Hi, good afternoon,
Thanks for the good expanation for the issue. In this case it sounds like the battery needs to be replaced;-(
But another issue these Neatos have is poor connections in the battery box.
Please check them with a bright light and look to see that the connection points or contacts are clean BRIGHT AND SHINY, if they aren't please clean them with a tiny bit of WD-40 or similar solvent on a Q tip to clean.
These seem to also cause this problem jut because they have a bad connection. this naturally corrode over time and need this basic maintenance.
Try cleaning the contact first before replacing the battery, but your battery is likely at its life's end and can no longer drive the current needed for proper operation.
In any case, I will stick with you until this is complete and working as it should.
Please rate kindly thus far, it won't cost you anything to rate my service to you kindly,
Then you can reach me by simply replying to this message anytime.
Thank you and have a wonderful day!