I’ve had a flir one for iPhone since the second generation I think, and it was fine until I tried to change out the battery - I bought a replacement, followed the ifixit guide and it never switched back on.
Does anyone know where to begin with this?
did you verify battery voltage on your replacement?
I did yes, it was the correct battery and tested at 1.3v IIRC.
Yeah 1.3V is way too low bud, probably low enough that it might even refuse to initialise charge also. I’d pull the battery out again, and charge it up yourself manually until battery voltage is at least around 3V independent and then connect it back up 