A 12V car battery dies not so much because its voltage drops

A 12V car battery dies not so much because its voltage drops but because chemical reactions increase its internal resistance. The dead battery has an internal resistance of 1.0 ohm. A good 8V charging battery (with internal resistance of 0.12 ohm) connected with jumper cables can both start the engine and recharge the dead battery - see figure below. The picture on the right is a representation of the starter motor circuit that is driven by the car battery.^* Good circuit diagrams are important here! How much current could the dead battery alone drive through the starter motor? With the jumper cables attached, how much current passes through the starter motor? With the jumper cables attached, how much current (and in what direction) passes through the dead battery? If the current that you determined in part b) is the required current for the starter motor (and the current that should flow from the battery), what should be the internal resistance of a good 12V car battery? What happens to the current through the dead battery and through the starter motor if the terminals of the good battery are switched (i.e. when the jumper cables are switched)?

Solution

The starter motor is powered by the car battery. To turn over the engine the starter motor requires a very high electric current, which means the battery has to have sufficient power.

Starting system problems are common and not all problems are caused by a faulty starter motor. To find the cause of the problem the starting system must be properly tested.