I want to make a binary watch, but I've got some power issues.
A coin cell can hold up to 610mAh @ 3V. However if I try to power the LEDs (there will be 20 of them, so let's say 13 are on simultanously), and we power them with 3V, in series with a 1kOhm resistor.
That makes 13 * 5 mA = 60 mA only for the LEDs.
This will drain the battery in a few hours, not very good. LEDs should be very efficient, what am I doing wrong?