When I bought a 7.4 V Li-Ion battery, little did I know that it would require a special charger for recharging.
While searching for chargers I found a simple circuit by Scott Henion that makes use of an LM317 Adjustable Regulator.
I found another interesting circuit by Bill Bowden which makes use of a 555 timer, LM339 comparator and TL431, band gap voltage reference.
This instructable makes use of Scotts design.
Step 1: List of Components
- LED x 1
- 470 Ohm Resistors x 3
- 47 Ohm Resistor x 1
- 2.2 K Ohm Resistor x 1
- 1 Ohm 5 W Resistor x 1
- 1 K Ohm Trimmer x 1
- 0.1 uF Ceramic Capacitors x 2
- 1N4004 Diode x 1
- LM317 Adjustable Regulator x 1
- 2N2222 NPN transistors x 2
- 7.4 V Li-Ion battery x 1
- 12 V wall DC adapter or Power Supply x 1
Step 2: Schematic
Here's my schematic and Layout designed in Eagle. The eagle files along with the layout as an eps file are available in the file "eagle_files.zip"
For details about the working of the circuit refer to Scotts Article
Step 3: My PCB
I transferred the layout to a copper clad board using the iron transfer method. Then I etched the layout using ferric chloride. The image shows my board after populating it with the components. I attached a heat sink to the LM317. The big white block is the 5 Watt 1 Ohm resistor.
Step 4: Testing the Circuit
- Before inserting the battery to be charged, we need to set the charging limit.
- Power on the circuit by connecting it to a 12 Volt power supply.
- The trimmer is used to set the output voltage limit. To set the limit, measure the voltage at the output (without connecting the battery) and adjust the trimmer R4 till the measured voltage equals the limit. I set my circuits limit to 7.2 Volts.
- Once the limit is set we can attach the battery to be charged.
- While the battery has a low charge the LED will glow brightly.
- Once the battery is sufficiently charged the LED will become dim and might turn off. The battery can then be disconnected from the circuit.
For more detailed instructions/modifications and safety instructions see Scott Henions article