Photon and back-up battery

[AndyW]

You might find this thread useful also.
It discusses using a supercap rather than a battery, but the use case is similar to yours.

Photon VBAT supercap circuit

When I get my Photon, I’d like to use a supercap to keep the RTC synchronised in the face of power outages. After some reading and some number crunching, I came up with this: Edit: added D2 as per discussion. My calculations below still reflect the circuit without it. [image] (Updated schematic from post #41) [vbat] The super cap will be a 1.5F 5.5V cheapie from eBay. The STM32F205 datasheet gives limits for VBAT operating voltage of Min 1.65V, Max 3.6V (p. 71) so that means the cap must…