Per the manual, self test error 32 is the real time clock. It also says the battery has an expected life of 7 years and is replaceable EXCEPT for the 300 Micro.
My last facility had some of those in service and had a battery fault on a spare unit we tried to install. After investigating repair options some, we opened it up and everything was encapsulated/potted to make it non-serviceable (by us). It may be possible to dig into it and find which bit is the battery and replace it, but it's also probably not worth the effort on these. We had a test board in the maintenance shop with a variety of PLCs and HMIs on it for the maintenance guys to practice with, so we had them add a section to hold 4 of these just to keep them powered up so the batteries wouldn't drain. It was a pain, but it kept our spares alive until they could be migrated.