Part of any self-respecting Smart Home, smart relays are useful for switching and monitoring loads that do not plug into an outlet. This also makes them a lot more integrated, and thus, a long lifespan is very welcome. Unfortunately, the popular Shelly 2.5 smart relays seem to be having a bit of a design flaw as they’re dying in droves once their 2-year warranty period is up. The cause and repair are covered in a recent [VoltLog] video on YouTube.

As noted in the Shelly documentation for the device, it’s a very compact form factor device, with screw terminals, two relays, and three fairly large electrolytic capacitors sharing very little space with the rest of the components. The apparent flaw comes in the form of these capacitors failing, with the video showing that one 100 µF capacitor has a massively increased ESR, likely due to electrolyte venting. This results in the observed symptoms, such as WiFi connectivity issues and audible hissing, the latter of which is demonstrated in the video.

Due to the cramped space, the replacement capacitors need to be at least as small as listed in the video and in the top screenshot, though mind the typo as ‘400µF’ has to be ‘100µF’. This limitation posed a bit of a problem, as for the two 400V, 4.7 µF capacitors, there aren’t that many options in that form factor. The original capacitors are definitely B- or C-grade ones, with the two large capacitors Chongx branded, being a well-known budget capacitor brand. The other capacitor’s branding cannot be made out in the video, but is likely also Chongx or a similar, less well-regarded Chinese brand.

For the replacements, a Nippon Chemicon capacitor was picked for the 100 µF capacitor, and Ymin-branded capacitors to fit within the size limitations. Picking Ymin over a second Nippon Chemicon set or similar was due to these unfortunate sizing limitations, but these Ymin replacement capacitors had the best datasheet of the options on LCSC. All of these capacitors have to be rated for 105°C, for obvious reasons.

Although it’s not easy to say for certain what caused these capacitors to fail so quickly without more data, it seems likely that having the SMPS circuitry for the 3.3V rail bunched up cozily with the three electrolytic capacitors and what looks like two load resistors inside the cramped enclosure with no clear ventilation holes does little to help the electrolytic capacitors hit their listed MTBF hours. Hopefully, using the new capacitors, these relays will last longer than 2-3 years before another recapping is needed.