I Always Feel Like Somebody’s Watchin’ Me
An engineer discovered his wireless switch's battery drain pattern through Amazon notifications and found temperature to be the culprit.
When the batteries run down in the electronics devices around our home, replacing them is just a simple matter of popping out the old ones, and then inserting a fresh set. Or at least it can be that easy. But for the curious sort of engineer that many Hackster News readers are, it can be a lot more complicated than that.
Electronics hobbyist Chris Greening has a battery-powered wireless switch that, after seven or so years, needed a battery change. But Greening did not find out about this in the usual way. Rather than noticing that the switch no longer worked, Greening instead received an email from Amazon stating that the device’s battery was running low and it was time to order a new one.
Never having received a notification of this sort before, Greening was intrigued and wanted to understand how it all worked. After finding that the Amazon website has a page that tracks battery levels of connected devices, he discovered that the switch was being polled every 15 minutes. With a little work, Greening was able to scrape this data from the website and plug it into a chart.
An interesting pattern immediately emerged. The battery level dropped off significantly every night, then started rising again in the morning. This correlated perfectly with the heating schedule Greening has in place — the heat in the room is turned off at night time. So to see if this was in fact the cause, some experiments were conducted in which a battery’s voltage level was measured before and after being placed in the refrigerator. The cold temperatures did prove to be the culprit.
Not wanting to throw batteries in the trash unnecessarily, Greening considered using a rechargeable coin cell battery in place of the existing CR2450. After tearing down the device and finding that the battery was directly connected to the microcontroller, however, it was determined that this was impossible. When fully charged, the battery sits at 4.2 volts, but the microcontroller can only handle 3.6 volts.
So, what did this knowledge do for Greening? Well, nothing really. The battery still needed to be replaced by the same type it previously used. But it is always interesting to understand how our devices work. Check out the video for a deeper dive into the power management of this smart switch.