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:
Atmega328 wakes up, starts sensors and takes readings: 17.5ms @ 8.4mA
Atmega328 goes back to watchdog sleep but leaves SPI initialised waiting for wait for RFM to initialise: 100ms @ 2mA - *now reduced (see code)
RFM69CW transmission: 4ms @ 44mA
Wait for RF transmission to finish: 100ms @ 0.06mA
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
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 @ 2700 mAhr each = 5400*
* The 2 x AA batteries are connected in series but since uses emonTH uses a DC-DC converter capacity is effectivly 2xAA capacity (mius losses), see forum post
Given the measured power consumption the emonTh2 is estimated to have a battery life of 7.6 years!
Plugging in the power consumption figures from the emonTh V1 (with DHT22) into the calculator a 1.4 year battery life is estimated.
From real world testing is been established that the emonTh V1 actually achieves a 8 month battery life. This is 57% less than the estimated figure.
Assuming the battery life calculator is overestimating by the same amount for the emonTH V2 we can expect a real world battery life of just over 5 years, which is not too bad :-p
I have setup a battery life test with the emonTh V2 with Si7021 sensor, I will report back in a couple of years :-)