While waiting for my new PCBs to come in, I started a new project; building a Delta 3D printer from scratch. That consumed all of my time, and when I was finished with that and returned to my ePaper Temperature and Humidity displays, I realized I made a mistake, so I need to get the PCBs re-manufactured again. After about 6 months of languishing on my bench, I decided to finish this thing up.

The tri-color display is on the left and the 2 color display is right.

Chugging through the numbers, about 75% of the lifetime on a CR2032 battery is going to be determined by the number of times the screen refreshed, with the rest being the idle current. According the my calculations, the tri-color ePaper disply should make it 103 days on a battery, while the 2 color display version should last 375 days.

I put fresh batteries on both units on Dec 2 2018, 12:00AM. My calculations tell me that the 3 Color display should die on March 15, 2019 and the 2 color display on December 12, 2019. Of course a lot of this will depend on how much energy the battery can really deliver out of its theoretical 225 mAh.

Things I have learned:

• Keep clock rates on the microcontroller low. It makes a big different in idle current.
• The multicolor ePaper display use a great deal more energy than the black and white ones.
• I2C clock stretching, or anything that pulls the lines down for extended periods of time, waste energy. Try to keep the amount of time the pull down resistors conduct to a minimum.
• Open Drain output on a microcontroller have high RDSon (on resistance) and make lousy switches. Use an external FET if you want to do something like switch power to a peripheral.