The existing topic under this header has not been posted to for 120 days so I'm starting another.
My 6-sensor beehive temperature monitor illustrated at www.dheaf.plus.com/monitor/ will save data to the SD with FTDI (3.3V) USB adapter connection to my PC, but not with the 18650 cell in the monitor. That cell is connected to a 3.3V voltage regulator on the on/off switch board.
All VCC terminals show a fraction over 3.4V with the FTDI, but 3.3V with the battery/regulator. All wires are tested for continuity. Anyway, the fact that the SD records with the FTDI indicates that the circuits are intact.
The above link lists all the boards etc.
Any suggestions welcomed.