Unless the mower isn't recharging the battery, and you have to put it on a charger before starting. the other possible causes are the area that you are using the equipment is rough and is breaking the plates in the battery. I have that problem with my Kioti DK45s. The other possible reason is the batteries are just old.
Batteries come in three levels of quality:
Wet charge/wet ship which is the most common like what Walmart, Rural King, and the like use must be put into use with 3-6 months of manufacture
Wet charge/dry ship must be put into use with 6-12 months of manufacture and are slightly more expensive than wet ship.
Dry charge/dry ship have a shelf life of 5 years unless they are charged with acid, which at that time must be put into use within 3-6 months. these are also the most expensive.
Had a customer a few years back that went through 6 batteries one summer due to the same issue you are having. Cause was old battery stock at the store
