EmonTH Battery Life

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I created feeds that record the transmitted battV of all 4x of my EmonTH sensors back when I originally got them two years ago. I used those feeds to create a multigraph of all the battery powered Emon devices in my shop. The data is interesting. Each one was in a slightly different environment (from inside conditioned office space, to semi-conditioned shop space, to outside). The batteries seem to last on average half a year to almost a year, with the longest being the ones inside and the shortest being the one outside (expected). What’s interesting is that once the BattV gets below 1.7volts, it drops off precipitously and you only have on average one to two weeks at most before the battery dies completely. With that in mind, I might create some LEDs on my primary dashboard that signal when a EmonTH batt is about to fail (because I rarely remember to take a look at this multigraph).


What a coincidence!

I too was looking at my emonTH battery life history today and came to the same conclusion about 1.75volts. Rather than a LED, I was actually thinking of setting an email notification at 2.0v as a 1st warning (to make sure I pick up some new batteries if I don’t have any in the house) and again at 1.75v that failure is imminent, to swap the batteries out before they fail for minimal data loss.

I did this today because I noticed one emonTH had stopped posting due to low battery voltage. I didn’t have any new batteries in the house as I had put new batteries in the TV remotes over Xmas, but I did still have the old batteries that I replaced in the remote and found at 2.2v, although they won’t power the TV remote, they will probably last a couple of months in the emonTH. So from here on in I will probably use the emonTH’s to get some more life out of “expired” batteries before disposal (recycling) and only ever put “new” batteries in more power hungry devices.


How would you set up an email alert? That’s a good idea.

I think I would set up my thresholds to be at 1.85v for the first warning and 1.7v for the final warning. There’s typically a couple weeks between those levels, and I’d be concerned that if I was warned too early I’d simply ignore it (like my alarm clock in the morning).

I must say that I’m impressed by the scavenging ability of this circuit. If my sensors were in more convenient locations and I had an alert system to keep from missing data, I too would use them to suck dry otherwise weak batteries.

I’m guessing by that question you are not running a self hosted emoncms? Although it’s not available on, self hosted emoncms instances have an eventp module (event process) which can be added into the input processing using the logic processes to send emails containing user defined messages and current feed values.

I know exactly what you mean, once the notification is in place the thresholds can be played with anyways.

I’m self hosting, but I haven’t messed with my setup in two years because its stable, does everything I need, and the data I’m recording is business related and mission critical. (No updates, because if it ain’t broke, don’t fix it)


I have used node red for this. Post battery data from emoncms which is a minor addition to the input. Do the level checking in node red and use whatever your preferred method of notification is to notify low battery.