While this can be operator error like running then engine at lower speeds than WFO or failing to clean the cooling fins
Agree on this. OP is trying to fix the problem overlooking what might of caused it to begin with. Need conformation on are the cooling fins and block CLEAN. As in a shiny new engine look down to bare aluminum. Pictures of everything assist us in helping you.
Agree with sgkent. Take the battery to an auto parts shop. Have the battery
load tested for free.
Agree with Scrubcadet10. Need to do voltage drop testing on the entire + and - sides of the battery, while CRANKING engine. Also make sure the ground strap is a stellar fella'. Check every cable and connection on the + and - sides. Polish every connection while you are there. Little dielectric grease is your friend here. Star grounding washers should be added.
Do you power wash this after mowing? If so that is another topic as what not to do.