Same battery as the original watch. Most of the watch's parts are reused, including the battery clip and the battery itself. Only the circuit board is replaced.
The original module lasts about 10 years on a single coin cell. The sensor watch can't match that incredible lifespan but it's still very respectable. Power profiling and testing has shown that it has over a year of endurance. There's a builtin battery voltage sensor which serves as a power reserve meter. Maximum power would be 3 V and low power would be something like 2.2 V.
I modified the low power mode timers to make it go to sleep after ten seconds in order to extend it as much as possible. Low power mode means the CPU wakes up once per minute instead of once per second. The only drawback is the lack of seconds display but the truth is I've come to consider that a feature. If I want to know the seconds, I push the alarm button to wake the watch up.
