next time you run it and shut it down, and then go to rtestart.. when you would expect it to hard-start.
park it next to something with a good bat. like your diesel pickup.. or a tractor with a big bat.
have charger on it to have it topped. when you shut down, put charger on your bat.
come back and kick the charger over to start and jumper cable the bats. and then see what it does. if it bang off.. i'd load tes the bat and check all your connections.
ps. when starting hard. is it making smoke?