Watch out for bricking core in battery application due to brown out condition which can happen if you let the battery run low.
It happened to me when I thought it would be simple to just run a Spark off a Lipo and solar charger, without thinking about it too much -> Ended up with a Spark flashing green (best I could tell) and completely unresponsive.
My most recent setup is configured Lipo -> Buck/Boost 5V -> MCP120 4.5V Voltage Supervisor [pull Reset to low if <4.3V] -> VIN to Photon. It’s been working so far.