I have a Photon connected to a 3.7V battery through a Battery Shield. Is it safe to connect a booster directly to VIN and GND to charge an external device (5V)?
I wouldn’t see an issue, but I’d go for a booster that can be switched off when the controller is off to prevent depleting the LiPo when not needed.
Also make sure the booster can copy with the 5V USB power when connected to USB.
But what do you mean with “charge an external device”?
By “charge an external device” I mean connecting a mobile phone to the booster and have it charged.
Important to mention that I also have a solar panel connected to the VIN and GND, as suggested in https://learn.sparkfun.com/tutorials/photon-battery-shield-hookup-guide:
If you don’t want to solder the barrel jack on, […] you can supply that voltage to the “VIN” and “GND” pins on the shield’s header.
I wonder what happens in VIN when there’s sunlight (so battery charging) and I’m charging the external device.
Here’s my diagram: