Note that you should not be wet or barefeet when handling batteries and powered stuff. Do Notice Swelling & cracks in lead acid batteries these are stone dead and are not revivable as they have 0 volts. Perhaps the Epsom Salts trick as noted on many YouTube videos will get some life from these but are junk to me.

Look for corroded and loose connections - Note the wiring diagram of how these four batteries are wired to make 48 volts. For 36 volts simply remove a battery from the connection.