Appendix A: Internal Resistance of Batteries and Battery Aging

Ideally for any source of electricity, it is desirable that the voltage is constant, irrespective of how much power is taken from the source (provided that it is within the capacity of the source). This is usually not the case, particularly for batteries. Available voltage at the terminals of a battery does not stay the same if the current (and consequently the power) taken from the battery varies. As current increases, voltage decreases; in other words, there is more voltage drop in the source. As long as this voltage drop due to an increase in the delivered current is small and acceptable, everything is fine.

Figure A.1 illustrates the decrease in voltage versus the increase in current for a battery. The graph for an ideal source is a horizontal line, and for a new battery this line has a small slope. For example, if the case of a car battery is considered, measurement by a voltmeter can show 14 V, but when starting a car if 200 A is taken from the battery, the voltage can only be 12 V. An old battery has a steep slope, and although it can look good for turning the lights on, when cranking a car, its voltage can drop to 5 to 6 V, which is not high enough to deliver the required torque to the engine.

We can always think of an internal resistance in the battery where when a battery is new, its value is low, but it gradually increases as the battery becomes older. In this sense, part of the battery’s power is consumed in this resistance, and as expected, it causes a voltage drop because it is in series with a load to the battery. This resistance, nevertheless, is very small. For instance, the value for this resistance can be on the order of 10 to 40 milliohm (mΩ). Figure A.2 shows the schematic for what can be represented as the internal resistance of a battery. Note that it is not accessible from outside a battery because it is an intrinsic part of the battery.

Figure A.1 Voltage drop in a battery due to the current drawn from it.

Figure A.2 Representation of the internal resistance of a battery.