Possitive cable from the Alternator to starter.

Take your meter and go from the battery to the starter. Positive bat post. Your looking for voltage drop. 0 is good. Measuer from the starter along the harness to the alt. If you see 2.2v thats your voltage drop and the harness is bad. You can make your own cable and run it to the battery. I've made custom alt harnesses for cars this way using wire bought from a marine supply store. Heavy duty cable and corrosion resistant.

 

I think the reason for that is that it effectively connects the alternator in parallel that way, without adding an extra connection right at the battery. But, excellent diagnostics on your part. You are one step away from a permanent solution. Replacing the faulty cable. (Either corroded or broken, or worn through, is my guess).

 

Many electrical connections originate at the starter. It's much easier then to clutter the battery box.

Connection need to be cleaned and the batteries need to be load tested.

 

They are going to fail very soon. I would have replaced them at 4 years.

You're lucky

 

Wiring and battery cables are actually a big expense for OEMs, so they try to reduce as much wiring as possible. They just route the alternator cable to the starter to save money.

 

Batteries used in solar panel systems will last up to 11 years, batterys fail because they are abused, a new battery has about a dozen dead battery cycles before it starts to deplete it's effectiveness. I am a firm believer in disconnecting, all the batteries and individually charging them with a regulated charger. Washing the batteries, that will extend their life, compared to ones that are not washed. Do a voltage drop test over the tops of the batterys, proves the benifits of doing that.
Doing a voltage drop test hooking the test leads to both ends of wire measuring the voltage drop is a great test, you also want to clean every available connection before you condemn a cable. Most cables have a little bit of extra length, I use to solder on new ends on many units. You also want to check the voltage drop from the alternator to the right frame rail, I use to run an extra ground cable, that stops all kinds of electrical gremlins.
Most of the electrical loads when the truck is running come from the alternator not the batteries, they are used as a buffer, so the starter is used as a junction block, going to the batteries would just add extra length, resistance and weight.
Just a thought!

 

I haven't noticed what size wire they used on my truck but oem usually goes with just enough to get the job done. Replacing it with a bigger guage might be a good idea while you are in there. I've heard that the wire count matters too but I can't really say why or what to look for. Something about the current traveling on the surface of the wires and the higher the wire count in the bundle the beter it flows current. Don't quote me on that. Just thought I should throw it in since I didn't see anyone else bring up wire size.