I spend hours and hours on a batch of DS1307 which I had bought on eBay from China for a student lab.
Some worked for a short time, then became hot - almost letting the gray smoke come out which drives then. Other did never work at all.
![Broken! :-BROKE](https://www.eevblog.com/forum/Smileys/default/smiliey_scope_broke.gif)
Running the same, exact same code on legitimate DS1307 from a commercial reseller in Sweden worked flawlessly!
![DMM :-DMM](https://www.eevblog.com/forum/Smileys/default/smiliey_dmm.gif)
It really took me a while to realize that I must have gotten counterfeit chips.
I was pulling all strings on possible circuit and software errors, tried my own I2C code and two or three external libraries. Replaced the crystals a dozen times, checked the battery connection, changed the battery, replaced the battery with a diode-circuit to fake the battery voltage...