Batteries chemically produce electricity that is used to power many portable devices including cell phones, tablet computers, flashlights, hearing aids, back-up power, and automobiles to mention just a few. Batteries are rated in both voltage and amperehours.

A typical car battery delivers a nominal 12 V for 70 A-hrs while a “D-cell”

delivers 1.5 V for 4.5 A-hrs. Batteries are not ideal sources; they are all real or practical sources of energy and must be modeled as an ideal voltage source in series with a resistor. The series resistor models the battery’s internal resistance, which can vary from a few milliohms for a car battery to as high as 100 ohms for a hearing-aid battery.

As a battery ages, especially in use, its internal resistance increases decreasing its usefulness.

Therefore, it is good practice to measure a battery’s voltage while under load since measuring the battery’s voltage with an open circuit could indicate a good voltage regardless of its internal resistance. The following example looks at the effect of a battery’s internal resistance on the current available to a load.

A car battery of 12.6 V and an internal resistance of 25mΩ delivers 100 mA to accessories and 210 A to the starter motor of a 6-cylinder car. Find the resistance of the starter motor