Disclaimer: that’s an overkill solution! And people having a lot of battery operated devices may smile…
As part of my home automation, I have a FIBARO FGMS001 Z-Wave motion sensor that controls a lamp in my lobby.
I was concerned by the fact that this sensor was powered by a non-rechargeable battery (3V model CR123A), so I investigated how to move to another source.
Solar cell was not an option with the place where this sensor is positioned.
However, I do have an intermittent AC supply very nearby, as the sensor is in fact hanged just below the lamp. So, each time the lamp is lit, there is mains power just around.
The idea was therefore to use that to charge a Lithium-ion battery during time where the light is on and uses that battery to power the sensor the rest of the time.
I tested supercapacitor, but without quick success and abandoned. I did not find the right setup that could give me power for several hours, even days in some cases.
Finally, I used an overkill solution: a 2000 mAH 18650 Lithium-ion battery cell that is controlled by a very well known module to charge it: it is often named 03962A and use a TP4056 IC.
Downstream, there is a cheap 3.3 – 6 V to 3.3 V converter that feeds the sensor. By the way, the sensor is now powered with 3.3 V instead of 3 V. I think the internal IC is designed to operate at up to this voltage as most of this kind of ICs, but did not check. It’s a voluntarily choice, I could have taken a 3V converter (although less common), but I thought this small 10 % increase will boost RF power, so increasing range a little bit.
For powering the battery controller module, I took the components from an unused mobile phone charger laying around, removing the plugs and the case.
Just a little remark: normally the TP4056 based module is not supposed to feed the output load while charging the battery. The doc says that it could fool the circuit in thinking that the battery is never charged as the output is drawing current. However, I considered the FIBARO sensor load being negligible in that case.
Everything has been soldered on a piece of ProtoBoard:
I made a 3D print case:
On the case, I let a little hole to glue a piece of transparent plastic sheet to have some visibility on the charger LED.
Another picture showing the board inserted in the case and the battery:
The FGMS001 Sensor is connected with a wire directly soldered on the original battery holder:
And here is the solution in place:
As a conclusion, this setup is overkill in several aspects, size and weight compared to the original sensor, price which nearly double the price of the original sensor, amount of power vs the need, but it works, and I just wanted to share it.
However, having so much margin in the power budget allows me now to set very low interval reports in the sensor settings which has some advantages.