backup battery for FeatherWing DS3231, RTC I am working with slightly modified example code for the Adafruit FeatherWing DS3231 RTC (real time clock) board. The DS3231 chip itself is from Maxim. Since the cell is described as a "backup battery" a literal interpretation suggests that you don't need the backup battery in order to operate the board if the board itself is powered and you don't mind that the device cannot remember the time when board power is lost. I might be taking the term "backup battery" too literally since it doesn't seem to work.

The Adafruit (Arduino) code:

&gt; #include &lt;Wire.h&gt;
&gt; #include "RTClib.h" 
&gt; RTC_DS3231 rtc; 
&gt; void loop() 
&gt;{
&gt;     rtc.adjust(DateTime(2018, 1, 1, 0, 0, 0));
&gt;     DateTime now = rtc.now();
&gt;     Serial.print(now.year());
&gt;     Serial.print(" ");
&gt;     Serial.print(now.month());
&gt;     Serial.print(" ");
&gt;     Serial.print(now.day()); 
&gt;}


The output is:

2165 165 165 


The Maxim datasheet says this about the VBAT line which is the line between the DS3231 chip and the missing backup battery:


  Backup Power-Supply Input. When using the device with the VBAT input
  as the primary power source, this pin should be decoupled using a
  0.1μF to 1.0μF low-leakage capacitor. When using the device with the VBAT input as the backup power source, the capacitor is not required.
  If VBAT is not used, connect to ground.


So I connected it to ground and it makes no difference. I can't buy a battery until tomorrow morning. Can somebody please confirm that the backup battery is actually essential. EDIT: I installed a battery and it makes no difference.

Aside: A good reason for not having a backup battery if possible is to have fewer user serviceable parts.
The problem was a bad connection for one of the serial lines. 

I found the solution at this link.

The battery is not necessary to make this work.