After assembling the third revision of the watch, I decided to test its battery life. Chronio is now running off of a CR2025 coin cell since August 7. During this time the display and RTC are on and the Atmega wakes up once per minute.

The calculated battery life is about 6 months, although that is not an accurate number. The varying current consumption caused by the 1 Hz generator makes it difficult to predict the actual battery life. Because of this there is no way around a real-time test to see how long the watch will last.

Next to the time and date, Chronio displays the battery voltage. For the past few weeks the voltage was between 2.84 and 2.86 V depending on the temperature of the room. Unfortunately Lithium-Ion batteries have a discharge curve that is anything but linear. Here is a graph from a very similar project with a CR2016:

The voltage stayed at 2.85 V for the first half of the test before it slowly began to drop. Based on this, it is very likely that the watch is still within the first half as well.

However, the test has already proven, that the watch will work for more than 2.5 months, which is more than twice of what I originally had hoped for. Also, this is the longest run of an Arduino I ever did. I had already expected some errors because of overflows. But Chronio even handled the overflow of millis() after 49 days without a problem.

I would assume that the battery will probably last until the end of the year, which would be a nice 4 months of runtime.