Intro: Cheap LIR2032 Coin Cell Charger
The LIR2032 is a rechargeable lithium coin cell battery. I use them in small robots.
For charging I hacked a lithium battery charger circuit board with a TP4056 chip.
They are available on the Internet for a very low price.
These circuit boards are designed to charge cells at 1A, this is way too much for the LIR2032.
Luckily they are easy to modify and obtain a different charging current.
In the TP4056 datasheet you can find that R4 is the resistor which determines the charging current. On a new purchased board, you can identify the resistor by it's marking 122 (R4 = 1.2K)
A save charging current for the LIR2032 is 35mA.
Also in the datasheet you can find a formula to calculate the replacement resistor for R4:
Ibat=(Vprog/Rprog)*1200 (Vprog = 1V)
So if you use a 33K resistor for R4 you get about 36mA, that should be good. (resistor marking: 333 or 3302)
I changed the resistor, mine was 0603, that's tiny! But this board is also available in different designs with 0805 components, making it a little easier.
As a last step i soldered a battery holder on a piece of prototype board. You can make this how you want, but keep the polarity in mind.
Now you can plug in the mini usb.
When charging, a red LED lights up.
And when the battery is fully charged, a green or blue LED will light up.