This section explains how the measurement of resistance differs depending on whether a tester or resistance meter is used, and on when to use each instrument. It also describes how a battery’s internal resistance can be measured with a battery tester as an example application (battery impedance measurement).
01. Resistance measurement and low-resistance measurement
Resistance measurement with an analog tester
The figure to the right illustrates an analog tester’s resistance measurement circuit.
Before measuring the resistance Rx, short the test leads and perform zero adjustment. This step serves to correct for the tester’s internal resistance value.
If there is any voltage across the Rx circuit, it will result in a short-circuit, so be sure to check.
The analog tester detects resistance values based on the change in ammeter A when connected to the resistance Rx.
Two-terminal measurement with a digital tester and four-terminal measurement with a resistance meter
Most digital testers measure resistance with two terminals. They use a voltmeter to detect the resistance value of resistance R0 while applying a constant current to the measurement target, and the resulting reading includes the wiring resistance r1 and r2. To reduce the impact of the wiring resistance on measured values, it is necessary to perform zero-adjustment by shorting the test leads before measuring the resistance R0.
However, this method is not able to eliminate the effects of the contact resistance between the test leads and the measurement target. In addition, it is not able to yield an accurate reading if the resistance R0 is small.
In four-terminal measurement, the circuit that applies the constant current and the circuit that includes the voltmeter are independent up to both ends of the measurement target. As long as zero-adjustment is performed by properly shorting the four test leads, it is possible not only to eliminate the effects of contact resistance, but also to ignore the effects of wiring resistance values r1 to r4.
Only certain Hioki products, including some bench-top digital multimeters and resistance meters, use four-terminal measurement to measure DC resistance.
Resistance meter temperature correction functionality
The resistance of all objects varies with temperature. Since all targets measured with a resistance meter will not necessarily be at the same temperature, it is necessary to eliminate the effects of temperature in order to ensure consistent testing.
Resistance meters’ temperature correction functionality calculates the difference between the temperature value t acquired from a temperature sensor that is connected to the instrument and the reference temperature t0, applies a correction to the measured resistance value, and displays the result.
In order to make use of this functionality, it is necessary to configure the resistance meter with a temperature coefficient.
In the case of annealed copper wire, a coefficient of 0.00393/°C is used. (This is the standard value used by Hioki resistance meters.)
For more information about temperature coefficients for different materials, please see the user manual that came with your Hioki resistance meter.
Resistance measurement of wires using a resistance meter
Since the resistance of wires varies with their length, a unit known as conductor resistance (Ω/m) is used to express wire resistance.
The 24 AWG (0.2 sq) cables used to carry weak electrical signals in distribution panels have a conductor resistance of 0.09 Ω/m, while 6 AWG (14 sq) power cables have a conductor resistance of 0.0013 Ω/m, and 150 sq. wires have a conductor resistance of 0.00013 Ω/m.
If S represents area (m2), L length (m), and ρ resistivity (ρ・m) in the figure to the right, the wire’s overall resistance value is given by R = ρ × L / S.
02. Measurement of a battery’s internal resistance with a battery tester and other measurement applications
Principle of battery internal resistance measurement
Battery testers (such as the 3561, BT3562, BT3563, 3555, and BT3554) apply a constant AC current at a measurement frequency of 1 kHz and then calculate the battery’s internal resistance based on the voltage value obtained from an AC voltmeter. As illustrated in the figure, the AC four-terminal method, which connects an AC voltmeter to the battery’s positive and negative electrodes, makes it possible to measure the battery’s internal resistance accurately while minimizing the effects of measurement cable resistance and contact resistance. This technique can be used to measure internal resistance as low as several milliohms. These instruments also enable high-precision DC voltage measurement (OCV), another application in which high accuracy is required for batteries, at 0.01% rdg. By allowing the measurement frequency to be set to a value other than 1 kHz, the Battery Impedance Meter BT4560 can be used to perform more fine-grained testing of internal resistance from Cole-Cole plot measurement. It also delivers measurement accuracy of 0.0035% rdg. for DC voltage measurement (OCV) of batteries.
Internal resistance, battery voltage values, and appropriate battery testers by battery type
The figure illustrates Hioki’s line of battery tester models that measure batteries’ internal resistance (IR) and voltage (open circuit voltage, or OCV) as well as which types of battery each instrument can be used to measure. The BT4560 and 3561 are well suited for use with battery cells designed for electric vehicles (EVs) and hybrid electric vehicles (HEVs) as well as with lithium-ion rechargeable batteries used in compact battery packs for mobile devices due to the low internal resistance of these types of cells. By contrast, the BT3562 and BT3563 should be used with battery packs (sets of multiple lithium-ion rechargeable batteries) due to the high battery voltage (OCV) of such configurations. Although the instruments can also be used to measure internal resistance and battery voltage for other rechargeable batteries such as nickel-metal-hydride, lead acid, and nickel-cadmium batteries, you should choose a battery tester on the basis of the battery voltage (OCV).
Measuring the internal voltage of a battery pack (also known as an assembled battery, battery stack, or battery module)
To obtain the required voltage, a battery is constructed by connecting multiple cells in series. To create such a battery pack (also known as an assembled battery, battery stack, or battery module), tabs or busbars are welded in place to connect the cells. The resulting weld resistance is included in measurements of the battery pack’s internal resistance. Since weld anomalies will prevent the battery pack from delivering its full level of performance, it is recommended to test assembled battery packs using a battery tester. The BT3562 can measure the internal resistance of battery packs of up to 60 V, while the BT3563 can measure the internal resistance of battery packs of up to 300 V.
Measuring a battery’s Cole-Cole plot
Broadly speaking, a battery’s internal resistance consists of three components: ohmic resistance (weld resistance), reaction resistance (charge transfer resistance), and diffusional resistance (Warburg impedance). These components are generally calculated by means of Cole-Cole plot (Nyquist plot) measurement. The Battery Impedance Tester BT4560, which allows the measurement frequency to be varied within the range of 100 mHz to 1.05 kHz, is ideal for Cole-Cole plot measurement. The instrument can measure a battery’s effective resistance R and its reactance X. It also ships with standard application software that can render Cole-Cole plots. In addition, LabVIEW can perform equivalent circuit analysis for simple batteries.
Other applications: Measuring the ESR of electric double-layer capacitors (EDLCs)
The internal resistance of electric double-layer capacitors (EDLCs) that belong to Class 1 and are used in backup applications is measured using AC. Hioki battery testers can also be used for simple measurement of Class 2, Class 3, and Class 4 capacitors. The BT3562 can measure ESR of up to 3.1 kΩ at a frequency of 1 kHz. JIS C5160-1 defines the measurement current for such applications, and the LCR Meter 3523 can be used in applications where the measurement current must conform to the JIS standard. With the BT3562, the measurement current is fixed for each measurement range.
Measuring the ESR of a lithium-ion capacitor (LIC)
As a result of a phenomenon known as transient recovery voltage, the potential of a lithium-ion capacitor (LIC) or electric double-layer capacitor (EDLC) does not stabilize immediately after the component charges or discharges. If the capacitor’s ESR is measured under those conditions, measured values may fail to stabilize due to the effects of the transient recovery voltage. The Battery HiTester BT4560’s potential gradient correction function can be used to cancel out the effects of transient recovery voltage, making stable ESR measurement possible. The instrument has a maximum resolution of 0.1 μΩ, and it can measure lithium-ion capacitors and electric double-layer capacitors with low ESR values of 1 mΩ or less.
Measuring the internal resistance of a Peltier device
Peltier elements can be used in cooling, heating, and temperature control through application of a DC current. When measuring a Peltier element’s internal resistance with a DC current, the measurement current causes heat flow and temperature changes inside the element, making it impossible to obtain a stable measurement. By using AC current to make the measurement, the amount of heat flow and temperature change can be reduced, enabling the component’s internal resistance to be measured in a stable manner. Since the BT3562 can measure internal resistance using an AC current at a measurement frequency of 1 kHz, the instrument is able to measure the internal resistance of Peltier elements with low resistance values on the order of several milliohms.
