So today I pulled out this project - I noticed that it was running too fast and I also wanted to build a nice case some day so it went in a project box. I checked the battery since I found a deviation of 2 hours leading a bit too much for this timespan. The battery read "LIR2032" and 3.6V on it and measured 4V which got me thinking and searching for some data-sheets. First I seem to have scored a rechargeable lithium cell (http://www.powerstream.com/p/Lir2032.pdf) which came with the module, but the datasheet of the ds1307 gives a maximal voltage of 2 - 3.5V for the battery input - I'm gonna grab a CR2032 rated 3V and hope their mistake didn't break the module board :D oh boy.