Today I swapped out a marginal battery on the driver side of my 07 2500 5.9 Cummins. Prior to changing the battery my charging system was working and with the new battery the passenger side is charging and the driver side is not. I assume that I probably need to replace the cables on the truck. Here is my question. On the negative side of the battery is a wire that attaches to the lug and appears to go down under the motor. Wire is in pretty bad shape at the terminal and I want to replace the end but have no idea what this wire is for or if it's ok to cut and solder on new terminal in.
Thank you in advance.

 

I would replace it as is. Could be an engine ground and may be why that side isn't charging. Try putting a jumper cable on that negative post of the battery that isn't charging and grounding it to something that you know is a good ground like the engine block. If it goes over 13 volts than you'll know that's the problem. Definitely ok to solder or clamp a new end on.

 

If you have the opportunity, switch everything over to cables with lugs on them. Switch the battery terminals to military style for those lugs. Life is much simpler that way.
A great places to buy the lugs or entire cables is 1998-2002 Dodge Ram 2500/3500 5.9L Cummins 24 valve Diesel Battery Cables. They have the lugs cheaper than any place else I have found.
I replaced the entire set of cables on my 96, from them, but for my 99, when I had the left positive terminal get on its last leg, I just replaced that one terminal with a mil style and lugs purchased from this place.

Chris

 

For sure get the cables in good shape. Power output from the alterntor goes to one battery, and the ecm gets power- and senses voltage to regulate the alternator- from the other battery.

As long as all the connections are good it works fine, but if not one battery might not charge, or the other battery may overcharge.

 

You ever figure it out?