I installed two fully charged 3Ah 18650s connected in parallel to the Postnote device back in mid January. I noticed that power was no longer being supplied to the device in early March. My goal is to achieve at least three months between battery charges so I am starting to consider putting the microcontroller into sleep mode at night, increasing battery capacity, and potentially adding a solar panel.

One other issue was that there was no battery management device to prevent voltage of the battery cells from discharging to below 2.5V. The two 18650s were measuring less than 1V after the month and a half period. My plan is to instead use a USB phone backup battery stick. It should prevent the battery from dropping below voltage. As a bonus, it already has the charging circuit so can be recharged from any 5.0V USB source.