I think the service manual is partially wrong to single out the charger is the only problem for Err5 error. The battery may generate the same error.
The following is the proof.
From the 500 Series Roomba service manual for the Err5:
Power Supply Failed/Overheated or Roomba lost connection on Home Base
This occurs when the full charging current is below 400 mA for more than 30 minutes.
Those members, who uses Li-Ion battery with the 500 Series Roomba, are familiar with the Err5 charging error. It is due to the Li-Ion battery internal controller shuts off (disconnects) itself from the charging source when the cells are fully charged. In this case, it has nothing to do with the charger. One member had the same error and it was the battery problem.
A few weeks ago, I was dealing with a Scooba giving "Alarm + 4 beeps" error. It was a Ni-Mh battery. But it generated the same error as my Li-Ion Scooba battery.
From the Scooba service manual for "Alarm + 4 beeps" error"
Power Supply has failed
•Need to test the Power Supply and Scooba.
It turns out that this error has nothing to do with the charger. It is the battery itself.
Therefore, don't write off the charger yet since the OP said the charger no-load voltage is 22.5V. We need to test the battery. [end of proof]
1. How old is your battery? If it is over 1 year old, you may need a new battery.
2. What type of battery are you using in your Roomba? Li-Ion or Ni-Mh?
3. If you have a Ni-Mh battery, do a load test on your battery with a car tail light bulb.