Is my battery drain correct? (Atmega bare @8Mhz + SI7021 + nrf)



  • Hi to all,
    I have a barebone Atmega328 @8MHZ reading every 10 minutes a SI7021 temp+hum and battery voltage with "secret voltmeter" workaround (no external voltage divider). Is it feeded directly from 2XAA and if last reading is not equal to previous, send to the gateway, otherwise no.

    It sends battery, % battery, temp and hum.

    I did also printed the pcb with oshpark but I did wrong the footprint for sensor. The rest is ok!

    By the way, I did read on my tester 10microA when sleep, and ~17milliA when transmitting (~2/3 seconds). For 1500mAH, battery calculator says about 1.5year with battery (http://oregonembedded.com/batterycalc.htm)

    But, after 24h, I did note a loss of 0.05V on battery (reported on Domoticz). From 3.09V to 3.04V, from yesterday at 18:00 to today at 18:00.

    I am worried that it seems too high the discharge!

    Thank you for your answer!

    PS this is first prototype of PCB...

    alt text


  • Hardware Contributor

    Hello,

    I have a few boards with similar setup (stripped down ProMini, sht21, nrf24) but powered with CR2032 and sending up to every minute (if changed) and they've been running for months with very low discharge so with your setup you should be able to run for many years as the capacity of 2 AA is about 10 times the one in a CR2032.

    It's hard to measure low current precisely with a multimeter so maybe it's just a measurement problem, but your 10ยตA seems on the high side, I measured around 6-7 with my boards. But this should not be a big problem with 2AA, I would not worry too much about it. You can try to help by using better capacitor with lower leakage current. If buying from aliexpress/ebay the only way you can reduce that is choosing lower voltage ratings. You can check quality of your solder joints too.
    But don't expect to gain much...

    What you can fix are those 2-3s at 17mA that seem way too much, you should use a 1MHz bootloader with internal oscillator, this will save some mA when running, and compute all your data first after you wake up, then send all messages in a row, so that on time of the radio will be limited.
    Also make sure you radio link quality is good as this can lead to radio having to resend messages.

    About the discharge you measure, I would not worry too much as the discharge curve of alkaline batteries is not linear, it drops quickly at the beginning, then will be more flat for a long time before dropping. Your load is way lower that even the blue curve so it will be even more flat.
    alt text



  • @Nca78

    Thank you for your answer! I wil test and I will update, if any...

    For the capacitors, yes, I use 40V rated. What's your hint on select them? Thank you another time ๐Ÿ™‚


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