USB Automatic Li-Ion Battery Charger Circuit

In this article we study a simple USB powered Li-Ion battery charger circuit having all the necessary features such as constant voltage, constant current, and overcharge cut-off with indication.


The circuit can be understood with the help of the following description:

As shown in the following automatic 3.7V USB li-ion automatic battery charger circuit, the IC 741 is configured as a standard comparator and becomes the over charge cut-off stage in conjunction with the transistor 2N2907.

It's non-inverting input is used as the trip voltage sensing input while the inverting input is clamped with a fixed reference voltage of 1.8V via the three 1N4148 diodes.

The 22k resistor is added for including some hysteresis in the circuit so that the cut-off action sustains the condition for a while and prevents threshold level oscillation across the battery.

The transistor 2N2907 basically does the power switching implementation in response to the triggering received from the opamp.

Another PNP device which is installed over the 2N2907 forms the current regulator device here. The resistor across the emitter/base of this transistor is selected such that a potential of around 0.6V develops at the emitter/base of the BC557 in case the current consumption of the Li-ion cell tends to rise above 200mA.

When this happens the BC557 triggers instantaneously and chokes the 2N2907 base by supplying a direct positive to it.

The operation momentarily stops the supply to the cell prompting a quick drop in the current level, this tends to restore the voltage condition back to the battery, as soon as this happens the current consumption yet again rises activating  the transistor. A continues switching is thus triggered keeping the current level in control at the set 200mA mark.

The diode at the output positive line drops about 0.6V from the source 5V, ensuring a steady 4.4V to the connected Li-Ion cell.

The continuously illuminated LED indicates charging process in progress, while a slow flashing of the LED could mean the battery has been fully charged and needs to be removed.

The flashing rate may be altered by altering the 22k resistor value, increasing its value produces faster flashing and vice versa.


Circuit diagram of USB Li-Ion Battery Charger Circuit with Auto Cut-off