EnOcean CR2032 Battery-backed Temperature Sensor

EnOcean temperature sensors are very power efficient thanks to their miniature solar panel and integrated battery storage. They can work on their own in any well-lit area. However, they might struggle during dark winter days, and will definitely not work as is in completely unlit rooms.

This project aims at adding a CR2032 backup battery to EnOcean STM331 / STM331U / STM333U temperature sensors (variants with helical antenna), which is supported by design and offered by some OEMs. According to the datasheet and user manual, a typical CR2032 backup battery should last ~15 years in complete darkness, and much more if the room is partially lit. In that case, the CR2032 cell will probably die before it runs out of power. See images for calculation details.

For those unfamiliar with EnOcean, you need an EnOcean USB dongle to integrate the sensors with your home automation system, e.g. using Home Assistant EnOcean integration. There's one frequency band per continent, so choose the appropriate dongle and sensor variant depending on your location. Second hand dongles are cheaper.

I used the following components to make this project:

• EnOcean STM331 sensor board (taken from battery-less sensor)
  • The cheapest option is usually to purchase a second hand sensor and take it apart
  • Alternatively, there's the EU board version available at Farnell (+Taxes and handling fees).
• 1.25mm male JST connector (salvaged, but should be this one at aliexpress)
• CR2032 Battery Holder (aliexpress)

Build instructions:

• Print all 3 parts on their flat side, without supports
• Install STM33x board into the case, should hold firmly in place
• Install battery holder into the midframe and solder connector with correct polarity
• Plug connector onto the STM33x board (see image for header pinout)
• Before installing CR2032 battery, check voltage and polarity with a multimeter
  • If wired correctly, your multimeter may display voltage from the internal battery. This is not always the case depending on board revision, so alternatively you can check continuity with pin header legs to make sure the battery slot is wired properly.
• Install midframe into the case, it should clip in place
• Install back cover by sliding it, should hold firmly in place
• Optionally mount on wall using the back cover variant with mounting holes
• Profit! You'll barely ever need to change the battery

Reference material for making this design:

• STM33x user manual for board dimensions, header pinout, power specs
• Avidsen and NodOn case designs for inspiration
• Energizer CR2032 datasheet for calculating autonomy