How to fix Roomba 500 series battery problem

My Roomba 520 has been out of service for more than six months. The wheels worked fine but the internal brushes didn't turn around anymore. I picked it apart and cleaned the internal gears and engine several times which made it work for a while but I finally gave in and bought a new engine.

Surprise, surprise, it refused to start even after charging it for more than one day. I always got this never ending loop:

It successfully undocked from its docking station but it just reset whenever it tried to start the brushes which led me to believe it was a low battery problem.

Mixing various instructions I found on the internet, I ended up with this procedure which seems to fix the problem:

1. Connect the Roomba and charge it completely (until the light turns  green)
2. Disconnect the Roomba from the charger (the light turns off)
3. Turn on the Roomba by pressing the Start button once (the light turns green)
4. Press and hold down the Dock and Spot buttons for 15 seconds to reset the Roomba (the Roomba turns off)
5. Release the Dock and Spot buttons (the Romba plays the 5 tone startup sound)
6. Go back to step 1

I have repeated the procedure 5 times so far and the run time keeps increasing.