RTC in Vbat Mode

[mccabehm]

Custom board with STM32H563 and 32.768KHz LSE and supercap for RTC in Vbat mode. I want RTC longevity in terms of weeks; it lasts 4 days. Is there anything SW can do to extend this time? For short intervals (minutes) the RTC updates fine. Disabling SRAM backup extends the time by 8 hrs. Schematic excerpts...

[Uwe Bonnes]

Sit down and calculate voltage drop  in the capacitor by the capacitor equation C = I * t / U. Use the IBat values in the STM32 datasheet. If calculation give substantial longer time, have a look at TZMB3V3. Datasheet only guarantees < 2 uA @ 1 Volt. If I rebverse is really at that limit, the diode may be the big sucker.