I have seen failures of parallel charge relays that PREVENTS FOREVER amperes from flowing by switch from priority HOUSE BATTERIES to chassis batteries. The engine start battery is drained and unless you take the time to manually connect cables together you is screwed. Jumping is rather a pain in the A$S on a crowded two lane highway, the middle of an intersection or straddling railroad tracks. The cure is to replace the relay. Then make the engine starting battery (chassis) the priority battery. Nothing in the way between the alternator and engine starting battery.
Wire a commercial fishing trawler where the main engines are isolated from the battery and before you know it you'll have a stout Danforth anchor chained to your ankles.
