I can happily report that the WSTL18 temperature logger can achieve its power consumption target. I had previously calculated a datasheet-based target consumption of 3.22uA during power-save mode, with the real-time counter running. I've currently achieved a power consumption of 2.92uA during power-save. I've already established communication with the temperature sensor and memory chip, and can verify they are operational and can successfully enter their lowest power-save mode.

This measurement was taken at 3.3V coming from the uart adaptor, and was taken with a simple multimeter, so I don't expect it to be very accurate. I also expect the power consumption to be slightly lower at 2.7-3.1V from the coin-cell battery but higher at 30°C in an incubating sea turtle nest.

Total power consumption

Power consumption goes up to slightly under 5mA when all chips and a couple of mcu peripherals are on, but this only lasts for under 0.1 seconds. I'll add another 0.1 seconds for every 10-minute log, because the mcu has to wake up every 8 seconds (75 times per 10 minutes) and check if it's been ten minutes. I'll round it up to 1 second per log, so 6 seconds per hour just to be sure. So in one hour, the device has consumed 3uA + 5mA* ( 6 seconds /3600 seconds/hour) = 11.33uA. I'll round this up to 20uA.

How much juice is in the battery?

Most brand-name CR2032 batteries report a total capacity between 200mA and 240mA. It's important to notice that this is the capacity the battery can give until its voltage drops to 2V. Luckily, coin cell batteries maintain their voltage until shortly before they die, so the total capacity isn't reduced much for lower voltage drops. The ATmega328PB can operate at that voltage, but the memory chip needs 2.3V and the temperature sensor needs at least 2.7V. The temperature sensor won't give any warning, but instead its temperature readings will become widely inaccurate. I'd rather stop recording temperatures when the voltage reaches 2.75V. When will this happen?

A power consumption of 20uA is roughly equivalent to 150kΩ load at 3V. According to Maxell's CR2032 datasheet, this should give more than 10000 hours of operation before the battery voltage starts dropping close to 2.8V. I don't expect the short high-consumption pulses to affect that by much. That's equal to 416 days, which is well over my original target of 180 days.

Instead of trying to extend the logger's operational time, I'm just going to say that the WSTL18 can safely maintain stated temperature accuracy for 180 days on a new coin-cell battery with a stated capacity of at least 200mA. That's good enough for the salesman.

Battery-Time test

I've actually designed the WSTL18 so that the ATmega328PB can measure the voltage of its power supply using its internal 1.1V reference. A capacitor is connected to the Vref pin, which allows the ADC to use Avcc as the voltage reference to measure the 1.1V refrence. This is a power-expensive operation and I won't have the logger do it during regular logging, but I do plan to have a few loggers run a voltage-time log, which will help verify the logger's stated duration of operation.