I noted yesterday that an extension cord going to pond had been chewed on (a bit of copper was showing). Since it was raining, decided not to do anything then. Taped over it this morning (unplugged cable, taped, plugged back in) from 09:02-09:14.
During this process, I note that Vdsm went from 12.4 (plugged in) to 11.9 (just battery). This tells me that the power supply voltage that charges the battery is a bit low. I went back to check if this power supply voltage could be adjusted, but it can't (at least without taking the supply apart, which I didn't want to do because it is quite hot). It has been running for weeks like this, so I would expect it to keep running.