Runs Slow, Dies quickly

[Dneon95]

So I bought a 1996 ezgo txt recently and the first thing I did, was change all the battery cables to 2 gauge. Other then playing around in the guys driveway, I never really tested it. I've been fighting with it for 2 weeks now, I charge it til the charger says full, run it a bit. It never goes over 8mph which is way off, and after a few minutes it slows down til it finally dies. It doesn't seem to be a battery issues, because when I plug the charger in, it fills quickly. I did notice one battery was reading 4v and noticed it was almost empty on water. I filled it and all read correctly. Same issues though. I noticed today a battery cable loose connected to nothing, and I am trying to figure out if I just forgot to pull it when I replaced it. The one end is still connected to B- on the control box. Along with my new cables. I don't see a reason for double grounds, am I missing something, or does someone have an idea about the issue? Thanks.