Q: DS3645 and DS3640B have a VBAT line. Is it safe to directly connect a CR2477 battery to this line?
A: Connecting a coin cell battery directly to VBAT is safe. A 10nF bypass capacitor could be added close to VBAT like the below diagram.
You should consider the temperature characteristics of CR2477. DS3640 and DS3645 will trigger a tamper if VBAT falls below VBTH, where VBTH is around 2.4V. The nominal voltage of CR2477 is 3.0V. It could fall below 2.4V depending on temperature and duration conditions.