Perhaps this would be a good hardware revision for the Spark Core… add the MCP1825 with the shutdown pin… and add a solder point for the VBAT pin (RTC module power in STM32) and a Schottky Diode on-board between the VBAT pin and the 3.3V output, anode on VBAT, cathode on 3.3V. Then all you have to do is wire up a coin cell or larger 3V battery to your VBAT input solder pad. Deep sleep like a baby.
