Update May 6, 2016: This kit is no longer offered until I get a new batch of QDSP-6064 displays (whose price seem to have jumped considerably).
The kit includes the following:
- DS1307 SMD
- 32kHz crystal
- CR1220 coin battery
- battery holder
- 330 ohm resistor 0805 (8x)
- tactile switch SMD (2x)
- machined female pins
The assembled clock can be fitted with a LiPo battery shield for ProMini, as shown in this post (source code also provided there).
The current draw (measured at 20mA with an unmodified ProMini) can be minimized by removing the 2 LEDs on the ProMini board, as well as dimming the 7-segment bubble display through software (SevSeg library). One other way of maximizing the LiPo battery life cycle is by waking the clock from sleep mode at the press of the "minutes" button (on D2).
Schematic and board layout are shown below.