Converting a lithium battery charging board into a constant voltage and current charger

The author bought a lithium battery charging board and modified it into a charger with adjustable output voltage and output current.

The circuit is shown in the attached figure: This lithium battery charging board was originally used for charging 7.2V lithium batteries in mobile phones from car power. The charging process is constant current charging first, constant voltage slow charging, and finally constant voltage floating charging. According to its principle, the power transformer, rectifier filter circuit, resistors R17~R21, W1, voltmeter, ammeter and other components are added to make it a charger with output voltage of DC2V~DC15V and output current of 100mA, 200mA, 500mA, and 1.5A.

The output voltage of this charger after rectification and filtering can be selected between DC18V and DC36V. The output voltage is adjusted by W1 and can vary within the range of DC2V to DC15V. If the output voltage is to exceed 15V, the withstand voltage value of C4 needs to be increased.

The output current is controlled by K2. If the output current is greater than 1.5A, the heat sink area of ​​T1 needs to be increased and the wire diameter of L1 needs to be thickened. The difficulty of the modification is to make resistors: R18, R19, R20, and R21. The author learned through inquiry that the resistance of φ0.13mm enameled wire is 1322Ω/km; the resistance of φ0.21mm enameled wire is 506Ω/km. After calculating the length of enameled wire required for the above resistors, it is wound on a cylindrical insulator and the length of the enameled wire is trimmed according to the output current during debugging.

After the above modification, this charger can not only charge various specifications of batteries with constant voltage and constant current, but also be used as a DC power supply.