Ezgo getting weak

[Ray83]

I've got a late 90's model ezgo that I bought a few months ago and this my 1st golf cart so I don't know much about them. It has a aftermarket alltrax controller, power converter and a motor out of a beast cart. It's 48v (6-8v batteries), 5 of the batteries are less than 14 months old and 1 is new. The cart has never run as long as I would think it should. I'm using it on the farm with some hills. Lately I been trying to figure out any problems. I have found that the batteries are fine but the cart will gets weak like the batteries are dying and is slow on hills. I noticed yesterday that as soon as I run it, i got off and tested the voltage it was only reading 20v but steady climbing and within 5 minutes it was back to 48v. I'm testing at the solenoid, not sure if this is where I should be checking and if so, is this normal? The same goes with testing the batteries separately, they will test a little higher after the cart has set a few minutes.The cart will also on occasion jerk when accelerating. I have checked connections for loose wires and checked for wires that are warm and everything seems fine. Would a bad solenoid, f&r switch or controller cause this?
There is also a yellow wire that comes out of the power converter that will arc bad hooking to battery even when the main cable is disconnected, not sure why? If you leave that wire disconnected the cart will still charge and run find but the lights won't work. Like I said, I don't know much about these things so I'm trying to provide as much information as I can. Any help would be greatly appreciated! This thing is driving me crazy and I'm about ready to sell it if I can't get it right! I can only ride it about 2-3 miles on the farm and it's gets too weak to go up a small hill but after it sits a few minutes the batteries test fine.
Thanks

[golfcartg]

Your description really pinpoints that the root of your cart’s struggles might not be the components themselves but rather the performance of your power source. Batteries are the heart of any electric system and even when they seem fine at rest, under load they can reveal hidden weaknesses that directly cause issues like voltage drops, weak performance or jerky acceleration. Since you’re already noticing that your voltage dips significantly during use and climbs back after resting, it strongly suggests the batteries aren’t maintaining their strength when under demand. Here's how you can confirm this yourself, it’s simple, and you’ll see exactly how your system is behaving.

Start by testing the whole pack voltage under load. Grab your digital voltmeter (DVM) and attach alligator clips to the main positive and negative terminals of your battery pack. Leave the meter in place and drive the cart, especially up a hill or in a way that stresses the system. Watch the voltage closely and jot down the lowest number it hits during the climb. This reading will show how your pack performs under pressure, something resting measurements can’t reveal. Compare your results to the attached chart, you never want a resting voltage below a 50% state of charge.

Next, load test each individual battery. Repeat the same steps but move the alligator clips to the terminals of one battery at a time. Take the cart for the same uphill ride and write down the lowest voltage each battery hits. This step will highlight if one or more batteries are disproportionately dragging down the system. A healthy 8V battery shouldn’t drop below around 7.5V under load, so anything significantly below that is a red flag.

Once you’ve got the numbers, write them down and post them here so we can figure out the best course of action.