Interesting.
What types of settings are stored in that SRAM? so far I did not manage to store calibration (open short load). It is always lost when the instrument is switched off. Just checked it:
calibrate in the NETWORK mode, then switch off and on again, and calibration is lost. It does not even keep the frequency settings, number of points or RBW. So it would be really interesting, what the battery is for. Obviously it must be something "important", that HP prevented the machine from booting when the battery was dead. But what?
it is not the calibration data, as that is stored in the EEPROM.

(By the way, has someone ever managed to find replacement parts? I am very nervous about the EEPROM to go nuts. It is almost unobtanium nowadays.)