Sure, coin cells usually last a long time — but do you really want to buy new ones and throw the old ones out? The LiR2032 coin cell is a rechargeable lithium battery, for which you can build a charger at around $1.
The 5 minute hack starts with a TP4056 lithium charging circuit, which is a great DIY board designed to charge high-capacity cells at about 1A. Luckily, it is pretty easy to modify the board to charge lower capacity batteries. It’s just a matter of replacing resistor R4, and a little bit of soldering!
The official TP4056 datasheet even tells you the range of resistors to use (page 3). [Tom] went below that range and found a 33KΩ will result in about a 50mA output — this results in a full (and safe) charge of the LiR2032 in just over an hour.
To finish up the hack he soldered a piece of strip board to the charging header, and attached a coin cell holder. Not bad for a buck. Now what if you added a few resistors and a dip switch too? It’d be pretty easy to make an all in one lithium charger!