Sounds to me like the ae old problem of corroded battery terminals and that should be the first place to look (KISS). Take the cables off the post and wire brush or use a terminal cleaning tool and clean both the posts and ther terminal clamps to a shiny clean. Maks sure that the terminals clamp down tight to the posts when done and that you can not turn them on the terminal by hand If you can an easy fix for that is to take out the olt and clamp the terminal together with a vise grips or put the terminal in a vise and cut through the mating surface with a hack saw making it able to clamp tighter. Reason there is lights etc is they don't draw near as much as the starter.