This is a circuit of a simple single cell li ion battery chager. This lithium ion battery charger circuit uses a LP2931 regulator IC. The diode is functioning as a current blocker to prevet the current flow back in the IC when there is no voltage on the IC input.The output voltage can be adjuted
with 50k potentiometer between 4.08V to 4.26V. The circuit provides 100mA of charging current. The charging current is 100mA due to which it will take some more time to charge the li ion battery so the circuit is good for using as a overnight charger.