Long ago, on a dark and stormy night in the Andes, Fred flipped the switch to turn on the spiffy new lights he had installed in the back of his Blazer, only to see, well, nothing. Clearly, his dual battery setup didn’t work, and he had the dead battery to prove it. Now, years later, when he flips the switch in his camper, he does get lights. And fans. And coffee. And even air conditioning.