[hatzie]

The year and engine type would be helpful...
Assuming it's an 80's Diesel with dual batteries and not a TP2 GM camper battery setup.
It's likely a corroded connection. Fixing it is tedious fussy work.

You can usually shotgun this one and fix the problem. It'll make the batteries, starter, and alternator happier too.

Looking at your pictures I can see a problem already...

The long battery bolt in your picture is a regular hardware store bolt... side post bolts are special parts. Especially the long bolt GM used to hook up the parallel battery to battery cable. Regular bolts will not work well and likely the special spacer GM used between the cable lugs is missing as well.
On a Dual Battery Diesel you need three regular length side post battery bolts and one long bolt with a special spacer to make contact between the positive cable connections.
Dorman 60310 is a "Long" "Diesel" Dual Battery Positive battery bolt and spacer. It's listed as a HELP! part...
You need three GM 8917444 or Dorman 60307, Short Regular side terminal battery bolts. Your RH battery negative has the correct bolt... I added this part number note because there isn't a picture of the LH battery so I can't tell what the previous owner did on that side.

You should clean up the battery lugs while they are off.
The puffy and crunchy crud on the cables is tin, arsenic, lead, and copper oxides and sulfate salts of some or all of the prior. Tin and sometimes arsenic were in the solder alloys used to tin the copper battery lugs. Your skin is not a serious barrier to Arsenic sulfate, lead sulfate, and copper sulfates and other similar metal salts. Swhy your hands turn a pretty blue and stay that way when you work with copper pipe on a hot day. Copper, lead, and arsenic are pretty nasty so it goes without saying you shouldn't breathe it or get it directly on your hands, near your eyes, or in your mouth etc. etc... Nuff said. We're all adults here.

GM battery cables are not overmolded so, after you remove the bolt, the cover can be removed from the battery lug and the connector can be cleaned with a brass brush. I usually clean off the connectors and re-install them with Vaseline or Battery cable protectant ( I use CRC 05046 because it's on the shelf locally but there are lots of them out there ) to keep out the weather and outgassing of Sulfuric acid from the battery vents.


Clean up the battery ground lugs at the alternator bracket and the LH side of the engine too. Spray them with battery protectant.

If that doesn't fix it move on to the starter BAT connections and the connections at the firewall J-studs. Remove, clean, and spray with battery protectant.

I would make a clean sweep of it with the braided ground straps... Rinse and repeat... remove, clean, re-install, and spray with battery protectant.

Last pull the 10ga BAT cable off the alternator (with the battery grounds disconnected), clean, re-install, and spray with battery protectant.