Testing out B4 puchasing new Batts

[NoMoreTime]

Mornin all. Ok so I got this 1997 TXT 36 volt cart. I installed 3 12 volt batteries taken from my cars so I know they are good. Cart runs nice for about 15 or 20 minutes then when the battey pack voltage drops to around 29 volts, no movement. Let is rest a few seconds, pack voltage comes back up to 36-37, cart will go a very short distance again. When the cart stops, and I attempt to go again, there is a strange noise like splines slipping against each other. I checked the hubs/axle shafts,,OK, pulled the diff cover,,OK. No mechanical issues found. I would have thought that when the pack voltage drops the cart would simply go slower and slower and eventually stop, but not make this strange noise. Any thoughts?

Also, I've read here a couple times that a person would be lucky to drive a cart more than 30 minutes with 3 good 12 volt batteries. Is there really that much of a difference between these and 6, 6volt batteries?? WHY? I don't understand where the difference lies.

Thanks