Found on the Internet, this came up because I gave a colleague one of these rtcs from my private stock.
Possible Bug and Fix
If you have a module like this…and it is unreliable or not working at all, here is a possible fix.
– Remove battery
– Remove D1, R6 and R4
– Solder jumper wire in place of R6
– Inspect crystal soldering – fix if necessary
– Replace battery
The module should now work properly. It requires 5V power, but will work with 3.3V logic.
The DS1307 can be replaced with the newer pin compatible DS1338 for operation from 1.8V to 5V, or the TI BQ32000 for 3.3V (only) operation.
Why this is so
The battery backup circuit on these modules is of poor design. D1 and R5 provide a charging current to the CR2032 coin cell. Lithium primary cells such as the CRxxxx series should not be (re)charged. Removing D1 fixes this. R4 & R6 create a voltage divider between the battery and the DS1307. This raises the source impedance (spec sheet warns against this) and increases the current draw from the battery. The voltage divider was necessary because the battery voltage would float at about 4.2V volts due to the D1/R5 circuit! That would keep the DS1307 disabled if present on the Vbat pin. Replacing R6 with a jumper allows the full battery voltage to Vbat.
This information is copied from here: forum.43oh.com/topic/4131-ds1307-rtc-eeprom-from-ebay-possible-unreliable-operation/
and full credit goes to it’s author. I’m writing this guide to hopefully save someone the 3+ hours it took me to learn this.