Powering SD breakout board with 3.7V battery So in a previous post I asked if I could charge an SD card breakout board with a 3.7V LiPo battery, and I got a positive response, so I did it.

The problem is that it seems not to be enough, since, with the exact same wiring, if I supply it with my Arduino Uno, it works perfectly, but when I change the supply to the battery it gives an error. The following text is the result of an Arduino example script:

When connected with the Arduino: 

Initializing SD card...Wiring is correct and a card is present.

Card type: SDHC

Volume type is FAT32

Volume size (bytes): 2696937472
Volume size (Kbytes): 2633728
Volume size (Mbytes): 2572

Files found on the card (name, date and size in bytes): 
SYSTEM~1/     2017-04-27 15:20:04
  WPSETT~1.DAT  2017-04-27 15:20:04 12
  INDEXE~1      2017-04-27 15:20:14 76
TEST0.TXT     2000-01-01 01:00:00 49050
TEST1.TXT     2000-01-01 01:00:00 60231
TEST2.TXT     2000-01-01 01:00:00 1044
TEST3.TXT     2000-01-01 01:00:00 34831
TEST4.TXT     2000-01-01 01:00:00 189736
TEST5.TXT     2000-01-01 01:00:00 12419


When connected to the battery:

Initializing SD card...initialization failed. Things to check:
* is a card inserted?
* is your wiring correct?
* did you change the chipSelect pin to match your shield or module?


or 

Initializing SD card...Wiring is correct and a card is present.

Card type: SD1
Could not find FAT16/FAT32 partition.
Make sure you've formatted the card


I've tried to supply the breakout board both through the 3V and 5V input. Can anyone notice what am I doing wrong?
You haven't connected the grounds together in your circuit. You need to connect all the grounds together so every connection has a return path back to where it came from.  That means connect together:


An Ardino GND pin
The GND pin of the SD card breakout board
The negative (or ground) pin of the battery