Yep, this is the classic symptom of the battery to battery-cable connection.

Clean the battery posts and the cable terminals. Scrape to bare shiny metal. Use a battery post brush if you want.

Then coat them to help slow down oxidation. You can buy some red stuff that's made for the purpose, it works well, or you can put a light coating of grease on the metail surface of the post and inside the cable end, which works fairly well.