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EmonTH/docs/power_consumption.md

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Power Consumption

The motivation behind replacing the DHT22 Temperature & Humidity sensor used on the emonTH V1 with the Si7021 Temperature & Humidity sensor on the emonTH V2 was the power saving benefits.

See the sensor folder in the emonth2 repo for a power consumption comparision between the DHT22 and Si7021.

This document contains real-world emonTH2 power consumption as measured using a multi-meter & scope resistor voltage drop method explained in previous blog post.

Here is a scope trace illustrating the emonTH power consumption during a sensor sample:

emonth2 power consumption

  1. Atmega328 wakes up, starts sensors and takes readings: 17.5ms @ 8.4mA

  2. Atmega328 goes back to watchdog sleep but leaves SPI initialised waiting for wait for RFM to initialise: 100ms @ 2mA

  3. RFM69CW transmission: 4ms @ 44mA

  4. Wait for RF transmission to finish: 100ms @ 0.06mA

  5. ATmega328 wakes up to Print data to serial UART: 5ms @ 7.2mA

Time taken for sample = 17.5 + 100 + 4 + 100 + 5 = 226.5ms

Average current per sample = (17.58.4) + (1002) + (444) + (1000.06) + (5*7.2) / 225.5 = 565 / 225.5 = 2.5mA

After sensor sample is complete the ATmega328 goes back to full watchdog sleep for consuming 0.06mA. This base quiescent consumption includes the quiescent power consumption of the LTC3525 DC-DC converter. See original emonTH hardware blog post for emonTH DC-DC converter design.

Assuming one reading per min (current default) the emonTH is off (sleeping for): number of ms in one min - on time = 60000ms - 225.5ms = 59774.5

To summarise:

  • On current = 2.5mA (average)
  • On time = 226ms
  • Off current = 0.06mA
  • Off time = 59774.5 ms

Oregon Embedded online battery life calculator tool can now be used to calculate battery live. Assuming using 2 x AA alkaline batteries in series = 2700 mAhr

emonth V2 estimated battery life

Given the measured power consumption the emonTh V2 is estimated to have a battery life of 3.8 years

Plugging in the power consumption figures from the old emonTh V1 (with DHT22) into the calculator a 1.4 year battery life is estimated.

emonth V1 estimated battery life

According to the calculator the emonTH V1 is estimated battery life of 0.7 years which matches the real world emonth v1 battery life.

I have setup a battery life test with the emonTh V2 with Si7021 sensor, I will report back in a couple of years :-)

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