First I removed old battery along with the 220 resistor, but this hadn't the effect I expected. With the power on the voltage on the anode of the 1S1588 was 0V, but on the cathode it was still 4.2V? The + side of the battery holder also give 4.2V.
Then I tried to find out what the HRP100 was. By the symbol it should be a zener diode but Google couldn't find a datasheet? So I desoldered it, and looked at it with my loupe. There it says: S4`3D. Can't find anything about that either. (S4 should be SMD?)
Then I took a leap and desoldered the 1S1588 and put this to replace the HRP100. Now I think this works, with power on the battery + from the cr2032 holder is 0V, the backup RAM had 5V. With a battery in the holder, the battery + gives 3.5V. The RAM still gets 5V of course.
With the power of and the battery in place, the RAM get 3.2V.
Is this really what needs to be done on the MV4FS? Throw out HRP100 and put 1S1588 in its place? Does 220 get removed as well?