demonstrated LED will close of when the 3.7V cell is nearly fully charged.
Caution: Turn off the power to the 3.7V cell the moment the red LED starts shutting off, with the intention to stop the cell from over charging and harm.
For providing better safety you really should set the output voltage to 3.9V, even though this may indicate the cell obtaining charged only upto 80% and not upto the optimal point.