you need to power V_VBAT_BKUP to have a working RTC if Edison is switched off
not sure if V_VBAT_BKUP is happy with 3.3 to 4.5V, some users provide 2.5V
the Module Hardware Guide should be extended to provide the required V_VBAT_BKUP voltage range
CMata_Intel, thanks for the feedback. The Compute Module Hardware Guide is not much help. Section 4.13 basically says you can hook up a 2.5V supercap or battery and section 5 does not mention anything about the RTC backup. I have searched (and searched and searched) for the SNB9024 and have been unable to find any more information for the device.
In the section 5.3 of the document you can see how the PMIC is powered inside the Edison Compute Module, and how the VSYS and VBAT are related. In the table 5, the VBAT pin is the input for the backup battery and VSYS is the system input power. The PMIC allows to use a rechargeable coin cell battery or a super-capacitor. You said that it could be powered down for a few months so you will need a coin cell battery that may work for that long, and always taking care of the voltage and current specifications for the PMIC as the document states in section 4.3.
About the PMIC datasheet, I haven't been able to find it either, I think you could contact their support team and ask for the datasheet.
I looked at section 5.3, and table 5. One refers to VBAT and the other to V_VBAT_BKUP. It is not clear if VBAT in section 5.3 is short for V_VBAT_BKUP (RTC backup) or for full battery operation of the Edison. I believe it is for full battery operation due to the reference is section 5.1 "Application of power to VSYS is interpreted as a battery insertion and will cause Edison to boot."
I have made a request to TI for the data sheet, and have not received any reply as of yet.