Hopefully it is. Though looking at the schematics, I'm not totally sure why the battery failing would quite cause this.
I could see that with it failing, some of the RAM bits would become corrupted; and so I would expect either the patches to go crazy, or the system just not booting up properly. However, it is mostly booting up fine and only failing after about half an hour. In the last case, it failed whilst I was just playing some midi notes on a custom sound (ie, not saved to patch memory). So that sounds not totally like it would be the fault of the RAM bits becoming corrupted whilst the machine is off. My only guess looking at the schematics, are the failing of battery causing the system to pull a bit too much on the +5v power line and get an undercurrent situation which then affects all the digital chips. Or maybe the RAM bits which are being corrupted are the settings memory but they're corrupting in a way that only manifests itself in rare occasions?
Regardless, I'll replace the battery when that arrives tomorrow since there's nothing to lose by doing so (particularly being a thirty year old battery...); but I will also try and check the following:
1 - The diodes in the battery to +5m line are working correctly (if the one by the battery is dead, then the +5v power line would be running through there - increasing the current usage which can't be good for it)
2 - Check the voltage level of the +5m line both with the system on and off
3 - Scope out the +5v line, and make sure it's relatively clean
My main reluctance to do these checks is just paranoia of the system dying from whatever is up with it (a minor paranoia), and zapping myself on the incredibly exposed psu board that is right below the processor board.
I'm pondering whether it might also be worthwhile to just replace the psu board with something more modern. However, since it requires a whole gamut of voltages (9v ac, +/-5v, +/-12v and that annoying +55v) it doesn't look like there's an off-the-shelf unit I can grab; but instead would have to get a basic switched mode power supply and then recreate the output transformer circuit from the xpander to get all those voltages. A lot of work, and really would need me to create a proper circuit board --- massively ambitious to the point of being impractical.
Of course, if there's an easier way or someone else has done the footwork for this, that would be amazing.
But enough rambling - let's hope the battery is the blame and the psu is still going strong.
