A wet cell battery has an open vent system. The vent is built into the cap of the battery that is to vent gases and let excess liquid out. The liquid in the battery is sloshing inside the battery while the mower is motion. When the mower is used at the maximum 20 degree angles and the battery liquid sloshing there is a good chance the liquid may be more prone to leak out the vent.
Operating this mower at a 20 degree angle is the maximum this product should be operated at for the operator safety and keeping control of the mower.
-The Toro Company