You can not read the charging voltage in SW. BBAT is supplying it. But
The PMIC’s builtin backup battery charging circuit can be configured using the CNFGBBC register of the PMIC.
The register exists at offset 4 for MAX77620 and MAX20024 parts.
Following configurations can be made:
Bit[0]: Enable backup battery charger
Bit[1:2]: Charging current setting
Bit[3:4]: Charging voltage limit setting
Bit[5]: Lower charging current enable
Bit[6:7]: Ouput resistor selection
Software configuration can be done using the MB1 PMIC cfg file. The file has sections for configuring PMIC registers.
Such sections have type=1 to indicate I2C transaction, i2c-controller-id=<> for instance of I2C controller and slave-add=0x78 for identifying PMIC.
[tegra194-mb1-bct-pmic-p3421-0000.cfg]
e.g,
T194:
pmic.system.block.type = 1; #I2C
pmic.system.block.controller-id = 4;
pmic.system.block.slave-add = 0x78; # 7BIt:0x3c