DS1307 running slow on Vbat


I’ve got a little custom project that uses a DS1307 to keep time. I’m using the 1Hz signal from the DS1307 to indicate it’s time to update the displayed clock (ie every second). Everything in this respect works fine. However when the device is off and the DS1307 is running off Vbat (a CR032 watch battery) it can lose many minutes over an hour or 2.

The fact the clock runs nicely when everything is on tells me the crystal (32.768kHz), load caps (12pF) etc are OK. The CR032 is new and is putting out 3V as expected.

Anyone else seen this before?