Simple lithium-ion/lithium polymer battery charger

Simple lithium-ion/lithium polymer battery charger

This is a lithium-ion charger for lithium polymer batteries. The circuit schematic shows a configuration for charging a single 3.7V lithium polymer battery, but the voltage can be set to charge multiple batteries connected in series. The lithium polymer charger uses an LM317 and a resistor to set the current limit, and a TL431 and two resistors to set the voltage limit. A 1kΩ resistor is needed to allow the TL431 to reduce the current of the LM317 and provide the necessary current to the TL431. An optional Schottky diode at the output will prevent current from flowing back from the battery to the charger circuit when the power is off and will prevent reverse bias of the LM317. A power supply with an output voltage higher than 3V is required.

This is a constant current/constant voltage charger, which means that in constant current mode it can generate 1.3A of current, and when the voltage reaches the constant voltage, it will reduce the current.

Component:

1x LM317

1x TL431 - 0.5%

1 x 6.8kΩ 1% resistor

1 x 10kΩ 1% resistor

1 x 1kΩ resistor

One current sensing resistor was used; for this 1.3A current, two 0.47Ω 3W resistors were connected in series.