From the document
Soft Start – Following a deep discharge applying the full current immediately may damage the
battery and cause overheating, the application of the current is tapered during the soft start
period to allow the recharge process to commence as efficiently as possible. Soft start is only
used following a deep discharge or when battery voltages are low
Give the batteries an hour rest before connecting the charger, particularly on hot days, and maybe consult with the charger supplier and confirm your charger meets the Soft Start requirements listed in the manual.