Error 5 after half an hour

Short: Roomba is returning an error 5 after charging for half an hour on original battery and new battery.

How do I fix this?

Long: I have a Roomba 557, used weekly for a few years, then wouldn't be able to run longer than 15 minutes on a full charge. After that, it was in storage (under the couch) for about 4 years. Figuring the issue was the battery, I got a new one from ifixit, easy install. The cord from the adapter to the dock has the ability to plug directly in the unit, and for the first battery charge you want it on there for quite some time, so I used that instead of the dock. Half an hour later, I get an audible error and "Error 5" on the display. Searching suggests this is an error with the battery. Unplug, replug, half an hour later, same error. Tried original battery, same error. Tried charging it using the dock, same error.
I'm getting 22-23 volts off the adapter, so near as I can tell, the issue is somewhere between the power coming in, and getting to the battery, but I don't know how to diagnose or correct that.