The internal resistance provides valuable information about a battery as high reading hints at end-of-life. This is especially true with nickel-based systems. Resistance measurement is not the only performance indicator as the value between batches of lead acid batteries can vary by 5–10 percent, especially with stationary units. Because of this wide tolerance, the resistance method works best when comparing the readings of a given battery from birth to retirement. Service crews are asked to take a snapshot of each cell or monoblock at time of installation and then measure the subtle changes as the cells age.
There is a notion that internal resistance is related to capacity, but this is false. The resistance of modern lead acid and lithium-ion batteries stays flat through most of the service life. Better electrolyte additives have reduced internal corrosion issues that affect the resistance.
Before exploring the different methods of measuring the internal resistance of a battery, let’s examine what electrical resistance means and understand the difference between pure resistance (R) and impedance (Z). R is pure resistance and Z includes reactive elements such as coils and capacitors. Both readings are obtained in ohms (Ω), a measurement that goes back to the German physicist Georg Simon Ohm, who lived from 1798 to 1854. (One ohm produces a voltage drop of 1V with a current of 1A.) The electric conductivity can also be measured in siemens (s). Combining resistance and impedance is known as reactance. Let me explain.
The electrical resistance of a pure load, such as a heating element, has no reactance. Voltage and current flow in unison and there is no advancing or trailing phase. The ohmic resistance is the same with direct current (DC) and alternating current (AC). The power factor (pf) is 1, providing the most accurate metering of the power consumed.
Most electrical loads are reactive and consist of capacitive reactance (capacitor) and inductive reactance (coil). The capacitive reactance decreases with higher frequency while the inductive reactance increases. An analogy of inductive reactance is an oil damper that stiffens when applying a fast back-and-forth action.
A battery has resistance, capacitance and inductance, and the term impedance includes all three in one model. Impedance can best be illustrated with the Randles model (Figure 2) that comprises resistors R1 and R2 as well as capacitor C. The inductive reactance is commonly omitted because it plays a negligible role in a battery, especially at a low frequency.
Measuring the battery by resistance is almost as old as the battery itself and several methods have developed over time, all of which are still in use.