Xiaomi Humidity and Temperature Sensor 2 Charging Back Cover 001

I've never quite understood why so many Mijia products insist on using button cell batteries or AA/AAA batteries. Especially the Xiaomi Humidity and Temperature Sensor, which originally uses button cell batteries. While simple, its battery life and replacement are not convenient enough. Many people have tried converting them into rechargeable versions, but either they use external AA batteries or are directly converted to direct power supply, making the overall volume too large, which I personally find unattractive.

It wasn't until I found a 3V charge-discharge module on Pinduoduo that I discovered a relatively ideal modification solution. The internal circuit is very simple: the lithium battery connects to the charge-discharge board, the charge-discharge board connects to a dummy button cell battery, and finally, it connects to the humidity and temperature sensor.

This project references and thanks the following two authors for their resource support:

• Xiaomi Humidity and Temperature Sensor 2 Back Cover by 柯西伴我行
• 2032 Button Cell Battery Modification by hellow

Currently, there are two issues:

1. Heating Issue: The circuit board generates noticeable heat during charging, and it's near the battery. Although no abnormalities have occurred during current use, there's still a potential risk. Furthermore, heat might interfere with the accuracy of temperature detection. Fortunately, the humidity and temperature sensor charges very infrequently (for example, mine hasn't been charged since early June).
2. Insufficient Casing Design: At the time, the focus was mainly on adjusting the internal dimensions, and the exterior design was rushed. The aesthetic effect of the printed finished product was also not ideal. The specific reason might be related to printing parameters or the model itself, but as a novice, I haven't found the cause.

Overall, although this modification still has room for improvement, it has achieved the goal of lithium battery charging while maintaining a compact and aesthetic form factor. I hope this attempt can provide some inspiration to friends who are also interested in optimizing energy for small devices. All suggestions for modification are welcome!