Final battery connect sparks

[Datawave]

Firstly, what a fantastic forum and participation.
I just bought a '90 36vdc Club Car from a friend. It was driven to my house from a few blocks down the road. When I took it later in the day it barely had enough power to get it to the charger. The batts were '03 t605's. I put it on charge for close to 12 hours to see if I could get anything out of them. The charger stayed at 25-30 amps. The forward/reverse swith was in forward during the charge, the key was off. I checked the SG and float levels and the batts weren't coming back. I replaced them with a new string of 105's and when connecting the last connection..which was the most positive I get a large spark. If I moved the F/R switch to neutral I can connect. I didn't change any wiring other than the batts. I don't want to ruin the new batts or do any further welding. Am doing something wrong procedurely? I'm afraid I may have shorted the motor during the charge? Any guidance is appreciated.Thanks.