Welcome, @sChik.1, to the community!
The STM32F777 does not provide a charging facility for capacitors or batteries connected to VBAT.
The STM32U585, on the other hand, has this built in, as do many other STM32 (e.g. L4, L5, G4, WB, H7A, H7B). The function is described for the STM32U585 in RM0456, section 10.4.7 Battery Backup Domain:
VBAT battery charging
When VDD is present, It is possible to charge the external battery on VBAT through an internal resistance.
The VBAT charging is done either through a 5 kΩ resistor or through a 1.5 kΩ resistor depending on the VBRS bit value in the PWR_BDCR2 register.
The battery charging is enabled by setting VBE bit in the PWR_BDCR2 register. It is automatically disabled in VBAT mode.
Since they are fixed resistors, the charging current depends on the state of the connected buffer connected to VBAT and the supply voltage and can easily be determined from the voltage difference.
Does it answer your question?
Regards
/Peter
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