Check the output from the alternator
Should be 12.5 to 14 V DC.
!2.5 can just keep the battery charged and 14 is needed if you have an electric PTO
So volt meter across the battery engine stopped and the same engine running.
If the later is not more than the former the you have charging problem.
In a dark place turn off the mower then remover one of the battery terminal.
If it sparks then you have a short somehwere draining the battery when not in use.