I would guess battery as well. You can replace the Lead batteries in the professional easily (contact your dealer or order from our site
http://paradiserobotics.com Or you can contact us about upgrading to the Lithium Ion battery (you only need 1). info@paradiserobotics.com
Lithium Ion mows longer time and the battery usually lasts way longer than Lead. Some customers can go up to 5 years (if you are lucky). The shortest fail time I've seen so far from our customers for Lithium is 3 years. I am sure it can also be shorter, but most last a very long time. You can increase battery life time by having your Lawnbott enter and leave the charging base less times. PS you get a 1 year warranty on Lithium, but only 6 months on Lead Acid.