Starting a car requires a massive amount of battery power, especially on six cylinder models. When the starter circuit is activated it cuts all the other circuits to give full power to the starter motor. If you car is displaying the symptoms of a low battery but the lights work on main beam without fading then the battery power is decent and is getting to the other electrical circuits but not to the starter circuit. A poor connection in the starter circuit will give the same symptoms as a low battery. The most common cause is a loose or corroded chassis to engine earth that runs from near the engine mounting to the block.