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Some GTM power consumption mucking

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
Paired: 45mA
Relay: 44-45mA

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.



In paired mode, if the paired smartphone is out of range, the LEDs blink continuously until the Bluetooth link is reacquired. While LEDs don’t require a lot of power, 4 or 5 of them blinking brightly eventually adds up.

Measured that. It seems to make no difference whether the GTM was paired and lost contact with the BT partner or was simply unpaired…power draw was the same. I’m sure some is LED blinking, but am guessing more is the extra energy running a constant “lescan” on the Bluetooth radio.

I’ve seen posts suggesting that relay mode is lower power than paired because of reduced (or no) BT power usage. If that’s true, it’s down in the noise for steady state.


I suspect we’re somehow missing something here. While LEDs are low power consumption vs bulbs, it’s not insignificant. I have a GTM mounted in a fairly tight plastic enclosure on top of our truck fed power via a USB cord. I accidentally bumped the connection loose, then reconnected it, causing it to lose pairing with the iPhone inside the truck. It didn’t repair, plus sat in the sun on top of the dark green truck. The combination of natural heat plus the constant blinking LEDs caused the plastic to distort from the heat on the side of the enclosure that was in closest contact with the GTM.

So I suspect either there’s another process that uses as much energy as the LEDs when not in paired mode or some other factor affected your measurement. Note that we may be talking about two different things as pairing to is different than having been paired but affected by loss of BT contact?