The DC-DC converter allows the AA batteries to be much further discharged, without the boost converter the atmega 328 would brown out at about 2.5V. With the converter the battery can be discharged to about 1V. Thus gains us about 3 months of extra battery life looking at the real work discharge curve:
Real World emonTH Battery Life - Blog | OpenEnergyMonitor
See the blog post for the boost converter design decisions:
The quiescent current draw of the converter is minimal (about 7uA).
In theory it looks fine, and accrording to my calculations, 2xAA cells should be able to drive the EmonTH for about 9-10 months with the DHT22. Assumptions: old booster setup, 2s uptime at each measurement, 1-minute intervals, about 28 uA sleeping current (7 uA MCU@8MHz + 7 uA booster, 50% efficiency), and about 10 mA average wake including 95% efficiency current including transmission. Yet you only seem to have 7 months. Naturally it’s a short span and a relatively high average consumption so 2-3 months theory vs. real lifie is not very big.
However, if you use any of the modern chips, their consumption is practically nonexistent. In that case, without the booster you can go down to about 1.9V or 0.95V/battery, but let’s be safe and say 2V. Standard specs are down to 0.8V usually. That’s about 73% of nominal mAh, 2200 mAh for an AA cell (3.1V - 1.6V vs. 3.1V - 2V). Assuming no booster, 7 uA sleep current, 50 mA wake current with transmission, 10 ms wake time, 1 measurements/minute, we get 14 years. There is only one kind of battery that lasts that long, the lithium primary. They’re 1.5V and a little more expensive. Every other kid of battery needs to be replaced in 3-4 years at most. No booster can help with that. Since a booster is not free amperage-wise, it wouldn’t prolong the lithium battery usage either.
Brown-out is not a problem, you can set it as low as you feel comfortable. In practice, 8 MHz is good down to ~1.8V.
Calculator I used: Oregon Embedded - Battery Life Calculator
Well, just my 2cents.
I am about six months late to the party, but for what it is worth, here are my comments.
I too have adopted the BME280 as my device of choice for the time being. It is more expensive but when only buying a handful for myself that is fine. Your considerations for emonTH are inevitably more complicated. In my tests which you have already seen (http://www.kandrsmith.org/RJS/Misc/Hygrometers/calib_many.html) it gave the most accurate results, particularly in terms of thermal and long term stability. Some of the others were great at a certain temperature but then drifted when the temperature changed. I am a little nervous making that statement publicly because I can only speak to the specific devices I tested, not the average device off-the-shelf. Also, when I look at my SHT71 data (http://www.kandrsmith.org/RJS/Misc/Hygrometers/calib_many.html#results_ht) the humidity vs temperature plane is extremely ‘flat’, in fact just as flat as the BME280. The problem is that it is tilted so on the basis of this one device the BME280 definitely wins, but one tiny tweak to the internal calibration parameters could fix it and make it just as good. Was I unlucky to get a bad one? I have no idea. Mind you, the SHT71 is even more expensive still and does not have a barometer so I still favour the Bosch device. The BME280 also has a variety of options for internal filtering and averaging which is all very nice.
On the temperature accuracy, the data sheet does as you say caution that they do not really consider the device to be intended for precision temperatures, but my tests have looked perfectly fine for my needs. I must stress this is just my own “home made” experiment, not based on proper lab certified references or anything, but the six BME280 I tried gave errors of +0.3, -0.1, +0.7, +0.2, 0.0 and -0.5 deg C. The write up, if you are interested is here:
http://www.kandrsmith.org/RJS/Misc/Hygrometers/absolutetemperature.html
Sorry to wake an ancient thread, but there is a packaged version of this that supports BLE and runs on a CR2032 - Avnet: Quality Electronic Components & Services
that’s impressive. Thanks for the link