Some of you familiar with my garage thread may recall that I’m using SmartThings to automate things. The door sensors in the garage see cold temps (-35C) and therefore their 3V lithium cells do not last long at all (like 4 months) in the cold weather. The other issue is that when they “die” the sensor just stops detecting, with no notication per se.
With my pool control project, I was able to power wireless temp sensors with a solar cell/rechargeable battery project, so I figured I’d try this out with the SmartThings sensors.
This is the older setup that powers three Fibaro door sensors that have external temperature probes attached. The three fibaro FGK-10x sensors and 3 AA cells are shielded from water using a standard outdoor electrical box. The lithium cells in the sensor (3.6 Volt) are removed, and + - leads soldered to the battery connectors inside each FGK-10x. I found reference to the 1st generation Fibaro door/window sensor (zwave) that has a connection block to allow use of an external temperature probe (DSB1820), so this is why they are being used to control my pool solar heating system.
This setup has run 100% reliable outside all year (with temps as low as -35C) since July 2019 with zero issues, resets, etc. They have always reported “low battery” but have remained powered up since installation.
Now the SmartThings multifunction sensors (I think these are gen2) use a 3 Volt lithium cell (CR-2450), so I figured 2 x AA batteries would work fine. These sensors work with an external magnet to detect open/close events but they also integrate temperature sensors and accelerometers as well. The first test was trying the sensor with 2 AA celles in series (2.9 volts) which work perfectly. They also powered up fine directly from the 5 volt solar cell in full sunlight. Once you connect the AA cells and solar panel, the voltage is maintained pretty much at 2.9 volts as this panel is only .5 watts@5V. In other words, with these very low power Zigbee sensors, the 5V panel has worked perfectly to keep 3 x AA cells charged…so we’ll see how well we fare with 2 x AA charged by the 5V panel.
The solar cell is a 5 Volt, 0.5 watt I found on Amazon.com. They have a blocking diode installed by the manufacturer, and claim to be waterproof. The older setup pictured above has been outside in the rain for 3 years.
I also ordered some holders for 2 AA cells, with an on/off switch on Amazon.
I’m just using Amazon Basics NIMH rechargeable AA batteries.
Bench testing the setup. Basically just connect up the + and - leads, and solder to the sensor. The sensor (with cover removed) is sitting between the AA batteries and solar cell.
Back of the solar cell (as provided by the manufacturer) where the diode is sealed under some silicon. This keeps the solar cell from draining the AA cells at night.
Battery holder (in series) with on/off
Here’s the panel spec.
A pretty cool way to have a wireless (temperature, contact, motion) sensor that never needs batteries and will reliably stay powered up even in very cold temps 
I’ll post a few pics of the latest setup installed. Cost to do this for one sensor is about $8 with just a few solder connections to do.
I have 4 Fibaro door sensors, and 3 Smarthings multipurpose sensors which have stayed powered up and happy through 3 winters (-35C and a lot of snow) using this solar setup.
