Battery based sensors: at what voltage level do you set an alarm ?

  • Hardware Contributor

    Last night one of my older sensors (v1) "flatlined". I let the battery die out.

    To be complete: my v1 sensor has NO step-up converter, everything on the sensor is directly connected to two batteries (AA or AAA).

    The last message with measured battery voltage was early this morning at 0:43am and the value was 1.644V. The temperature sensor (a TMP36 on this node) had given up already at around 1.9V, but radio and atmega328p kept going until this morning.


    This sensor started with two AA batteries which were already considered flat by another tool (first voltage reading was 2.469V on 15 september). My "alarm" threshold is at 2.3V which was reached last saturday morning, so it worked for another 4 days. I will set my alarm at 2.4V for future sensors, to have a bit more time to buy and replace batteries, but 4 days should have been enough.

    So here is the question: how are other battery based sensor owners faring ? At what voltage do you set your alarm to replace batteries ?
    We have the well documented case of "sensor 105", but I was wondering if a low-battery alarm was set on the controller side, and at what battery voltage.

    So this question is not about how long a node has worked on a set of batteries, but how to best set the alarm to change the batteries.

  • Hardware Contributor

    Hi! I report batterylevel 0% =1.9v and 100%=3.0v . I use 2x aa and the nrf radio which has Vmin as 1.9v i think. Domoticz will alert at 10%.

    I have same setup as node 105.

  • Hardware Contributor

    @sundberg84: thanks for the feedback. As you see, my NRF24L01+ kept running wel below 1.9V (down to 1.65V). Spec says indeed 1.9V as a minimum.

    The atmega328p is specked at 1.8V minimum for stable operation. So 1.9V seams a reasonable 0% level (there is some buffer, but it lasted only 24 hours).

    I want to keep some time between alarm and the actual battery change, as some nodes may be in places were I need to look for some time before finding them (just anticipating memory loss).

    Anyone else with battery operated sensor nodes ? Setup (AA,AAA,18xxx,LiPo,...), alarm levels ?

  • hello
    well... i have an pro mini 3.3v 8mhz (red ones) +nrf24+ motion sensor, running for 6 months or more .
    Its powered by 4 18650 batteries (laptop cells) , 2 in parallel, 7.2v direct to arduino's voltage regulator and probably it will last years.

    Its not running mysensors library,yet because i'm changing my sensor by this marvellous project called mysensors 🙂 connected to vera 3 lite by serial gw.

    But backing to my battery powered sensor... depending which way you wake up your sensor, battery can last years. I'm waking up my by motion detection. Arduino its sleeping until get some input from the motion sensor .
    If it sleeps and only wake up during 1 second each 30minutes to send temperatures ,humidity and other kind of "non real" time sensors it can work for month as well.

    ps: i didn't implement any alarm or voltage sensor on this one , because it just stops working when voltage is less than 6volts on Vregulator (its what they say ,never happens). but on new ondes for mysensor project will have voltage dividers for voltage monitoring... in that case alarm will be at 3.2v per cell or that cell start to lose life time

  • I have five battery powered MySensors nodes powered with CR123 lithium batteries. I set the limits at 100% at 3.0v and 0% at 1.9v. Now I just need a method for Vera to generate an alert when they drop near 0%.