Hi mweal
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.
Regards;
CMata
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