Low Current LIPO Cell Charger for My Alert Button Project

This project is designed to charge a small 100mAh 3.7v LIPO cell at a low 21mA approx. constant current until it reaches 4.2V

My Alert Button project uses a small type 601220 100mAh 3.7V LIPO cell which measures about 22 x 12 x 6 mm

*** WARNING *** LIPO batteries may overheat or catch fire and cause damage or injury if shorted, over-charged, over-discharged, connected incorrectly or damaged in any way!

Do not attempt this project if you are unsure or inexperienced in working with surface-mount components or LIPO batteries!

This project uses mostly SMD components.

1. LTC4054ES5-4.2 (LTH7R) SMD LIPO charger IC (from Aliexpress)
2. 47K SMD (1206) resistor
3. 1K SMD (1206) resistor
4. 1N5819 (SS14) SMD 40V 1A Schottky diode
5. 1uF SMD (1206) Ceramic Capacitor 16V working
6. 0.1A SMD fuse (resettable) 60V working
7. 3mm Red LED
8. USB plug and lead (For supply power)
9. JST PH 1x02_P2.00mm Vertical sockets and plugs X 2 (for power and battery connections)
10. M2*8mm screws X 2
11. Small custom two-layer PCB (20 x15mm)
12. PLA filament for 3D printed case

The main component is a LTC4054ES5-4.2 SMD LIPO charger IC

The charge current is programmed by the value of resistor R2.

The chosen 47K value provides approx. 21mA charge current.

During charging, LED D2 is lit until the cell reaches 4.2V when charging stops and the LED turns off.

I designed the small 20 x 15mm PCB using KiCad and had it manufactured by JLCPCB

The PCB Gerber Zip files can be downloaded from my website.

The SMD parts are carefully soldered first. then the JST sockets and LED.

Double check polarities of all components, The diode D1 has a cathode white bar marking which should be near the bottom edge of the PCB as shown.

The LED D2 cathode (shortest lead or flat side) should be on the right (nearest the IC (U1))

Clean off flux residue using flux-remover spray and examine the PCB with a magnifying glass - look for any solder splashes( particularly around the IC).

It must be tested BEFORE connecting to a LIPO cell.

Connect socket J1 to a 5V DC regulated supply or a USB socket and the LED should light.

Measure the output voltage at socket J2 - it should be 4.2V.

Now switch your meter to the current mA range and measure the current across the output, it should be about 6mA.

(This is because the charger has detected that a LIPO cell is either not connected or it's voltage is below 2.9v and automatically switched to "trickle charge mode"

If the voltage and current readings are correct, check polarity and plug-in a LIPO cell and it should start charging.

The LED should light until the LIPO reaches 4.2V volts, fully charged.

The charger PCB should be cased to protect it.

I designed the case using RS Designspark Mechanical. The top and pcb are held in place with two M2 8mm long screws.

Paper labels can be glued on to the lid show the correct connections.