Normally kapag umiinit ang wire, either overloaded or loose ang terminal. But in your case, tingin ko wala ka namang additional heavy electrical load which will cause the abnormal temperature of wire. So its the terminal connection of the battery. Pero dapat hindi lang yung wire ang mainit, pati na rin yung terminal post. Kung hindi naman mainit yung terminal post, maybe you have a loose connection somewhere after the battery terminal.