So I’ve played some with an amp meter inline for the power to a GTM. The results are not precise, due to the way GTM handles inbound USB power. That is…once the internal battery is fully charged, the charging circuit appears to cut itself off from outside power and run on the internal. I’ve observed this before on various experiments with “always on” supplies…the GTM gets charged, outside power cuts off, battery discharges to some level (haven’t determined that yet, but will try…that’s a painful diligence task), and then the charging circuit kicks back in. I’m sure this provides power-buffering for the innards, but it seems like a lot of charge/discharge cycles for a non-replaceable battery in a long-running relay.
Baseline charging seemed to be right around 27mA…that’s with the GTM off, but needing a top-off charge. Other readings were also made with the battery at 99%+ charge, which wasn’t much fun to do.
OFF (charging): 27mA
Pairing mode: 65mA
So based on the calculations of others (solar folks, mostly), at 5V the nominal power draw should be 18mA, which is 45-27, so expected. The “unpaired” 65-27 gives 38mA, just over twice the paired consumption, which agrees with anecdotal reports. My amp meter was dancing around and never settling on values for all of these, so there’s windage that should be applied to all of them. I’m not convinced there’s a meaningful power consumption difference between paired and relay, unless the paired node gets a lot interaction with the app. I think the value in relay mode is really not having the node grope for pairing when unattended.
Stretch goal was also to measure power consumption with the GTM connected to a Pi over USB, but having the amp meter in the USB power path was one insult too many for the data lines. At least so far. It has been a lot of playing with fairly small wires with old eyes.